Class 9 Science Chapter Wise Topic Wise Notes Chapter 4 Structure of The Atom

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CLASS IX
SCIENCE

INTRODUCTION
STRUCTURE OF THE ATOM
04
Class 9th Science (Chemistry)
INTRODUCTION
Discovery of protons
The proton is the positively charged subatomic particle present inside the nucleus of the atom.
Proton carries a positive charge.
The mass of the proton is about 1840 times the mass of the electron and its mass is almost equal to
the mass of the neutrons. The number of protons inside the nucleus of any atom is called the atomic
number and the properties of any element are the periodic function of its atomic number. Generally,
the number of protons and the number of electrons for any atom is equal which maintains the
electrical neutrality of any atom.
Mass of Proton
As we know that all the mass of any atom is concentrated in the nucleus of the atom as both Protons
and Neutrons reside inside the nucleus. The mass of the atom is measured in amu or Atomic Mass
Units.
The mass of a proton is 1.6726219 × 10-27
kilograms or 1.6726219 × 10-24
grams. It is slightly equal to
1 amu.
In 1886 Eugene Goldstein (1850–1930) discovered evidence for the existence of a positively charged
particle. Using a cathode ray tube with holes in the cathode, he noticed that the rays were travelling
in the opposite direction from the cathode rays. He called these canal rays and showed that they
were composed of positively charged particles.
The nucleus of the atom was discovered by Ernest Rutherford in the year 1911 in his famous Gold
Foil experiment. He concluded that all the positively charged particles in an atom were concentrated
in a singular core and that most of the atom’s volume is empty. He also stated that the total number
of positively charged particles in the nucleus is equal to the total number of negatively charged
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INTRODUCTION
04
electrons present around it. To support his theory he performed a famous experiment known as the
Gold foil experiment.
Discovery of Electron
The electron is a negatively charged, low-mass particle. As a result, passing close to other electrons
or the positive nucleus of an atom might readily deflect it. The first basic particle identified was the
electron.
J.J. Thomson and The Discovery of Electron
J.J. Thomson, a scientist, began working with cathode ray tubes in the early 1900s. Cathode ray tubes
are vacuum-sealed glass tubes that have had most of the air removed. At one end of the tube, a high
voltage is placed between two electrodes, causing a stream of particles to flow from the cathode
(the negatively charged electrode) to the anode (the positively charged electrode) (the positively
charged electrode).
Because the particle beam, or cathode ray, starts at the cathode, the tubes are termed cathode ray
tubes. The beam may be detected by painting phosphors on the tube’s far end, beyond the anode.
Thomson surrounded the cathode ray with two oppositely charged electric plates to investigate the
particles’ characteristics. The cathode ray was redirected from the negatively charged electric plate
to the positively charged plate.
The cathode ray was made up of negatively charged particles, according to this.
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INTRODUCTION
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Thomson also installed two magnets on either side of the tube and noticed that the cathode ray was
diverted by the magnetic field. Thomson used the findings of these tests to calculate the mass-to-
charge ratio of cathode ray particles, which led to a surprising discovery: each particle’s mass was
much, much lower than any known atom. Thomson continued his tests with several metals as
electrode materials and discovered that the characteristics of the cathode ray were consistent
regardless of the cathode material.
Thomson made the following findings based on the evidence:
Negatively charged particles make up the cathode ray.
Because the mass of each particle is ∼1/2000 that of a hydrogen atom, they
must be part of the atom.
Within the atoms of all elements, these subatomic particles may be found.
Thomson’s discoveries were initially contentious, but they were progressively
accepted by scientists. His cathode ray particles were eventually given a more
common name: electrons. The discovery of the electron contradicted Dalton’s atomic
theory’s assumption that atoms were indivisible. An altogether new atomic model
was required to account for the presence of electrons.
The charge to mass ratio of an electron
The charge to mass ratio of the electron is given by:
e/m = 1.758820 × 1011
C/kg
Where,
m = mass of an electron in kg = 9.10938356 × 10-31
kilograms.
e = magnitude of the charge of an electron in coulombs = 1.602 × 10-19
coulombs.
Experimental setup for the determination of charge to mass ratio of the electron
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INTRODUCTION
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While carrying out the discharge tube experiment, Thomson observed that the particles of the
cathode deviate from their path. He noticed the amount of deviation in the presence of an electrical
or magnetic field depends on various related parameters.
❖ Particles with a greater magnitude of the charge experienced greater
interaction with the electric or magnetic field. Thus, they
exhibited greater deflection.
❖ Lighter particles experienced greater deflection. Thus,
deflection is inversely proportional to the mass of the particle.
❖ Deflection of particle from their path is directly proportional to the
strength of the electrical and the magnetic field present.
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STRUCTURE OF AN ATOM
04
Atoms are tiny particles that comprise all the things in the known universe. Atoms of an element are
responsible for all chemical reactions occurring in nature.
Atoms are made up of three fundamental particles namely,
• Electrons
• Protons
• Neutrons
These particles are also called subatomic particles as they are found inside atoms. An atom is the
basic building block of matter and all the thing around us is made up of combing atoms in different
proportions.
Atoms are the smallest part of an element and between the 18th and 19th centuries, many scientists
proposed their theories regarding the structure of atoms based on their studies. These theories were
published to elaborate on the structure of atoms, and its constituent.
The most appreciated theories about the atomic model were proposed by,
• John Dalton’s Atomic Theory
• J.J Thomson’s Atomic Model
• Niels Bohr’s Model of an Atom
• Ernest Rutherford’s Atomic Model
These theories give us an idea about the structure of the atom and the general output of all the
theories is that,
“An atom consists of two parts a nucleus which lies at the center and carries a proton (positively
charged particle) and neutron (negatively charged particle) and the other part is the outer shell that
carries electrons (negatively charged particles). The electrons present in the outer shells
continuously revolve around the nucleus without losing any energy.”
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Thomson’s Model of an Atom
According to Thomson,
❖ An atom consists of a positively charged sphere and the electrons are embedded in it.
❖ The negative and positive charges are equal in magnitude. So, the atom as a whole is
electrically neutral.
❖ The first model of an atom to be put forward and taken into consideration.
❖ He proposed a model of the atom be similar to that of a Christmas pudding/watermelon.
❖ The red edible part of the watermelon is compared with the positive charge in the atom.
❖ The black seeds in the watermelon are compared with the electrons which are embedded on
it.
Postulates of Thomson’s Atomic Model
❖ An atom consists of a positively charged sphere or cloud in which negatively charged electrons
are embedded. The positive charge of the sphere is uniformly distributed to balance the
negative charge of the electrons.
❖ An atom is an electrically neutral quantity, and the total positive
charge of the atom is balanced by the total negative charge in the
atom.
Limitations of Thomson’s Atomic Model
❖ This model did not account for the presence of positively charged particles
in the atom, which were later identified as protons by Ernest Rutherford.
The model assumed that the positive charge was distributed uniformly
throughout the atom.
❖ The model could not explain the results of the gold foil experiment
conducted by Rutherford, which showed that most of the mass and
positive charge of an atom is concentrated in a small, dense nucleus at the
centre of the atom.
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❖ The model could not explain the phenomenon of atomic spectra, which is the emission and
absorption of specific wavelengths of light by atoms.
❖ The model did not explain the presence of Isotopes.
Rutherford’s Model of an Atom
The plum pudding model of J.J. Thomson could not able to explain certain experimental results about
the atomic structure of elements. still, there was no clear model that defines atoms, therefore in
1909, Ernest Rutherford, a British scientist conducted an experiment and based on the experiment
he observed and proposed the atomic structure of elements and Rutherford Atomic Model.
❖ The positively charged particle was concentrated in an extremely small volume and most of
the mass of an atom was also in that volume. He called this a nucleus of an atom.
❖ Rutherford proposed that there is negatively charged electrons around the nucleus of an
atom. the electron surrounding the nucleus revolves around it in a circular path with very high
speed. He named orbits to these circular paths.
❖ Nucleus being a densely concentrated mass of positively charged particles and electrons being
negatively charged are held together by a strong force of attraction called electrostatic forces
of attraction.
Observations of Rutherford’s Alpha Scattering Experiment
❖ First, he observes that most of the α-particles that are bombarded towards the gold sheet
pass away the foil without any deflection, and hence it shows most of the space is empty.
❖ Out of all, some of the α-particles were deflected through the gold sheet by very small angles,
and hence it shows the positive charge in an atom is non-uniformly distributed. The positive
charge is concentrated in a very small volume in an atom.
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❖ Very few of the alpha-particles (1-2%) were deflected back, i.e., only a very less amount of α-
particles had nearly 180° angle of deflection. this shows that the volume occupied by the
positively charged particles is very small as compared to the total volume of an atom.
Drawbacks of Rutherford Atomic Model
❖ He explained that the electrons in an atom revolve around the nucleus in well-defined orbits.
Particles in a circular orbit would experience acceleration.
❖ Thus, the revolving electron would lose energy and finally fall into the nucleus.
❖ But this cannot take place as the atom would be unstable, and the matter would not exist in
the form we know.
Bohr’s Model of an atom
Bohr’s model consists of a small nucleus (positively charged) surrounded by negative electrons
moving around the nucleus in orbits where he found out that an electron located away from the
nucleus has more energy as compared to electrons close to the nucleus.
Bohr’s model of an atom can be described where an atom is made up of three sub-atomic particles
– protons, electrons, neutrons. Proton is a positively charged particle that is a fundamental
constituent of all atomic nuclei. It is the lightest and most stable and has a charge which is equal in
magnitude to that of the electron. An electron is a particle that is a fundamental constituent of
matter that exists independently or as the component outside the nucleus of an atom. A neutron is
an elementary particle that has no charge, and which has a mass slightly greater than that of a
proton. Protons and neutrons are housed in a tiny nucleus at the atom’s core.
The electrons spin rapidly around the nucleus in circular routes known as energy levels or shells. The
number of electrons that each shell can hold is limited. The formula is, 2n2
where n is the orbit
number or energy level index.
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Each energy level is associated with a fixed amount of energy and there is no change in the energy
of the electrons as long as they keep revolving in the same energy level, and the atom remains stable.
Postulates of Bohr’s Model of an Atom
❖ Electrons revolve around the nucleus in stable orbits without the emission of radiant energy.
Each orbit has a definite energy and is called an energy shell or energy level.
❖ An orbit or energy level is designated as K, L, M, and N shells. When the
electron is in the lowest energy level, it is said to be in the ground state.
❖ An electron emits or absorbs energy when it jumps from one orbit or energy
level to another.
❖ When it jumps from a higher energy level to a lower energy level, it
emits energy, while it absorbs energy when it jumps from a lower energy
level to a higher energy level.
Limitations of Bohr’s Model of Atom
❖ It failed to explain the Zeeman Effect. In the presence of a static magnetic field,
the Zeeman effect causes a spectral line to break into numerous components.
It’s similar to the Stark effect.
❖ It also failed to explain the Stark effect. The Stark effect is the splitting of a
spectral line into several components in the presence of an electric field.
❖ It violated the Heisenberg Uncertainty Principle and couldn’t explain the spectra of bigger
atoms. This concept asserts that an object’s position and velocity cannot be determined
precisely at the same time. In fact, in nature, the concepts of absolute position and exact
velocity have no relevance.
Neutron
Neutrons are subatomic particles that are one of the primary constituents of atomic nuclei within
Electrons and Protons. Neutron, neutral subatomic particle that is a constituent of every atomic
nucleus except ordinary hydrogen.
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Neutrons and protons are together termed nucleons since they behave in a very similar manner
within the nuclei of atoms. The number of nucleons is also called the Mass number. The mass of a
nucleon may be roughly approximated to at least one mass unit (often abbreviated to a.m.u.). The
branch of science that deals with the study of the properties of neutrons and also the interactions
of those subatomic particles with different matter and nonparticulate radiation is named atomic
physics.
A neutron is present inside the nucleus of an atom, Hydrogen atom does not contain a nucleus. As
protons and neutrons, both are present inside the nucleus, they are combinedly and known as
nucleons.
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DISTRIBUTION OF ELEMENTS IN DIFFERENT ORBITS SHELLS
04
Orbits are energy shells surrounding the nucleus in which electrons revolve.
Electrons move around Nucleus of an atom in discrete orbits.
These are also called energy levels or shells.
According to Bohr-bury scheme
According to the Bohr Bury scheme, a system is stable when it has less
energy.
Thus, the electrons first occupy the first shell i.e., K which has the lowest energy
then go to the second shell i.e. L, then M, N, and so on.
The first shell K shell accommodates 2 electrons, the second shell L shell
accommodates 8 electrons, the third shell M shell accommodates 18 electrons, and
the fourth shell N shell accommodates 32 electrons.
The scheme also states that the outermost shell cannot hold more than 8 electrons.
What is the number of Electrons in a Shell?
The distribution of the electrons in the energy shells is known as electronic configuration.
This is based on the Bohr-Bury scheme according to which, the maximum number of electrons that
can be present in a particular energy shell of an atom is given by 2n2
, where “n” is the number of
energy shells.
The energy shells K, L, M, N are the first four energy shells given by Bohr.
.
So, the maximum number of electrons which can be accommodate in each shell is given as:
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Electron Shell Maximum Capacity
K shell 2 electrons
L shell 8 electrons
M shell 18 electrons
N shell 32 electrons
How is Maximum Number of Electrons in a shell Calculated
For K shell, n = 1, using formula 2n2
, the maximum number of electrons that can be filled in K shell
= 2 × (1)2
= 2 × 1 = 2.
For L shell, n = 2, So number of Electrons = 2n2
= 2 × 22
= 2 × 4 = 8
For M shell, n = 3, So number of Electrons = 2n2
= 2 × 32
= 2 × 9 = 18
For N shell, n = 4, So number of Electrons = 2n2
= 2 × 42
= 2 × 16 = 32
Examples:
Helium has 2 Electrons: All these are in K Shell (as K Shell can hold max 2 electrons)
Lithium has 3 Electrons: 2 of these are held in K Shell (as K Shell can hold max 2 electrons), Remaining
1 are held in L Shell
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Sodium has 11 Electrons: 2 of this are held in K Shell (as K Shell can hold max 2 electrons), 8 of this
are held in L Shell (as L Shell can hold max 8 electrons), Remaining 1 is held in M Shell
Octet Rule
As per the chemical rule of thumb, which is considered to be the octet rule, it is asserted that an
electron should have eight electrons in its outermost shell. Moreover, it is seen that this rule is
followed by most of the elements from s-block and p-block apart from a few elements from the p
block which are known as hydrogen, lithium, and helium. For example, Carbon Dioxide is a
compound that follows binding information the 'Octet Rule'. As per the chemical rule of thumb which
is the octet rule it is asserted that an electron should have eight electrons in its outermost shell.
The octet rule can be observed in the bonding between the carbon and oxygen atoms in a carbon
dioxide molecule, as illustrated via a Lewis dot structure.
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For Example:
NaCl (Sodium Chloride)
• This compound features an ionic bond between the sodium ion (Na+
) and the electronegative
chloride ion (Cl–
).
• The chlorine atom holds 7 electrons in its valence shell and can attain an octet configuration
by gaining an electron.
• The outermost shell of sodium has one electron. If it loses this electron, the second shell
would become the valence shell (which is already filled with 8 electrons). Thus, the Na+
ion is
more stable than metallic sodium.
• The sodium cation and the chloride anion now form an ionic bond, and the resulting molecule
features octet configurations for both the participating atoms.
MgO (Magnesium Oxide)
• The bond between magnesium and oxygen in magnesium oxide is ionic in
nature.
• The magnesium atom readily loses two electrons to obtain the stable
electronic configuration of neon. This leads to the formation of the Mg2+
• Similarly, oxygen gains two electrons to form the O2-
• The electrostatic attraction that arises between these two ions leads to the
formation of an ionic bond between them.
• Both the atoms in a molecule of MgO have stable octet configurations.
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VALENCY
04
VALENCY
The valency of an atom is equal to the number of valence electrons that this atom can gain or lose
during chemical reactions. For example, the amount of hydrogen atoms, chlorine atoms, or double
the number of oxygen atoms that one atom of an element may combine with is referred to as its
valency.
The valency of an element refers to the number of valence electrons in that element that participates
in chemical processes. A chemical compound is created when two or more components are
combined in a certain mass proportion. A stable compound is formed when one atom of one element
unites with a specific number of atoms of another element. All components have different
capabilities for mixing.
valency of an element is equal to the number of electrons present in the outermost shell and if it is
greater than 4, then the valency of an element is determined by subtracting the total number of
electrons present in outermost shell from 8.
Calculation of valency:
Valency of chlorine: The electronic configuration of chlorine = 2, 8, 7.
Since, chlorine has 7 (more than 4) electrons in its outermost shell.
Therefore, valency of chlorine = 8 - number of electrons in outermost shell = 8 – 7 = 1
Valency of Sulphur: The electronic configuration of Sulphur = 2, 8, 6.
Since, Sulphur has 6 (more than 4) electrons in its outermost shell.
Therefore, valency of chlorine = 8-number of electrons in outermost shell = 8 – 6 = 2.
Now, Valency of magnesium: The electronic configuration of Magnesium =2, 8, 2.
Since, magnesium has 2 (less than 4) electrons in its outermost shell.
Therefore, valency of magnesium = Number of electrons in its outermost shell = 2
Composition on of atoms of first eighteen (18) elements distribution in various shell
Name of
Element
Sym
bol
Atomic
Number
Number of
Protons
Number of
Neutrons
Number of
Electrons
Distribution of
Electrons
K L M N
Vale
ncy
Hydrogen H 1 1 - 1 1 - - - 1
Helium He 2 2 2 2 2 - - - 0
Lithium Li 3 3 4 3 2 1 - - 1
Beryllium Be 4 4 5 4 2 2 - - 2
Boron B 5 5 6 5 2 3 - - 3
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VALENCY
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Carbon c 6 6 6 6 2 4 - - 4
Nitrogen N 7 7 7 7 2 5 - - 3
Oxygen 0 8 8 8 8 2 6 - - 2
Fluorine F 9 9 10 9 2 7 - - 1
Neon Ne 10 10 10 10 2 8 - - 0
Sodium Na 11 11 12 11 2 8 1 - 1
Magnesium Mg 12 12 12 12 2 8 2 - 2
Aluminium Al 13 13 14 13 2 8 3 - 3
Silicon Si 14 14 14 14 2 8 4 - 4
Phosphorus P 15 15 16 15 2 8 5 - 3, 5
Sulphur s 16 16 16 16 2 8 6 - 2
Chlorine Cl 17 17 18 17 2 8 7 - 1
Argon Ar 18 18 22 18 2 8 8 - 0
Atomic Number and Mass Number
Atomic Number
The number of protons that a chemical element has in its centre (nucleus) is called the atomic
number. The mass number is determined by the number of protons and neutrons combined.
Atomic numbers and mass numbers are always whole numbers as they are obtained by counting
protons, neutrons, and electrons. The sum of the mass number and the atomic number for an atom
corresponds to the total number of subatomic particles present in the atom.
Mathematically, the atomic number is defined as.
Atomic Number (Z) = No of protons (p+)
The mass number reports the mass of the atom’s nucleus in atomic mass
units (a.m.u).
Properties of the Atomic Number:
• The total number of protons in the nucleus of an atom gives us the atomic
number of an atom.
• It is represented with the letter ‘Z.’
• Atoms of different elements have different atomic numbers. For
example, all carbon atoms have the atomic number of 6, whereas all atoms of Oxygen have
an atomic number of 8.
• This term was first introduced by Henry Gwyn-Jefferies Moseley.
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Mass Number
Rutherford discovered that an atom’s nucleus, which is made up of protons and neutrons, contains
the vast bulk of its mass. The total number of protons and neutrons in an atom is known as the mass
number.
Properties of the Mass Number:
The number of protons and neutrons combined give us the mass number of an atom.
It is represented using the letter ‘A.’ For example, an atom of carbon has 6 protons and 6 neutrons.
Thus, its mass number is 12.
While the number of protons remains the same in all atoms of an element, the number of neutrons
can vary. Thus, atoms of the same element can have different mass numbers, and these are called
isotopes.
Mathematically, the mass number is defined as.
Mass Number (A) = No. of Protons + No. of Neutrons
Isotopes
Isotopes are variations of the same element with a different number of nucleons. In other words,
nuclei with the same number of protons and different numbers of neutrons are classified as isotopes.
For example, all carbon atoms contain six protons, but some have 6, 7, or 8 neutrons in the nucleus,
which classify these carbon atoms as C-12, C-13, and C-14.
Isotopes of Hydrogen
Hydrogen has three naturally occurring Isotopes. Isotopes of Hydrogen are
1
1
H Protium, 2
1
H Deuterium, and 3
1
H Tritium, but in a lab environment, we
can create more isotopes with different numbers of neutrons in the
nuclei of it. These synthetic isotopes of Hydrogens are all radioactive
and have a half-life of the order of yotta seconds.
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Isotopes of Carbon
Carbon-13 and Carbon-14 are both isotopes of carbon, one with 7 neutrons and one with 8
neutrons (but both with 6 protons). Carbon-12 is a stable isotope, while carbon-14 is a radioactive
isotope (radioisotope). Other than this, many synthetic carbon isotopes are synthesized in a lab
environment and can only sustain for a very brief period of time (order of 200 ms).
Isotopes of Oxygen
There are only three stable isotopes of oxygen which are 16
O, 17
O, and 18
O. Other than this oxygen
nuclei with a different number of neutrons such as 3, 4, 5, 6, 7, etc. can be synthesized in labs but
they are not stable in nature and only can exist in the lab environment.
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VALENCY
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Average Atomic Mass of elements
When an element has an isotope, the mass number can be calculated by the different proportions
it exists in.
For example, take 98% Carbon-12u and 2% Carbon-13u
(12 ×
98
100
) + 13 ×
2
100
= 12.02u
This does not mean that any Carbon atoms exist with a mass number of
12.02u. If you take a certain amount of Carbon, it will contain both isotopes of
Carbon, and the average mass is 12.02 u.
Applications of isotopes
• An isotope of uranium acts as a fuel in nuclear reactors.
• In the treatment of cancer, an isotope of Cobalt is used.
• In the field of medical, an isotope of Iodine is used in the whole-body scan.
Isobars
Isobars are a group of elements from the periodic table that have different atomic numbers, but their
mass number are the same. We can say that in isobars the number of protons in their nucleus is
different, but the sum of the number of protons and neutrons is the same. For example, Argon
(18Ar40
), Potassium (19K40
), and Calcium (20Ca40
) are isobars as they all have 40 as their mass number
but their atomic number are different. This happens because they have different atomic numbers
but the sum of protons and neutrons in their nucleus is different.
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Atom Atomic Number Number of Neutrons Mass Number
Argon (Ar) 18 22 40
Potassium (K) 19 21 40
Calcium (Ca) 20 20 40
The group of Isobar of Argon, Potassium, and Calcium.
Isobar Examples
There are various examples in the periodic table that are isobars, i.e., they have
the same mass number but different atomic numbers. Various examples of the
isobars are discussed below,
11Na24
and 12Mg24
Sodium (24) and Magnesium (24) are the isobars of each other, and we can
represent their condition as,
Sodium (Na) 11 13 24
Magnesium (Mg) 12 12 24
13Al27
and 14Si27
Aluminium (27) and Silicon (27) are the isobars of each other, and we can represent their condition
as,
Aluminium (Al) 13 14 27
Silicon (Si) 14 13 27
16S34
and 17Cl34
Sulphur (34) and Chlorine (34) are the isobars of each other, and we can represent their condition
as,
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VALENCY
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Sulphur (S) 16 18 34
Chlorine (Cl) 17 17 34
26Fe58
and 27Ni58
Iron (58) and Nickel (58) are the isobars of each other, and we can represent their condition as,
Iron (Fe) 26 32 58
Nickel (Ni) 27 31 58
Isotones
Two nuclides are said to be isotones if they contain similar neutron number N, but with different
proton numbers Z. For example, carbon-13, and boron-12 nuclei both have 7 neutrons, and so they
are called isotones. In the same way, 37
Cl, 36
S, 39
K, 38
Ar, and the 40
Ca nuclei are all
isotones of 20 since they all hold 20 neutrons. About its similarity to the Greek word
for "same stretching," the word "isotope" was created by German
physicist K. Guggenheimer by changing the letter "p" in "isotope" from
"proton" to "neutron."
Observationally, the largest numbers of the stable nuclides there exist for
isotones 50 (which are five: 88
Sr, 86
Kr, 90
Zr, 89
Y, and 92
Mo) and 82 (which are six: 139
La,
138
Ba, 141
Pr, 140
Ce, 144
Sm, and 142
Nd). The neutron numbers where there are no
stable isotones are given as 19, 21, 35, 39, 45, 61, 89, and 115 or even more. In
contrast, the proton numbers, where there are no stable isotopes, are given as 43,
61, and 83 or even more.
This is related to the nuclear magic numbers, which are the number of nucleons
that form complete shells within the nucleus. For example, 2, 8, 28, 50, 82. Not
more than one stable nuclide contains a similar odd neutron number, except for 1 (it means 2
H and
3
He), 5 (which means 9
Be and 10
B), 55 (the 97
Mo and 99
Ru), and also 107 (the 179
Hf and 180
mTa). 27
(50
V), 65 (113
Cd), 81 (138
La), 85 (147
Sm), and 105 are the odd neutron numbers of a primordial
radionuclide and a stable nuclide (176
Lu). Neutron numbers, where there exist two primordial
radionuclides, are given as 88 (151
Eu and 152
Gd) and 112 (187
Re and 190
Pt).
For example, boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones. Similarly,
36
S, 37
Cl, 38
Ar, 39
K, and 40
Ca nuclei are all isotones of 20 because they all contain 20 neutrons.
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Structure of the Atom
DPP-01
[Topic: Charged Particles in Matter, The Structure of an Atom]
Very Short Answer Type Questions
1. Name three subatomic particles present in an atom.
2. Name the negatively charged particles present in all the atoms of all the elements.
3. Which part of the atom was discovered by Rutherford's alpha particle scattering experiment?
4. What are Canal rays?
5. If an atom contains one electron and one proton, will it carry any charge or not?
6. Why is an atom neutral inspite of the presence of charged particles in it?
7. How does a proton differ from an electron?
8. Helium atom has atomic mass of 4𝑢 and has 2 protons in its nucleus. How many neutrons does it
have?
9. Who discovered electrons?
10. What is a proton? Who discovered it?
11. What was the main drawback of Rutherford's model of atom?
12. Name an element which does not have any neutron.
Short Answer Type Questions-I
13. Mention any two points which Rutherford put forward to explain the nuclear model of an atom.
14. Compare the properties of electrons, protons and neutrons.
15. What are the limitations of J.J. Thomson's model of the atom?
16. What are the limitations of Rutherford's model of atom?
Short Answer Type Questions-II
17. Describe Thomson's model of atom. Which subatomic particle was not present in Thomson's
model of Atom?
18. Describe Rutherford's model of atom.
19. Describe Bohr's model of atom.
Long Answer Type Question
20. Compare all the proposed models of an atom given in this chapter.

DPP-02
[Topic: Distribution of Electrons in Shells, Valency, Atomic Number, Mass Number, Isotopes and
Isobars]
Very Short Answer Type Questions
1. The total number of electrons in Nitrogen is 7. What is its valency?
2. What name is given to the pair of atoms: 7
14
N and 7
15
N ?
3. Oxygen has 8 protons and 8 neutrons whereas sulphur has 16 protons and 16 neutrons. What is
the mass number of oxygen and sulphur.
4. The atomic number of neon is 10. Write its electronic configuration.
5. What happens to the element ' 𝑍 ' if it can gain three electrons?
6. Helium has 2 electrons in its valence shell but its valency is not 2. Explain
7. According to Bohr Bury Scheme what is the maximum number of electrons present in M-shell of
an atom?
8. In the notation 7
14
N, what do the numbers 14 and 7 denote?
9. If Mg2+
has 12 protons and 12 neutrons, what is its atomic number and mass number?
10. What is the difference between Na atom and Na+
ion in terms of number of electrons?
11. Why are the shells in which electrons revolve are called energy levels?
12. Why is Co − 60 used in the treatment of cancer?
13. Which isotope is used to find the age of a mummy?
14. Which isotope is used as a fuel in Nuclear reactors?
15. What is the formula of the compound when ' X ' with atomic number 12 combines with an
element with atomic number 17?
16. An ion of an element has 3 units positive charge. Mass number of the atom is 27 and the number
of the neutrons is 14. What is the number of electrons in this ion?
17. In a sample of ethanoate (CH3COOC2H5), the oxygen atoms have the same number of electrons
but different number of neutrons. What is a relationship between two atoms of oxygen?
18. Elements with valency equal to one are
(a) always metals
(b) always metalloids
(c) either metals or non-metals
(d) always non-metals
19. Which of the following statement is always correct?
(a) An atom has equal number of electrons and protons.

(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.
20. Will 35
Cl and 37
Cl have different valencies? Justify your answer.
21. Why did Rutherford select a gold foil for his 𝛼-ray scattering experiment?
22. One electron is present in the outermost shell of the atom of an element X. What would be the
nature and value of charge on the ion formed, if this electron is removed from the outermost
shell?
23. In the atom of an element X, 6 electrons are present in the outermost shell. If it acquires noble gas
configuration by accepting requisite number of electrons, what would be the charge on the ion so
formed?
Short Answer Type Questions-I
24. For the symbol H, D and T tabulate three sub-atomic particles found in each of them.
25. Write the electronic configuration of any one pair of isotopes and isobars.
26. Summarise the rules for writing of distribution of electrons in various shells for the first eighteen
elements.
27. Define valency by taking the examples of silicon and oxygen.
28. Na+
ion has completely filled K and L shells. Explain.
29. If bromine atom is available in the form of, say, two isotopes 35
79
Br(49.7%) and 35
81
Br(50.3%).
Calculate the average atomic mass of bromine (Br) atom.
30. The average atomic mass of a sample of an element X is 16.2u. What is the percentage of isotopes
8
16
X and 8
18
X in the given sample?
31. If 𝑍 = 3, what would be the valency of the element? Also, name that element.
32. Composition of the nuclei of two atomic species X and Y are given as under
Nuclei X Y
Protons 6 6
Neutrons 6 8
Give the mass numbers of X and Y. What is the relation between the two species?
33. The following data represents the distribution of electrons, protons and neutrons in atoms of the
elements: A, B, C, D:

Elements Protons Neutrons Electrons
A 19 21 19
B 17 18 17
C 17 20 17
D 18 22 18
Answer the following questions:
(a) Describe the electronic configuration of B.
(b) Is element ' B ' a metal or a non-metal, why?
(c) Which two elements are pair of Isotopes?
(d) Which two elements are pair of Isobars?
34. (a) What is the similarity in the electronic structure of the following set of elements?
(i) Lithium (ii) Sodium (iii) Potassium
(b) Which of the above element is most reactive and why?
35. An atom of an element has two electrons in its outermost ' M ' shell, state the (𝑎) electronic
configuration, (b) Atomic number, (c) Number of protons, (d) Valency of this element
36. In response to a question, a student stated that in an atom, the number of protons is greater than the
number of neutrons, which in turn is greater than the number of electrons. Do you agree with the
statement? Justify your answer.
37. Show diagrammatically the electron distributions in a sodium atom and a sodium ion. Give their
atomic numbers.
38. In the Gold foil experiment of Geiger and Marsden, that paved way for Rutherford's model of an
atom, ∼ 1.00% of the 𝛼-particles were found to deflect at an angles > 50∘
. If one mole of 𝛼-particles
were bombarded on the gold foil, compute the number of 𝛼-particles that would deflect at an angle less
than 50∘
.
39. Find the electron distribution for the element that has atomic number 20 and write its valency.
40. If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic
number of the atom? (ii) What is the charge on the atom?
Short Answer Type Questions-II
41. Write the distribution of electrons in an atom of element whose atomic number is 18. What is
special about the outermost shell of an atom in this element?
42. How will you find out the valency of chlorine, sulphur and magnesium atoms?

43. Explain with examples: (i) Atomic number, (ii) Mass number, (iii) Isotopes, (iv) Isobars. Give
any two uses of isotopes.
44. State the observations in 𝛼-particle scattering experiment that led to Rutherford making the
following conclusions:
(a) Most of the space in an atom is empty.
(b) Almost whole of the mass of atom is concentrated in its centre.
(c) Centre is positively charged.
45. The number of electrons, protons and neutrons of 5 elements are given below:
Element A B C D E
Electron 4 18 17 11 17
Protons 6 18 17 9 17
Neutrons 6 22 20 10 18
(a) Which of them is a cation?
(b) Which of them is an anion?
(c) Which is an atom of inert gas?
46. Fill in the blanks
Element
Atomic
Number
P E N
Mass
Number
Na+ 11 11 - 12 23
Ca2+ 20 20 18 - 40
Oxygen 8 8 - 8 16
47. In the following table, the mass number and atomic number of certain elements are given.
Elements A B C D E F G H
Mass No. 2 3 3 6 9 11 19 23
Atomic No. 1 1 2 3 4 5 9 11
(a) How many neutrons are present in " F "?
(b) Which atoms are the isotopes of the same element?

(c) Which atom will form single positive charged ion?
(d) Which is an atom of inert gas?
(e) Which will form single negative charged ion?
(f) Which one of these have 11 electrons?
48. (a) Calculate the average atomic mass of chlorine if it exists in two isotopes 17
35
Cl(75%) and
17
37
Cl(25%).
(b) Write the main drawback of Rutherford model of atom.
Long Answer Type Question
49. Complete the following table.
Atomic
Number
Mass
Number
Number
of Neutrons
Number of
Protons
Number of
Electrons
Name of the
Atomic Species
9 - 10 - - -
16 32 - - - Sulphur
- 24 - 12 - -
- 2 - 1 - -
- 1 0 1 0 -

INTRODUCTION
04
(Practice Sheet)
1 What is the charge of a proton?
A. Positive B. Negative
C. Neutral D. No charge
2 Who discovered the proton?
A. J.J. Thomson B. Ernest Rutherford
C. Eugene Goldstein D. Dalton
3 Who discovered the nucleus of the atom?
A. J.J. Thomson B. Ernest Rutherford
C. Eugene Goldstein D. Dalton
4 What experiment led to the discovery of the nucleus?
A. Cathode Ray Experiment B. Gold Foil Experiment
C. Discharge Tube Experiment D. Dalton's Experiment
5 What is the charge of an electron?
A. Positive B. Negative
C. Neutral D. No charge
6 Who discovered the nucleus of the atom, and what experiment led to this discovery?
7 What is the electrical neutrality of an atom dependent on?
8 Who observed canal rays and provided evidence for the existence of a positively charged
particle?
9 What is the charge of the particles making up the cathode ray?
10 What does the atomic number represent?
Page 1

04
(Practice Sheet)
1 What are the three fundamental particles that make up an atom?
A. Electrons, Protons, Neutrons B. Photons, Protons, Neutrons
C. Electrons, Positrons, Neutrons D. Electrons, Protons, Isotopes
2 Which atomic model proposed a positively charged sphere with embedded electrons,
comparing it to a Christmas pudding?
A. Rutherford's Atomic Model B. Bohr’s Model of an Atom
C. Dalton’s Atomic Theory D. Thomson’s Model of an Atom
3 Who conducted the alpha scattering experiment leading to the proposal of the atomic
nucleus?
A. Niels Bohr B. Ernest Rutherford
C. J.J. Thomson D. John Dalton
4 What did Rutherford's experiment with alpha particles reveal about the atom?
A. Electrons are evenly distributed in the
atom.
B. Most of the atom is empty space.
C. Protons are concentrated in the nucleus. D. Electrons revolve in well-defined orbits.
5 Which model of an atom introduced the concept of energy levels or shells for electrons?
A. Rutherford’s Atomic Model B. Thomson’s Model of an Atom
C. Bohr’s Model of an Atom D. Dalton’s Atomic Theory
6 Explain the major limitation of Thomson’s Model of an Atom.
7 What did Rutherford's observations in the alpha scattering experiment reveal about the
structure of an atom?
8 Describe the key postulates of Bohr’s Model of an Atom.
9 Why did Bohr’s Model fail to explain the Zeeman Effect?
10 How do electrons behave in Bohr’s Model when they transition between energy levels?
Page 1

04
(Practice Sheet)
1 What is the maximum number of electrons that can be accommodated in the K shell
according to the Bohr-Bury scheme?
A. 4 B. 8
C. 2 D. 16
2 How many electrons can the L shell accommodate according to the Bohr-Bury scheme?
A. 2 B. 8
C. 18 D. 32
3 Which compound follows the octet rule in its bonding configuration?
A. NaCl (Sodium Chloride) B. H2O (Water)
C. CO2 (Carbon Dioxide) D. CH4 (Methane)
4 What is the maximum capacity of the N shell according to the Bohr-Bury scheme?
A. 2 electrons B. 8 electrons
C. 18 electrons D. 32 electrons
5 What type of bond is formed between the sodium ion (Na+
) and the chloride ion (Cl–
) in
NaCl (Sodium Chloride)?
A. Covalent bond B. Metallic bond
C. Ionic bond D. Hydrogen bond
6 What is the maximum number of electrons that can be held in the M shell?
7 If an atom has 3 energy shells, how many electrons can be accommodated in the third shell
according to the Bohr-Bury scheme?
8 According to the octet rule, how many electrons should an atom ideally have in its outermost
shell for stability?
9 In the Bohr-Bury scheme, which shell has the lowest energy?
10 Who is elements violates the octet rule?
Page 1

VALENCY
04
(Practice Sheet)
1 How is the valency of an element determined if it has more than 4 electrons in its
outermost shell?
A. Subtracting the number of electrons
from 4
B. Adding the number of electrons to 4
C. Subtracting the number of electrons
from 8
D. Adding the number of electrons to 8
2 Which element has a valency of 2 with the electronic configuration 2, 8, 6?
A. Carbon B. Nitrogen
C. Oxygen D. Sulphur
3 How is the atomic number defined mathematically?
A. Atomic Number (Z) = No of neutrons (n) B. Atomic Number (Z) = No of protons (p+)
C. Atomic Number (Z) = No of electrons (e-) D. Atomic Number (Z) = No of isotopes (I)
4 What is the mass number of an atom?
A. The sum of protons and electrons B. The sum of neutrons and electrons
C. The sum of protons and neutrons D. The sum of atomic and mass numbers
5 What are isotopes?
A. Elements with the same number of
protons but different neutrons
B. Elements with the same number of
protons and neutrons
C. Elements with the same atomic and mass
numbers
D. Elements with the same valency
6 Explain the difference between atomic number and mass number.
7 What is the valency of Chlorine with an electronic configuration of 2, 8, 7?
8 Which element has three naturally occurring isotopes, including Protium, Deuterium, and
Tritium?
9 What is valency, and how is it determined for an element?
10 Define isotones and provide an example.
Page 1

Chapter 4
Structure of the atom
Multiple Choice Questions
1. Which of the following correctly represent the electronic distribution in the Mg atom?
(a) 3, 8, 1
(b) 2, 8, 2
(c) 1, 8, 3
(d) 8, 2,
2 Soln:
Answer is 2, 8,
2 Explanation :
Atomic number of Mg is 12 hence electronic distribution will be 1s22s22p63s2.
2. Rutherford’s ‘alpha (α) particles scattering experiment’ resulted in to discovery of
(a) Electron
(b) Proton
(c) Nucleus in the atom
(d) Atomic mass
Soln:
Answer is (c) Nucleus in the
atom Explanation:
Rutherford’s ‘alpha (α) particles scattering experiment’ experiment concludes that alpha particles returned to their
original path. This showed the presence of nucleus in the centre.
3. The number of electrons in an element X is 15 and the number of neutrons is 16. Which of the following
is the correct representation of the element?
(a) 31 15 X
(b) 31 16 X
(c) 16 15 X
(d) 15 16 X
Soln:
Answer is (a) 31 15
X Explanation:
Number of protons in an element depicts atomic number. Number of protons and electrons are equal in an
element. Hence atomic number is written in subscript whereas mass number is written in the subscript before the
symbol of the element.
EXEMPLAR SOLUTION

4. Dalton’s atomic theory successfully explained
(i) Law of conservation of mass
(ii) Law of constant composition
(iii) Law of radioactivity
(iv) Law of multiple proportion
(a) (i), (ii) and (iii)
(b) (i), (iii) and (iv)
(c) (ii), (iii) and (iv)
(d) (i), (ii) and (iv)
Soln:
Answer is (d) (i), (ii) and (iv)
Explanation:
Dalton’s theory explains Law of conservation of mass, Law of constant composition, Law of multiple proportion.
But it never give any details of Law of radioactivity .
5. Which of the following statements about Rutherford’s model of atom are correct?
(i) considered the nucleus as positively charged
(ii) established that the α–particles are four times as heavy as a hydrogen atom
(iii) can be compared to solar system
(iv) was in agreement with Thomson’s model
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) only (i)
Soln:
Answer is (a) (i) and (iii)
Explanation:
Positively charged alpha particles were deflected by nucleus. This shows nucleus is positively charged.
Rutherford also postulated that electrons are arranged in an atom around the nucleus like planets arranged around
sun.
6. Which of the following are true for an element?
(i) Atomic number = number of protons + number of electrons
(ii) Mass number = number of protons + number of neutrons
(iii) Atomic mass = number of protons = number of neutrons
(iv) Atomic number = number of protons = number of electrons
(a) (i) and (ii)
(b) (i) and (iii)
(c) (ii) and (iii)
(d) (ii) and (iv)
Solution:
Answer is (d) (ii) and (iv)

Explanation:
Atomic number Z is the number of proton present in an electron which is also equal to number pf electron in an
atom. Since the mass of neutron is negligible. Number of protons and electron are added to obtain mass number of
an element.
7. In the Thomson’s model of atom, which of the following statments are correct?
(i) the mass of the atom is assumed to be uniformaly distributed over the atom
(ii) the positive charge is assumed to be uniformaly distributed over the atom
(iii) the electrons are uniformaly distributed in the positively charged sphere
(iv) the electrons attract each other to stabilise the atom
(b) (i) and (iii)
(c) (i) and (iv)
(d) (i), (iii) and (iv)
Soln:
Answer is(a) (i), (ii) and (iii)
Explanation:
Thomson proposed that negatively charged electron are stabilised by positively charged protons in the nucleus.
Hence option II) the positive charge is assumed to be uniformly distributed over the atom is wrong statement and
other statements are part of Thomson’s model of atom.
8. Rutherford’s α–particle scattering experiment showed that
(i) electrons have negative charge
(ii) the mass and positive charge of the atom is concentrated in the nucleus
(iii) neutron exists in the nucleus
(iv) most of the space in atom is empty Which of the above statements are correct?
(a) (i) and (iii)
(b) (ii) and (iv)
(c) (i) and (iv)
(d) (iii) and (iv
Soln:
Answer is (b) (ii) and (iv)
Explanation:
An atom consists of a positively charged, dense and very small nucleus which have all the protons and neutrons.
Positive charge is due to protons, as neutrons have no charge. Here the space is empty because alpha particles
pass straight through the gold foil without any deflection.
Thomson explained that electron s have negative charge. Existence of neutron was discovered by Chadwick.

9. The ion of an element has 3 positive charges. Mass number of the atom is 27 and the number of neutrons
is 14. What is the number of electrons in the ion?
(a) 13
(b) 10
(c) 14
(d) 16
Soln:
Answer is (b) 10
Explanation:
Mass number (A) of the atom = 27
Number of neutron in the atom =14
Number of Electrons=Mass number-Number of neutrons=27-14
Number of electrons= 13
Since ions of the element has 3 positive charges number of electron in the ion is 13-3 which equal 10.
Hence the answer is 10
10. Identify the Mg2+ ion from the Fig.4.1 where, n and p represent the number of neutrons and protons
respectively.
Soln:
Answer is d)
Explanation:
Number of protons in Mg atom = 2+ 8 + 2 = 12
Number of neutrons in Mg atom = 24 -12 = 12
[as mass number of Mg atom = 24 and number of neutrons = mass number – number of protons]
Therefore, option (d) is the correct answer

11. In a sample of ethyl ethanoate (CH3COOC2H5) the two oxygen atoms have the same number of
electrons but different number of neutrons. Which of the following is the correct reason for it?
(a) One of the oxygen atoms has gained electrons
(b) One of the oxygen atoms has gained two neutrons
(c) The two oxygen atoms are isotopes
(d) The two oxygen atoms are isobars.
Soln:
Answer is(c) The two oxygen atoms are isotopes
Explanation:
Two Oxygen atoms in CH3COOC2H5 can have different number of neutrons only if the two O-atoms are isotopes.
Isotopes of an element have same number of protons (and electrons) but different number of neutrons.
12. Elements with valency 1 are
(a) always metals
(b) always metalloids
(c) either metals or non-metals
(d) always non-metals
Soln:
Answer is (c) either metals or non-metals.
Explanation:
If element shows positive valency it is a metal and if the element shows negative valency it will be a non-metal.
13. The first model of an atom was given by
(a) N. Bohr
(b) E. Goldstein
(c) Rutherford
(d) J.J. Thomson
Soln:
Answer is (d) J.J. Thomson
14. An atom with 3 protons and 4 neutrons will have a valency of
(a) 3
(b) 7
(c) 1
(d) 4
Soln:
Answer is (c) 1
15. The electron distribution in an aluminium atom is
(a) 2, 8, 3
(b) 2, 8, 2
(c) 8, 2, 3
(d) 2, 3, 8

Soln:
Answer is (a) 2, 8, 3
Explanation:
Atomic number of Aluminium is 13, First shell can have maximum of 2 electron and second shell holds a
mximum of 8 electrons. Hence option a is right answer.
16. Which of the following in Fig. 4.2 do not represent Bohr’s model of an atom correctly?
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (i) and (iv)
Soln:
Answer is ii and iv
Explanation :
First shell can have maximum of 2 electron and second shell can have maximum of 8 electron hence ii and iv do
not represent Bohr’s model of an atom correctly.
17. Which of the following statement is always correct?
(a) An atom has equal number of electrons and protons.
(b) An atom has equal number of electrons and neutrons.
(c) An atom has equal number of protons and neutrons.
(d) An atom has equal number of electrons, protons and neutrons.
Soln:
Answer is (a) An atom has equal number of electrons and protons.
Eplanation:
An atom is electrically neutral because the number of protons are always equal to number of electrons. Hence
option a) is right.
18. Atomic models have been improved over the years. Arrange the following atomic models in the order of
their chronological order
(i) Rutherford’s atomic model
(ii) Thomson’s atomic model
(iii) Bohr’s atomic model

(b) (ii), (iii) and (i)
(c) (ii), (i) and (iii)
(d) (iii), (ii) and (i)
Soln:
Answer is (c) (ii), (i) and (iii)
Explanation:
Thomson’s atomic model was proposed in the year 1904
Rutherford’s atomic model was proposed in the year 1911
Bohr’s atomic model was proposed in the year 1913
Short Answer Questions
19. Is it possible for the atom of an element to have one electron, one proton and no neutron. If so, name the
element.
Soln:
Yes, Hydrogen is the element which is having only 1 proton and 1 electron and no neutron hence there is no
repulsive force in the nucleus hence it is stable.
20. Write any two observations which support the fact that atoms are divisible
 Ionic compounds are formed because of formation of ions that involves transfer of electrons.
 Difference in the number of make isotopes. This shows that atom is formed by different particles such as
electrons, protons and neutrons and element is divisible.
21. Will 35Cl and 37Cl have different valencies? Justify your answer.
Soln:
35Cl and 37Cl cannot have different valencies because they are the isotopes of same element.
22. Why did Rutherford select a gold foil in his α–ray scattering experiment?
Soln:
Rutherford used gold for his scattering experiment because gold is the most malleable metal and he wanted the
thinnest layer as possible.
Therefore, Rutherford selected a Gold foil in his alpha scatttering experiment.

23. Find out the valency of the atoms represented by the Fig. 4.3 (a) and (b).
Soln:
Atom (a) has zero valency as it has 8 electron in its valence shell making the configuration stable.
Atom (b) has valency of +1 as it has 7 electrons in it outermost shell . It can accept 1 electron to achieve octet
configuration.
24. One electron is present in the outer most shell of the atom of an element X. What would be the nature
and value of charge on the ion formed if this electron is removed from the outer most shell?
Soln:
If an electron is removed from the outermost shell a cation will be formed and the charge of the element will be
+1.
25. Write down the electron distribution of chlorine atom. How many electrons are there in the L shell?
(Atomic number of chlorine is 17).
Soln:
Atomic number of chlorine atom = 17
So, its electronic configuration is
K L M
2 8 7
L shell of chlorine contains 8 electrons.
26. In the atom of an element X, 6 electrons are present in the outermost shell. If it acquires noble gas
configuration by accepting requisite number of electrons, then what would be the charge on the ion so
formed?
Soln:
In this atom 6 electrons are already present in its outermost orbitals. In order to attain noble gas configuration
element has to accept two electron hence its charge is -2.

27. What information do you get from the Fig. 4.4 about the atomic number, mass number and valency of
atoms X, Y and Z? Give your answer in a tabular form.
Soln
Atomic number, mass number and valency of atoms X, Y and Z
Atomic no Mass no Valency
X 5 11 3
Y 8 18 2
Z 15 31 3.5
28. In response to a question, a student stated that in an atom, the number of protons is greater than the
number of neutrons, which in turn is greater than the number of electrons. Do you agree with the
statement? Justify your answer.
Soln
Statement is wrong because number of protons can never be greater than number of neutrons. Number of protons
will always be less than or equal to number of neutrons. Number of electrons and protonsare always equal in a
neutral atom.
29. Calculate the number of neutrons present in the nucleus of an element X which is represented as 𝒙
𝟏𝟓
𝟑𝟏
Soln:
Mass number = No. of protons + No. of neutrons = 31
∴ Number of neutrons = 31– number of protons
= 31–15
= 16

30. Match the names of the Scientists given in column A with their contributions towards the
understanding of the atomic structure as given in column B
(A) (B)
(a) Ernest Rutherford (i) Indivisibility of atoms
(b) J.J.Thomson (ii) Stationary orbits
(c) Dalton (iii) Concept of nucleus
(d) Neils Bohr (iv) Discovery of electrons
(e) James Chadwick (v) Atomic number
(f) E. Goldstein (vi) Neutron
(g) Mosley (vii) Canal rays
Soln:
(A) (B)
(a) Ernest Rutherford (iii) Concept of nucleus
(b) J.J.Thomson (iv) Discovery of electrons
(c) Dalton (i) Indivisibility of atoms
(d) Neils Bohr (ii) Stationary orbits
(e) James Chadwick (vi) Neutron
(f) E. Goldstein (vii) Canal rays
(g) Mosley (v) Atomic number
31. The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these
elements is 40. What is the name given to such a pair of elements?
Soln:
Elements with different atomic numbers but same mass numbers are known as isobars. Calcium and argon are
isobars.
32. Complete the Table 4.1 on the basis of information available in the symbols given below
(a) 35 17 Cl (b) 12 6 C (c) 81 35 Br
Element np nn
Soln:
Element np nn
Cl 17 18
C 6 6
Br 35 46

33. Helium atom has 2 electrons in its valence shell but its valency is not 2, Explain
Soln:
Helium has 2 electrons in its outermost shell thereby completing duplet configuration. Hence it has no valence
shell left empty making its valency 0.
34. Fill in the blanks in the following statements
(a) Rutherford’s α-particle scattering experiment led to the discovery of the ———
(b) Isotopes have same ———but different———.
(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be———and——
—respectively.
(d) The electronic configuration of silicon is ———and that of sulphur is ———
Soln:
(a) Rutherford’s α-particle scattering experiment led to the discovery of the atomic nucleus
(b) Isotopes have same atomic number but different mass number
(c) Neon and chlorine have atomic numbers 10 and 17 respectively. Their valencies will be 0 and 1
respectively.
(d) The electronic configuration of silicon is 2.8.4 and that of sulphur is 2.8.6
35. An element X has a mass number 4 and atomic number 2. Write the valency of this element?
Soln:
Mass number = 4
Atomic number = 2
X is Helium.
It has 0 valency and it will not react with any other atom because it has its outer shell filled.
Long Answer Questions
36. Why do Helium, Neon and Argon have a zero valency?
Soln:
Helium has 2 electron in its outermost orbit thus filling shell 1 and forming duplet configuration in valence shell
.Neon has 8 electron in their valence orbit hence completing duplet configuration. In the same way Argon and
Neon has 8 electron in its outermost shelling completing octet configuration. As these elements have maximum
electron in their valence shell thus reach stable electron configuration and they will not take part in any sort of
chemical reactions.

37. The ratio of the radii of hydrogen atom and its nucleus is ~ 105. Assuming the atom and the nucleus to
be spherical,
(i) What will be the ratio of their sizes?
(ii) If atom is represented by planet earth ‘Re ’ = 6.4 ×106 m, estimate the size of the nucleus.
Soln:
38. Enlist the conclusions drawn by Rutherford from his α-ray scattering experiment.
Soln:
Rutherford drawn following conclusion from his α-ray scattering experiment.
 α-particles passed through the gold foil without any deflection concluding the empty space inside the
atom.
 Deflection is observed in few particles which proves positive charge of the atom occupies very little
space.
 Deflection in A very small fraction of α-particles indicates that all the positive charge and mass of the
gold atom were concentrated in a very small volume within the atom.

39. In what way is the Rutherford’s atomic model different from that of Thomson’s atomic model?
Soln:
Rutherford proposed that electron revolve around the nucleus in well differentiated orbits. Nucleus is the centre
which is positively charged. Rutherford proposed that nucleus is very small and nearly all the mass of an atom is
centred in the nucleus.
.
Thompson proposed that electron are scattered positively charged spheres like a Christmas pudding and the mass
of the atom was supposed to be uniformly distributed.
40. What were the drawbacks of Rutherford’s model of an atom?
Soln:
Rutherford’s model could not explain the stability of the atom.
Revolving electrons would lose energy as they are the charged particles and due to acceleration, charged particles
would radiate energy.
Orbit of the revolving electron will reduce in size, following a spiral path as shown in figure and ultimately the
electron should fall into the nucleus. In other words, the atom should collapse.
41. What are the postulates of Bohr’s model of an atom?
Soln:
Postulates of Bohr’s model of an atom are
(i) Only certain special orbits known as discrete orbits of electrons, are allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy. These orbits or shells
are called energy levels. These orbits or shells are represented by the letters K,L,M,N,… or
the numbers, n=1,2,3,4,….

42. Show diagramatically the electron distributions in a sodium atom and a sodium ion and also
give their atomic number.
Soln:
Atomic number of sodium (Z) =11 Mass number of sodium (A) = 23
Number of protons in the nucleus = 11 Number of neutrons in the nucleus =23-11 = 12
Number of electrons = 11
Electronic configuration of Na-atom = 2, 8, 1(K,L,M)
Na+ ion is formed from s,odium atom by loss of an electron (present in the outermost shell). Hence, its
electronic configuration is 2, 8(K,L). However, number of protons and neutrons remains the same.
43. In the Gold foil experiment of Geiger and Marsden, that paved the way for Rutherford’s model of an
atom, ~ 1.00% of the α-particles were found to deflect at angles > 50º. If one mole of α-particles were
bombarded on the gold foil, compute the number of α-particles that would deflect at angles less than 500.
Soln:
Total number of α particles used for bombardment = 1 mole
1 mole=6.022x1023
particles
number of α particles deflected at angles greater than 50°(>50°)= 1%
Number of α particles deflected at angles greater than 50° =100-1= 99%
Actual number of α particles deflected at angles less than 50°=
99
100
× 6.022 × 1023
=5.96× 1023

Chapter 4
Questions:
Q. 1 What are canal rays?
Ans:
• The positively charged radiations produced in the discharged tube
at low pressure and high voltage are called as canal rays.
• It was noticed by German scientist ‘ Eugen Goldstein’.
Q.2 If an atom contains one electron and one proton, will it carry any
charge or not?
Ans: One proton has one unit positive. Similarly, one electron has one
unit negative charge. An atom with one proton and one electron will be
electrically neutral. The charges will be balanced inside an atom.
NCERT TEXTBOOK SOLUTION

Questions:
Q. 1 On the basis of Thomson's model of an atom, explain how the atom
is neutral as whole.
Ans: On the basis of Thomson’s model of an atom, an atom consists of
a positively charged sphere. The electrons are embedded in the sphere.
The negative and the positive charges are equal in magnitude. Hence, the
atom is electrically neutral.
Q.2 On the basis of Rutherford's model of an atom, which subatomic
particle is present in the nucleus of an atom?
Ans: According to the Rutherford’s model of an atom, there is a
positively charged center called nucleus. The subatomic particles called
protons are embedded in the nucleus. Almost all the mass of an atom
resides in the nucleus. The electrons revolve around the nucleus in well
defined orbits.
Q.3 Draw a sketch of Bohr's model of an atom with three shells.
Ans: A sketch of Bohr's model of an atom with three electron shells is
shown below:
According to Bohr’s model of an atom:
(i) Only certain special orbits known as discrete orbits of electrons, are
allowed inside the atom.
(ii) While revolving in discrete orbits the electrons do not radiate energy.
These orbits are called energy levels. Energy levels in an atom are
shown by circles.

These orbits are represented by the letters K,L,M,N,… or the numbers,
n=1,2,3,4,….
Further, the distribution of electrons into different orbits of an atom was
suggested by Bohr and Bury. The following rules are the rules followed
for writing the number of electrons in different energy levels or shells: (i)
The maximum number of electrons present in a shell is given by the
formula 2n2, where ‘n’ is the orbit number or energy level index, 1,2,3,….
Hence the maximum number of electrons in different shells can be written
as : first orbit or K-shell will be = 2 × 12 = 2, second orbit or L-shell will
be = 2 × 22 = 8, third orbit or M-shell will be = 2 × 32 = 18, fourth orbit
or N-shell will be = 2 × 42= 32, and so on. The maximum number of
electrons that can be accommodated in the outermost orbit is 8. Electrons
are not accommodated in a given shell, unless the inner shells are filled.
That is, the shells are filled in a stepwise manner.
Q. 4 What do you think would be the observation if the a-particle
scattering experiment is carried out using a foil of a metal other than gold?
Ans: On using any other metal other than gold would not have affected
the observations of the experiment. Since, the structure of an atom is
same for all states of atom. All other metals are not as ductile as gold, so
a thin foil would have been difficult to obtain. If a thick foil is used in
this experiment, then more alpha particles would have bounced back. As
a result, no idea about the location of nucleus and the presence of empty
space in an atom would be available with such a certainty.
Questions:
Q. 1 Name the three subatomic particles of an atom.
Ans.: Electron: It is negatively charged with one unit negative. The mass
of an electron is considered to be negligible.
Proton: Proton has a charge, equal in magnitude but opposite in sign to
that of the electron. Its mass is considered to be approximately 2000 times
as that of the electron.

Neutron: Neutrons are present in the nucleus of all atoms, except
hydrogen. It is subatomic particle which had no charge and a mass nearly
equal to that of a proton.
Q. 2 Helium atom has an atomic mass of 4u and two protons in its nucleus.
How many neutrons does it have?
Ans: We know that atomic mass is numerically equal to mass number of
an atom. Since the helium atom has an atomic mass of 4u, therefore, the
mass number of helium atom will be 4. And the number of protons in the
helium nucleus has been given to be 2. Now,
Mass number = No. of protons + No. of neutrons
4 = 2 + No. of neutrons
And, No. of neutrons = 4 – 2 = 2
Thus, the helium has 2 neutrons.
Questions:
Q.1 Write the distribution of electrons in carbon and sodium atoms.
Ans: (i) The atomic number of carbon is 6. It means carbon has 6
protons and 6 electrons. Out of these 6 electrons, 2 electrons will occupy
the first electron shell which is K shell and the remaining 4 electrons
will occupy the second electron shell which is L shell. So, the electron
distribution in a carbon atom will be:
K L
2, 4
(ii) The atomic number of sodium is 11. It means sodium atom has 11
protons and 11 electrons in it. Out of these 11 electrons, the first 2
electrons will go to K shell, the next 8 electrons will go to L shell and
the remaining 1 electron will go to M shell. So, the electron distribution in
a sodium atom will be:
K L M

8, 1
Q. 2 If K and L shells of an atom are full, then what be the total number
of electrons in the atom?
Ans: K shell becomes full with 2 electrons and L shell becomes full
with 8 electrons. So, if the K and L shells of an atom are full, then it will
have a total of 2 + 8 = 10 electrons in it.
Questions:
Q.1 How will you find the valency of chlorine, sulphur and magnesium?
Ans: The number of electrons gained, lost or shared so as to make the
octet of electrons in the outermost shell, gives the combining capacity of
the element which is called the valency of an atom.
(i) Valency of chlorine: The Atomic number of chlorine atom is 17. This
means it has 17 protons and 17 electrons.
K shell -2, L shell -8 and M shell-7.
A chlorine atom can accept 1 more electron to achieve configuration of
8 valence electrons. The valency of chlorine is 1.
(ii) Valency of sulphur: The atomic number of sulphur is 16. The
sulphur atom has 16 protons and 16 electrons.Its electron configuration
is K shell-2, L shell- 8 and M shell- 6.
A sulphur atom can accept 2 more electrons to achieve configuration of
eight valence electrons. The valency of sulphur is 2.
(iii) Valency of magnesium: The atomic number of magnesium is 12.
The means it has 12 protons and 12 electrons.
Its electron configuration is K shell-2, L shell-8 and M shell- 2 .
A magnesium atom can lose 2 outermost electrons to achieve
configuration of 8 valence electrons, The valency of magnesium is 2.

Questions:
Q.1 If the number of electrons in an atom is 8 and the number of protons
is also 8, then (i) what is the atomic number of the atom? And (ii) what is
the charge on the atom?
Ans: (a) Atomic number is equal to the number of protons in one atom.
Since this atom contains 8 orotons, so the atomic is 8.
b) This atom contains an equal number of positively charged and
negatively charged electrons (8 each), so it has no overall charge. That is,
the charge on this atom is 0 (zero).
Q.2 With the help of Tab. Is given below, find out the mass numbers of
oxygen and sulphur atoms:
Name of
Element
Symbol Atomic
Number
No. of
Protons
No. of
Neutrons
No. of
Electron
Oxygen
Sulphur
O
S
8
16
8
16
8
16
8
16
Ans.: The mass number is defined as the sum of the total number of
protons and neutrons present in the nucleus of an atom.
(i) Mass number of oxygen
= No. of protons + No. of neutrons = 8 + 8 = 16
Mass number of Oxygen is 16.
(ii) Mass number of sulphur
= No. of protons + No. of neutrons = 16 + 16 = 32
Mass number of sulphur is 32

17 17
18 20
Questions:
Q. 1 For the symbols H, D and T, tabulate three subatomic particles
found in each of them.
Ans: Hydrogen has three isotopes, protium (𝐻1), deuterium ( 𝐻2or 𝐷2)
1 1 1
and tritium (𝐻3or 𝑇3). The chemical properties of isotopes are similar
1 1
but their physical properties are different. They have same atomic
number but different mass numbers.
The three subatomic particles, electrons, protons and neutrons, in the
three isotopes of hydrogen are given in the following table:
Isotope Symbol Mass
number
Atomic
number
Number
of
protons
Number
of
electrons
Number
of
neutrons
Hydrogen H 1 1 1 1 0
Deuterium D 2 1 1 1 1
Tritium T 3 1 1 1 2
Q.2 Write the electronic configuration of any one pair of isotopes and
isobars.
Ans: (a) A pair of isotopes of chlorine is 35 Cl and 37 Cl. The atomic
number of both the isotopes is the same 17. So, the electronic
configuration of both these isotopes will be 2, 8, 7.
(b) A pair of isobars is 40 Ar and 40 Ca. The atomic number of orgon
(Ar) is 18, so its electronic configuration will be 2, 8, 8. The atomic of
calcius (Ca) is 20, so its electronic configuration will be 2, 8, 8, 2.

Exercise:
Questions:
Q. 1 Compare the properties of electrons, protons and neutrons.
Ans: Comparison between proton, Neutron and Electron
Subtomic
particle
Relative mass Relative charge Location in the
atom
Proton
Neutron
Electron
1u
1u
1/1840u
+1
0
-1
in the nucleus
In the nucleus
Outside
nucleuus
Q. 2 What are the limitations of J.J. Thomson's model of the atom?
Ans:
1. J.J. Thomson's model of the atom could not explain the results of
alpha particle scattering experiment carried out by Rutherford.
2. It also failed to explain the deflections and rebounding of the alpha
particles.3. It also failed to explain an atom's stability.
4. The theory did not mention anything about the nucleus of an atom.
Q.3 What are the limitations of Rutherford's model of the atom?
Ans: Limitations:
• The orbital revolution of the electron is not expected to be stable.
• According to Rutherford’s model, the electrons, while moving in
their orbits, would give up energy.
Q.4 Describe Bohr’s model of the atom.
Ans: The present concept of atom was given by Neils Bohr. The bohr’s

model of atom can be described as follows:
1) An atom is made up of three particles, called as electrons, protons and
neutrons.
2) The protons and neutrons are located in a small nucleus at the centre of
the atom.
3) The electrons revolve rapidly round the nucleus in fixed circular paths
called energy levels or shells.
4) There is a limit to the number of electrons which each energy level (or
shell) can hold.
5) Each energy level (or shell) is associated with a fixed amount of energy.
6) There is no change in the energy of electrons as long the keep revolving
in the same energy level, and the atom remains stable.
Q.5 Compare all the proposed models of an atom given in this chapter.
Ans: A comparison between the models of an atom proposed by J.J.
Thomson, Rutherford and Bohr is given below:
Model Thomsens model of an
atom
Rutherford's
model of an
atom
Bohr's model
of an atom
Property atom in which the atom
consisted of a positively
charged sphere. The
electrons are embedded
in the sphere. The
negative and the
positive charges are
equal in magnitude.
Hence, the atom is
electrically neutral.
Rutherford put
forward the
model of an
atom which
had a
positively
charged center
called nucleus.
It also had the
electrons
which
revolved
around the
i) Only certain
special orbits
known as
discrete orbits
of electrons,
are allowed
inside the atom
.(ii) While
revolving in
discrete orbits
the electrons do
not radiate
energy. (ili)The

nucleus in
well defined
orbits.
maximum
number of
electrons
present in a
shell is given
by the formula
2n2 (iv)The
maximum
number of
electrons that
can be
accommodated
in the
outermost orbit
is 8.
Limitations J.J. Thomson's model of
the atom could not
explain the results of
alpha particle scattering
experiment carried out
by Rutherford
he orbital
revolution of
the electron is
not expected
to be stable.
Any particle
which is in a
circular orbit
would
undergo an
acceleration.
Charged
particles
would radiate
energy during
acceleration.
Electrons are
negatively
charged. Thus,
the revolving
(i) Heisenberg
principle -
Position and
momentum of a
particle cannot
be determined
at same time,
accurately.
Result of both
of them is
greater than
h/4n. (ii) In
Bohr's theory,
orbits were
circular but
now we know
that they are 3-
D and not 2-D.

electron would
lose energy. It
will finally
fall into the
nucleus. If this
was the
situation, then
the atom
should be
highly
unstable.
Hence, matter
would not
exist in the
form that we
know.
Q.6 Summarise the rules for writing of distribution of electrons in
various shells for the first eighteen elements.
Ans:
(1) The maximum possible number of electrons which can be found in
any level of the atom of an element is given by 2n2 (where n is the number
of that energy level). By using this relation, we will find that :
(a) the maximum possible number of electrons which can be
accommodated in 1st energy level K shell, where n= 1,
2n2 = 2 X (1)2 = 2 X 1 = 2. Hence, K shell can have a maximum of 2
electrons.

(b) the maximum possible number of electrons which can be found in
2nd energy level, L shell, where n = 2 , 2n2 =2 X (2)2 = 2 X 4 = 8. L shell
can have a max 8 electrons.
(c) the maximum possible number of electrons which can be found in
3rd energy level M shell, where n = 3, 2n2 = 2 X (3)2 = 2 X 9 = 18. M shell
(d) the maximum possible number of electrons which can be found in 4th
energy level N shell, where n = 4. 2n2 = 2 X (4)2 = 2 X 16 = 32. N shell
(2)The electrons in an atom can not occupy a newer shell unless all the
inner shells filled with electrons. This means that first, all the electrons
fill K shell, then L shell, then M shell, then N shell, and so on.
(3) The electron in the outer shell of the first 18 elements cannot be more
than 8 electron in the outermost shell
Q. 7 Define valency by taking the examples of silicon and oxygen.
Ans: In order to achieve an octet in the outermost shell, atoms react. This
is done by gaining, sharing or losing electrons. The number of electrons
gained, lost or shared so as to make the octet of electrons in the outermost
shell, gives the combining capacity of the element which is called the
valency of an atom.
Valency of silicon: The atomic number of silicon is 14. It has 14 electrons
in its atom. The electronic configuration of silicon atom will be K shell-
2, L shell- 8 and M shell- 4. Silicon has 4 electrons in its valence shell.
Now, Silicon atom can neither lose 4 electrons nor gain 4 electrons to
complete its the octet due to energy considerations. Hence, silicon atom
will share its 4 electrons with the 4 electrons of atoms of other elements
to complete its octet. Since one silicon atom will share 4 electrons to
achieve inert gas electron arrangement, therefore, the valency of silicon is
4. Valency of oxygen: The atomic number of oxygen is 8. It has 8
electrons in its atom. The electronic configuration of oxygen atom will be

1 1
K shell- 2 and L shell- 6. Oxygen has 6 electrons in its outermost shell.
Now, an oxygen atom having 6 outermost electrons can gain 2 electrons
from some other atom to achieve the nearest noble gas configuration. Since
one atom of oxygen can gain 2 electrons to complete its octet. Therefore,
the valency of oxygen is 2.
Q. 8 Explain with examples (i) Atomic number (ii) Mass number (iii)
Isotopes, and (iv) Isobars. Give any two uses of isotopes.
Ans: (i) Atomic number: The number of protons in an atom of an element
is known as atomic number of that element. For example, one atom of
sodium element has 11 protons in it, so the atomic number of sodium is
11. Similarly, one atom of carbon element has 6 protons in it, so the
atomic number of carbon is 6.
(ii) Mass number: The total number of protons and neutrons present in
one atom of an element is known as its mass number. For example, one
atom of sodium element contains 11 protons and 12 neutrons. Hence, the
mass number of sodium is 11 + 12 = 23. Similarly, a normal carbon atom
has 6 protons and 6 neutrons, so the mass number of carbon is 6 + 6 = 12.
(iii) Isotopes: Atoms of same element with same atomic number but
different mass number are called isotopes of that element. For example,
Hydrogen has three isotopes, protium (H1
1), deuterium (H2
1 or D2
1) and
tritium (H3 or T3 ). Many elements consist of a mixture of isotopes. Each
isotope of an element is a pure substance. The chemical properties of
isotopes are similar but their physical properties are different.
(iv) Isobars: Atoms of different elements which have the same mass
number but have different atomic numbers are called isobars. For
example, calcium, atomic number-20 and argon, atomic number 18 are
isobars. Both these elements have same mass number which is equal to
40. Therefore, the total number of nucleons is the same in both the
elements.

Uses of Isotopes: Isotopes of some elements have special properties
makes them useful in various fields. For example, (i) An isotope of
uranium is used as a fuel in nuclear reactors. (ii) An isotope of cobalt is
used in the treatment of cancer. (iii) An isotope of iodine is used in the
treatment of goitre.
Q.9 Na+ has completely filled K and L shells. Explain.
Ans: A sodium ion Na+, has 10 electrons in it. Now, the maximum
capacity of K shell is 2 electrons and that of L shell is 8 electrons. Taken
together, the maximum capacity of K and L shells is 2 + 8 = 10 electrons.
A sodium ion Na+ has completely filled K and L shells because its 10
electrons can completely fill up K and L shells.
Q.10 If bromine is available in the form of, say, two isotopes 7935Br
(49.7%) 8135Br (5,3%) calculate the average atomic mass of bromine
atom.
Ans: We know that upper digit in the symbol of an isotope represents its
mass (which is the same as its mass number). Now

35
35
8 8
8
8
(i) The mass of
(ii) The mass of
79𝐵𝑟 isotope is 79 u and its abundance is 49.7%.
81𝐵𝑟 isotope is 81 u and its abundance is 50.3%.
So, Average atomic mass of bromine =
79 ×
49.7
+ 81 ×
50.3
100 100
=
3926.3
+
4074.3
100 100
= 39.263 + 40.743
= 80.006
= 80u
Thus, the average atomic mass of bromine is 80 u.
Q.11 The average atomic mass of a sample of an element X is 16.2 u.
What are the percentages of isotopes
Ans.:
16𝑋 and 18𝑋 in the sample.
In order to solve this problem, we will have to suppose that the percentage
of one of the isotopes in the sample is x, so that the percentage of the other
isotope in the sample will be (100-x). Now:
(i) The mass of
is x%.
(ii)The mass of
(100-x)%.
16𝑋 isotope is 16u. Suppose its percentage in the sample
18 𝑋isotope is 18 u. Its percentage in the sample will be
So, Average atomic mass of X = 16 ×
𝑋
100
+ 18 ×
(100−𝑋)
100
But the average atomic mass of X has been given to be 16.2 u. Therefore,
16.2 = 16 ×
𝑋
100
+ 18 ×
(100−𝑋)
100

8
8
16.2 =
16𝑋+1800−18𝑥
100
16.2 × 100 = 1800 − 2𝑥
2x = 1800-1620
2x = 180
x =
180
2
X = 90
Thus, the percentage of the 16𝑋 isotope in the sample is 90%. The
percentage of the isotope 18
𝑋 in the sample will be 100 – 90 = 10%.
Q.12 If Z = 3, what would be the valency of the element? Also name the
element.
Ans: The symbol Z stands for the atomic number of an element. By
saying that Z = 3, we mean that the atomic number of this element is 3.
The electronic configuration of the element having atomic number 3 is K
L. 2, 1 It has 1 electron in its outermost shell (L shell). So, one atom of this
element can lose 1 electron to achieve the nearest inert gas electron
arrangement of K (which is the same 2 as that of helium gas). Since one
atom of this element loses 1 electron to achieve the inert gas electron
configuration, therefore, its valency is 1. The element having atomic
number 3 is lithium.
Q. 13 Composition of the nuclei of two atomic species X and Y is given
as under:
X Y
Protons = 6 6

Neutrons = 6 8
Give the mass numbers of X and Y. What is the relation between the two
species?
Ans.: We know that:
Mass number = No. of protons + No. of neutrons
So, Mass number of X = 6 + 6
= 12
Mass number of Y = 6 + 8
= 14
Thus, the mass number of X is 12 and that of Y is 14.
Q. 14 A For the following statements, write T for true and F for false:
J. J. Thomson proposed that the nucleus of an atom contains only
nucleons.
Ans.: False.
J,J Thomson proposed the model of an atom in which the atom consisted
of a positively charged sphere. The electrons are embedded in the sphere.
The negative and the positive charges are equal in magnitude.
Q.14(B) For the following statements, write T for true and F for false:
A neutron is formed by an electron and a proton combining together.
Therefore, it is neutral.
Ans.: False.

Neutrons are present in the nucleus of all atoms, except hydrogen. It is
subatomic particle which had no charge and a mass nearly equal to that of
a proton.
Q. 14C For the following statements, write T for true and F for false:
The mass of an electron is about 1/2000 times that of a proton.
Ans.: True.
Proton has a charge, equal in magnitude but opposite in sign to that of the
electron. Its mass is considered to be approximately 2000 times as that of
the electron.
Q. 14D For the following statements, write T for true and F for false:
An isotope of iodine is used for making tincture iodine, which is used as a
medicine.
Ans.: False
An isotope of iodine is used in the treatment of goitre.
Q.15 Rutherford's alpha-particle scattering experiment was responsible
for the discovery of:
(a) Atomic nucleus (b) Electron
(c) Proton (d) Neutron
Ans: (a) Atomic nucleus
According to the Rutherford’s model of an atom, there is a positively
charged center called nucleus. Almost all the mass of an atom resides in
the nucleus. The electrons revolve around the nucleus in well defined
orbits which can be compared to the solar system.

1
1 1 1 1
Q. 16 Isotopes of an element have :
(a) the same physical properties
(c) different number of neutrons
(b) different chemical properties
(d) different atomic numbers
Ans: (c) different number of neutrons.
Atoms of same element with same atomic number but different mass
number are called isotopes of that element. For example, Hydrogen has
three isotopes, protium (H1 ), deuterium (H2 or D2 ) and tritium (H3 or
T3 ).
Q. 17 Number of valence electrons in Cl- ion are:
(a) 16
(b) 8
(c) 17
(d) 18
Ans:
Chlorine atom with 17 protons(+) Chloride ion with 17 protons (+)
and 17 electrons (-) Charge =0 and 18 electrons (-) Charge = -1.

Q. 18 Which of the following is a correct electronic configuration of
sodium?
(a) 2, 8 (b) 8, 2, 1
(c) 2, 1, 8 (d) 2, 8,1
Ans: (d) 2, 8, 1
Atomic number of sodium is 11. Its electronic configuration is K shell- 2,
L shell- 8 and m shell- 1. It has 1 electron in its valence shell.
Q. 19 Complete the following table:
Atomic
Number
Mass
number
Number
of
neutrons
Number
of protons
Number
of
electrons
Name of
the atomic
species
9 - 10 - - -
16 32 - - - Sulphur
- 24 - 12 - -
- 2 - 1 - -
- 1 - 1 0 -
Ans.:
(a) First row:
(i) The given atomic number is 9. This shows that the number of protons
is 9 and the number of electrons is also 9.
(ii) Mass number is equal to the sum of the number of protons and
neutrons. Therefore, mass number = 9 + 10 = 19.
(iii) The atomic species having atomic number 9 is fluorine.
(b) Second row:
(i) The given atomic number is 16. This shows that the number of protons
is 16 and the number of electrons is also 16.

1
(ii) The given mass number is 32. Number of neutrons can be calculated
as , Mass number minus the number of protons. Therefore, the number of
neutrons = 32 - 16 = 16
(c) Third row:
(i) The number of protons as 12. Hence,the atomic number is 12. The
number of electrons is also equal to 12.
(ii) Number of neutrons can be calculated as, Mass number minus the
number of protons. Therefore, the number of neutrons = 24-12 = 12.
(iii) The atomic species with atomic number 12 is magnesium.
(d) Fourth row:
(i)The number of protons is given as 1. Hence, the atomic number is 1
and the number of electrons is also equal to 1.
(ii) Number of neutrons can be calculated as, Mass number minus the
number of protons. Therefore, the number of neutrons = 2-1=1.
(iii) The atomic species of atomic number 1 and mass number 2 is an
isotope of hydrogen called deuterium H2
1 or D2 .
(e) Fifth row
(i) The number of protons is given as 1. Hence,the atomic number is also
1.
(ii) The atomic species of atomic number 1 and mass number 1 is
hydrogen or protium.
We can now write the completed Table as follows:
Atomic
number
Mass
number
Number
of
neutrons
Number
of protons
Number
of
electrons
Name of
the atomic
species
9 19 10 9 9 Fluorine

16
12
1
1
32
24
2
1
16
12
1
0
16
12
1
1
16
12
1
0
Sulphur
Magnesium
Deutrium
Protium
(hydrogen)

04
MIND MAP
Page 1

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