Structure of the Atom.pptx.pdf

❏ Atoms & Molecules are the fundamental
building blocks of matter.
❏ The existence of different kinds of matter
is due to different atoms constituting
them.

Dalton assumed that atom is indivisible,
i.e., it has no constituent particles.
But a series of experiments proved
otherwise.

Some other particles smaller than atom are
also present, which are called subatomic
particles.
Subatomic Particles
Electrons Protons Neutrons

Indication that Atoms are
DIVISIBLE
1. Static electricity (electrical charge caused by
an imbalance of electrons on the surface of a
material), and
2. The condition under which electricity is
conducted by different substances.

It was known by 1900 that the
atom was indivisible particle
but contained at least one
sub-atomic particle– the
electron identiﬁed by J.J.
Thomson. in his cathode ray
experiment.

Structure of the Atom.pptx.pdf

THIS IS HOW ELECTRONS WERE
DISCOVERED!
BUT WAIT, ATOMS ARE SUPPOSED
TO BE NEUTRAL, NO?

Before the electron
was identiﬁed, E.
Goldstein in 1886
discovered the
presence of new
radiations in a gas
discharge and called
them canal rays.

❏ It led to the
discovery of
another
sub-atomic
particle, the
proton.

Phosphorescence Material
A material absorbs energy with a short
wavelength (such as UV light) and then
emits it as visible light.

The mass of a proton is taken as one unit
and its charge as plus one.
The mass of an electron is considered to be
negligible and its charge is minus one.

RECAP
❏ Every matter has charged particles.
❏ The charged particles are present in the
constituent atoms.
❏ Each atom has two types of charged
particles, electrons (‘e—
’) with a negative
charge and protons (‘p+
’) with a positive
charge.
❏ Sir J.J. Thomson discovered- Electrons.
E. Goldstein discovered- Protons.
❏ The mass of an electron = 9.1 x 10 — 31
Kg.
❏ The mass of an proton = 1.6 x 10 — 27
Kg.

J.J. Thomson- The First Scientist to
Propose a Model for Structure of an Atom.

Thomson’s Model of an Atom
The electrons, in a sphere of positive charge,
were like currants (dry fruits) in a spherical
Christmas pudding.
Model of an atom was similar to that of a
Christmas pudding/ Watermelon

Thomson proposed:
1. An atom consists of a positively
charged sphere and the electrons are
embedded in it.
2. The negative and positive charges are
equal in magnitude. So, the atom as a
whole is electrically neutral.

Limitations of Thomson’s Model
1. His model could not explain the
experimental results of other scientists
such as Rutherford, as there is no
nucleus in the atomic model proposed
by Thomson.
2. It could not explain the stability of an
atom, i.e., how could positive and
negative charge remain so close.

Rutherford’s Model of an Atom
Ernest Rutherford DISCOVERED arrangment
within an atom. Rutherford designed an
experiment for this. In this experiment, fast moving
alpha (α)-particles were made to fall on a thin gold
foil.

He selected a gold foil because he wanted as thin a
layer as possible. This gold foil was about 1000
atoms thick.
It was expected that α-particles would be deﬂected
by the sub-atomic particles in the gold atoms. Since
the α-particles were much heavier than the protons,
he did not expect to see large deﬂections.
α-particles are doubly-charged helium ions. Since
they have a mass of 4 u, the fast-moving α-particles
have a considerable amount of energy.

But, the α-particle scattering experiment
gave totally unexpected results.

Observations made by Rutherford
● Most of the fast moving α-particles passed
straight through the gold foil.
● Some of the α-particles were deﬂected by the
foil by small angles.
● Surprisingly one out of every 12000 particles
appeared to rebound.
In the words of Rutherford, “This result was
almost as incredible as if you ﬁre a 15-inch
shell at a piece of tissue paper and it comes
back and hits you”.

Rutherford’s Conclusion
● Most of the space inside the atom is empty
because most of the α-particles passed through
the gold foil without getting deflected.
● Very few particles were deflected from their path,
indicating that the positive charge of the atom
occupies very little space.
● A very small fraction of α-particles were deflected
by 180o
, indicating that all the positive charge and
mass of the gold atom were concentrated in a very
small volume within the atom.
From the data he also calculated that the
radius of the nucleus is about 105 times
less than the radius of the atom.

Rutherford’s Nuclear Model of an Atom
1. There is a positively charged centre in
an atom called the nucleus. Nearly all
the mass of an atom resides in the
nucleus.
2. The electrons revolve around the
nucleus in circular paths.
3. The size of the nucleus is very small as
compared to the size of the atom.

Limitations of Rutherford’s Model
● The revolution of the electron in a circular orbit is not
expected to be stable. Any particle in a circular
orbit would undergo acceleration. During
acceleration, charged particles would radiate energy.
● Thus, the revolving electron would lose energy and
ﬁnally fall into the nucleus.
● If this were so, the atom should be highly unstable
and hence matter would not exist in the form that we
know. We know that atoms are quite stable.

Bohr’s Model of an Atom
In order to overcome the objections raised
against Rutherford’s model of the atom,
Neils Bohr put forward his hypothesis about the
model of an atom.

Postulates of Bohr’s Model
● Atom consists of positively charged nucleus
around which electrons revolve in discrete orbits,
i.e., electrons revolve in certain permissible
orbits and not just in any orbit.
● While revolving in discrete orbits the electrons
do not radiate energy. These orbits or shells are
called energy levels

Postulates of Bohr’s Model
● These orbits or shells are represented by
the letters K, L, M, N,… or the numbers, n=
1,2,3,4,….
● The electrons present in the ﬁrst energy
level (E1
) have lowest energy. Energies
increases on moving towards outer
energy levels.
● Energy of an electron remains same as
long as it remains in discrete orbit and it
does not radiate energy while revolving.
● When energy is supplies to an electron, it
can go to higher energy levels. While an
electron falls to lower energy level, when
it radiate energy.

In 1932, J. Chadwick discovered another subatomic
particle which had no charge and a mass nearly
equal to that of a proton.
It was eventually named as
neutron (n). Neutrons are
present in the nucleus of all
atoms, except hydrogen.

The mass of an atom is therefore given by the sum
of the masses of protons and neutrons present in
the nucleus (because e- have negligible mass).
Nitrogen Atom
Protons = 7
Neutrons = 7
7 + 7 = 14
Atomic Mass = 14

How are Electrons
Distributed in Different
Orbits (Shells)?

Electronic Conﬁguration
It describes how electrons are distributed in its
atomic orbitals.

The distribution of electrons into
different orbits of an atom was
suggested by Bohr and Bury.
There are rules to be followed for
writing the number of electrons in
different energy levels or shells.

Rule 1
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 are as follows:
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.

Rule 2
The maximum number of electrons that can be
accommodated in the outermost orbit is 8.
Rule 3
Electrons are not accommodated in a given shell, unless
the inner shells are ﬁlled. That is, the shells are ﬁlled in a
step-wise manner.

Element Name
Atomic
Number
Hydrogen 1
Helium 2
Lithium 3
Beryllium 4
Boron 5
Carbon 6
Nitrogen 7
Oxygen 8
Fluorine 9

Element Name
Atomic
Number
Neon 10
Sodium 11
Magnesium 12
Aluminium 13
Silicon 14
Phosphorous 15
Sulphur 16
Chlorine 17
Argon 18

The electrons present in the outermost
shell of an atom are known as the
valence electrons.
They govern the chemical properties of
atoms.
Valence Electron

● The atoms of elements having completely
ﬁlled outermost shell means which has
eight electrons show little chemical
activity, i.e., they are highly stable. Such
elements are called inert elements.
● Their valency is zero.
● Of these helium atom has two electrons
in its outermost shell and all other
elements have atoms with eight electrons
in the outermost shell.

Atoms react with other atoms in order to attain
fully-ﬁlled outermost shell.
Octet- An outermost shell, which has 8 electrons.
Atoms thus react, so as to achieve an octet in the
outermost shell.
This is done by sharing, gaining, or the loss of
electrons.

Valency of Element
The number of electrons lost or gained
or shared by an atom to become stable
or to achieve an octet in the outermost
shell.

FOR EXAMPLE
Hydrogen/ lithium/sodium atoms contain 1
electron each in their outermost shell, therefore
each one of them can lose one electron. So, they
are said to have valency of 1.
Na
H Li

EXAMPLE:
The valency if each of Mg, Ca, and Be is 2
because all of these have 2 valence electrons and
they can lose these 2 electrons to become stable.

● If the number of electrons in the
outermost shell of an atom is close to its
full capacity, then valency is determined
in a different way.
● For example, the fluorine atom has 7
electrons in the outermost shell, and its
valency could be 7. But it is easier for
fluorine to gain one electron instead of
losing seven electrons.
● Hence, its valency is determined by
subtracting seven electrons from the
octet (8 - 7 = 1) and this gives you a
valency of one for fluorine.

How will you ﬁnd the valency of chlorine, sulphur and
magnesium? - NCERT
Solution:
i) Chlorine = 2, 8, 7
Valency of Chlorine = 8 - 7 = 1
ii) Sulphur = 2, 8, 6
Valency of Sulphur = 8 - 6 = 2
iii) Magnesium = 2, 8, 2
Valency of Magnesium = 2

1. It is deﬁned as the number of protons
present in the nucleus of an atom.
2. All the atoms of the same element have
same number of protons in their nuclei
and hence, they have the same atomic
number.
3. It is denoted by Z. All atoms of an element
have the same atomic number, Z.
ATOMIC NUMBER

For hydrogen, Z = 1, because in
hydrogen atom, only one proton is
present in the nucleus.
For carbon, Z= 6.
Therefore, the atomic number is
deﬁned as the total number of protons
present in the nucleus of an atom.

1. It is deﬁned as the sum of the total number
of protons and neutrons present in the
nucleus of an atom.
2. Protons & neutrons together are called as
nucleons.
3. Mass number is denoted by A.
Mass number (A) = Number of protons +
Number of Neutrons.
MASS NUMBER

In the notation for an atom, the atomic
number, mass number and symbol of the
element are to be written as:

ISOTOPES
Sometimes some elements have different atomic species.
They then have same atomic number (Z) but different mass
number (A) due to different number of neutrons in it.
Example: hydrogen atom, it has three atomic species, namely
protium (1
1
H), deuterium ( 2
1
H or D) and tritium ( 3
1
H or T).
The atomic number of each one is 1, but the mass number is
1, 2 and 3, respectively.

● On the basis of these examples, isotopes
are deﬁned as the atoms of the same
element, having the same atomic number
but different mass numbers.

● 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.
FEATURES- ISOTOPES

Average Atomic Mass
If element has no isotopes, the mass of its
atom would be the same as the sum of
masses of protons and neutrons.
But if element occurs in isotopic forms, then
from the percentage of each isotopic form,
the average mass is calculated.

Average Atomic Mass of
an element:
[ ( Atomic mass of isotopes I x percentage of isotope I ) +
( Atomic mass of isotope II x percentage of isotope II)
+..........]

Isotopic Form of Chlorine
Chlorine occurs in nature in two isotopic forms, with
masses 35 u and 37 u in the ratio of 3:1.
The atomic mass of chlorine on the basis of the
formula is:
Here, 35.5 u is not the atomic mass of any one
atom of chlorine but it shows that its given
amount contains both the isotopes and their
average atomic mass is 35.5 u.

Applications of Isotopes
Some isotopes have special properties which ﬁnd
them useful in various ﬁelds. Some of them are:
An isotope of cobalt is used in the treatment of
cancer.
An isotope of iodine is used in the treatment of
goitre.
An isotope of uranium is used as a fuel in
nuclear reactors.

ISOBARS
Atoms of different elements with different
atomic numbers, which have the same
mass number, are known as isobars.
Isobars are the atoms of different
elements that have same number of
nucleons (protons + Neutrons) but differ
in the number of protons.

EXAMPLE OF ISOBARS
ARGON CALCIUM

1. The ﬁrst model of an atom was given by:
A. N Bohr
B. E. Goldstein
C. Rutherford
D. J.J. Thomson

2. The electron distribution in an aluminium
atom is:
A. 2, 8, 3
B. 2, 8, 2
C. 8, 2, 3
D. 2, 3, 8

Structure of the Atom.pptx.pdf

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