Grade-09_Science_Chapter04_Structure-of-the-Atom.pdf

Grade 09
Science Chapter Notes
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Chapter Notes
Structure of
the Atom
GRADE 09
103
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Topics to Be Covered
1. Sub-Atomic
Particles
Electron
1.1
Proton
1.2
Neutron
1.3
3. Distribution of
Electrons in Different
Shells
Bohr-Bury Scheme
3.1
Atomic Number
5.1
Mass Number
5.2
Notation of an Atom
5.3
5. Atomic Number
and Mass Number
6. Isotopes
and Isobars
Introduction to
Isotopes
6.1
Introduction to
Isobars
6.2
4. Valency
Valence Electrons
4.1
Introduction to
Valency
4.2
Thomson’s Model of
an Atom
2.1
Rutherford’s Atomic
Model
2.3
Bohr’s Model of
an Atom
2.4
2. Structure
of an Atom
Calculation of
Valency
4.3
Electronic
Configuration
3.2
Rutherford’s Gold
Foil Experiment
2.2
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Mind Map
Bohr-Bury
Scheme
Structure
of the Atom
Electronic
Configuration
Valency
Thomson’s Model
Rutherford’s Model
Bohr’s Model
Atomic
Models
Electron
Proton
Neutron
Sub-Atomic
Particles
Isotopes
and
Isobars
Atomic
Numberand
MassNumber
105
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1. Sub-Atomic Particles
• Discovered by J.J. Thomson.
• Has a charge of -1.
• Its mass is considered to be negligible as compared to
the mass of the atom.
Electron (e-
)
1.1
Proton (p+
)
1.2
• Discovery of canal rays by E. Goldstein in a gas
discharge led to the discovery of protons.
• Has a charge of +1.
• Its mass is considered as one unit.
Neutron (n)
1.3
• Discovered by J. Chadwick in 1932.
• It is neutral as it does not contain any charge
• The mass of a neutron is nearly equal to the mass of
a proton.
The mass of a proton is approximately
2000 times the mass of an electron.
Neutrons are present in the nucleus of all
atoms, except hydrogen.
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Thomson’s Model of an Atom
2.1
• Thomson proposed the model of an atom that was
similar to a Christmas pudding.
Thomson proposed that:
• 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.
2. Structure of an Atom
Fig. Thomson’s model of an atom
-
-
-
-
-
-
- -
-
-
-
-
-
+
+
+
+
+
+
+ + +
+
+ +
Electron
Positive
sphere
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Rutherford’s Gold Foil Experiment
2.2
• Rutherford performed an alpha (α) particle
scattering experiment to know the arrangement of
electrons within an atom.
• In his experiment, fast-moving α-particles were
bombarded on a thin gold foil.
• α-particles are the doubly-charged helium ions,
having a mass of 4 u.
• He expected that the 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.
Gold foil
Alpha
beam
source
Detector
Fig. Rutherford's Gold Foil Experiment
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• Most of the α-particles passed through the gold foil
undeflected, indicating that most of the space inside
the atom is empty.
• Few α-particles underwent slight deflections 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
180°, indicating that all the positive charge and mass
of the gold atom were concentrated in a very small
volume within the atom.
• Most of the α-particles went across the gold foil
without any deflection.
• Few α-particles underwent deflections by small angles.
• 1 in 12,000 α-particles got deflected by almost 180°.
Gold atom
Observations of Gold Foil Experiment
Fig. Scattering of α-particles by a gold foil
Conclusions of Gold Foil Experiment
Alpha
particles
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Rutherford’s Atomic Model
2.3
• Atom consists of a dense positively charged centre
called nucleus. Nearly all the mass of an atom resides
in the nucleus.
• Electrons revolve around the nucleus in circular paths.
• The size of the nucleus is very small as compared to
the size of the atom.
Nucleus
Electron
Fig. Rutherford’s atomic model
Drawbacks of Rutherford’s Atomic Model
The electrons in the circular orbit would lose energy due to
acceleration and finally fall into the nucleus.
If this were so, an atom would have been highly unstable,
and matter would not exist. But atoms are quite stable
Hence, Rutherford failed to explain the stability of atoms.
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Bohr’s Model of an Atom
2.4
+ Nucleus
M-shell (n=3)
L-shell (n=2)
K-shell (n=1)
N-shell (n=4)
Fig. Energy levels in an atom
• Electrons revolve around the nucleus in discrete
orbits. These orbits or shells are known as energy
levels.
• While revolving in discrete orbits, the electrons
do not radiate energy.
• Orbits or shells are represented by the letters K, L, M,
N, … or the numbers, n=1, 2, 3, 4, ….
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3. Distribution of Electrons
in Different Shells
3.1 Bohr-Bury Scheme
Shell (n)
Maximum number of
electrons (2n
2
)
K (n = 1)
L (n = 2)
M (n = 3)
N (n = 4)
2 x 1
2
= 2
2 x 2
2
= 8
2 x 3
2
= 18
2 x 4
2
= 32
The following rules are followed for writing the number of
electrons in different energy levels or shells:
• The maximum number of electrons present in an
orbit is given by the formula 2n
2
, where ‘n’ is the
orbit number or energy level index, 1, 2, 3,….
• 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 step-wise manner.
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3. Distribution of Electrons
in Different Shells
3.2 Electronic Configuration
The distribution of electrons in different shells is known
as the electronic configuration of an element.
Hydrogen (H)
Carbon (C)
Nitrogen (N)
Oxygen (O)
Sodium (Na)
Magnesium (Mg)
Elements
Atomic
number
1
6
7
8
11
12
Electronic
configuration
1
2, 4
2, 5
2, 6
2, 8, 1
2, 8, 2
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Valence Electrons
4.1
4. Valency
• The electrons present in the outermost shell
(valence shell) of an atom are known as the valence
electrons.
• An atom with 8 valence electrons is said to possess
an octet.
Argon
(2, 8, 8)
Valence shell
Valence electron
Introduction to Valency
4.2
• Valency is the combining capacity of an atom of an
element.
• The number of electrons gained, lost or shared by an
atom to complete the octet in its outermost shell, gives
the valency of that atom.
Number of valence electrons = 8
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Calculation of Valency
4.3
4. Valency
If an atom contains 4 or less than 4 valence electrons, then
Valency = Number of valence electrons
If an atom contains more than 4 valence electrons, then
Na
Valency of sodium = 1
Valency of chlorine = 8-7 = 1
Cl
Valency = 8 - Number of valence electrons
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• The atomic number is defined as the total number of
protons present in the nucleus of an atom.
• All atoms of an element have the same atomic number.
5.1 Atomic Number (Z)
5. Atomic Number
and Mass Number
Mass Number (A)
5.2
• The mass number is defined as the sum of the total
number of protons and neutrons present in the
nucleus of an atom.
• Protons and neutrons reside in the nucleus of an
atom. Hence, they are collectively known as nucleons.
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• In the notation for an atom, the atomic number,
mass number, and symbol of the element are to be
written as:
5.3 Notation for an Atom
Atomic number
A
Z
Mass number
Symbol of
element
• For example, nitrogen is written as N.
14
7
5. Atomic Number
and Mass Number
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6. Isotopes and Isobars
Introduction to Isotopes
6.1
• Isotopes are the atoms of the same element, which have
the same atomic number but different mass numbers.
For example,
Protium
H
1
1
Deuterium Tritium
H
2
1 H
3
1
• The chemical properties of isotopes are same, but their
physical properties are different.
M1 and M2 = Atomic mass of isotopes 1 and 2
P1 and P2 = Percentage abundance of isotopes 1 and 2
Average atomic mass =
(M1P1 + M2P2 + M3P3 +….)
100
• If an element occurs in different isotopic forms, then
we need to know the percentage of each isotopic
form in nature and then the average atomic mass is
calculated.
Average Atomic Mass of Isotopes
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Used as fuel in nuclear
reactors.
Isotope of uranium
Used in the treatment
of cancer.
Isotope of cobalt
Used in the treatment
of goitre.
Isotope of iodine
Applications of Isotopes
119
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Introduction to Isobars
6.2
• Isobars are the atoms of different elements, which
have different atomic numbers but same mass
numbers.
For example,
Calcium
Ca
40
20
Argon
Ar
40
18
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