NCERT Textbook: Structure of Atom | General Science(Prelims) by IRS Divey Sethi - UPSC PDF Download

 Page 1


In Chapter 3, we have learnt that atoms and
molecules are the fundamental building
blocks of matter. The existence of different
kinds of matter is due to different atoms
constituting them. Now the questions arise:
(i) What makes the atom of one element
different from the atom of another element?
and (ii) Are atoms really indivisible, as
proposed by Dalton, or are there smaller
constituents inside the atom? We shall find
out the answers to these questions in this
chapter. We will learn about sub-atomic
particles and the various models that have
been proposed to explain how these particles
are arranged within the atom.
A major challenge before the scientists at
the end of the 19th century was to reveal the
structure of the atom as well as to explain its
important properties. The elucidation of the
structure of atoms is based on a series of
experiments.
One of the first indications that atoms are
not indivisible, comes from studying static
electricity and the condition under which
electricity is conducted by different
substances.
4.1 Charged Particles in Matter
For understanding the nature of charged
particles in matter, let us carry out the
following activities:
Activity ______________4.1
A. Comb dry hair. Does the comb then
attract small pieces of paper?
B. Rub a glass rod with a silk cloth and
bring the rod near an inflated balloon.
Observe what happens.
From these activities, can we conclude
that on rubbing two objects together, they
become electrically charged? Where does this
charge come from? This question can be
answered by knowing that an atom is divisible
and consists of charged particles.
Many scientists contributed in revealing
the presence of charged particles in an atom.
It was known by 1900 that the atom was
not a simple, indivisible particle but contained
at least one sub-atomic particle – the electron
identified by J.J. Thomson. Even before the
electron was identified, E. Goldstein in 1886
discovered the presence of new radiations  in
a gas discharge and called them canal rays.
These rays were positively charged radiations
which ultimately led to the discovery of
another sub-atomic particle. This sub-atomic
particle had a charge, equal in magnitude but
opposite in sign to that of the electron. Its
mass was approximately 2000 times as that
of the electron. It was given the name of
proton. In general, an electron is represented
as ‘e
–
’ and a proton as ‘p
+
’. 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.
It seemed highly likely that an atom was
composed of protons and electrons, mutually
balancing their charges. It also appeared that
the protons were in the interior of the atom,
for whereas electrons could easily be peeled
off but not protons. Now the big question was:
what sort of structure did  these particles of
the atom form? We will find the answer to
this question below.
4 4
4 4 4
S S S S STRUCTURE TRUCTURE TRUCTURE TRUCTURE TRUCTURE     OF OF OF OF OF     THE THE THE THE THE A A A A ATOM TOM TOM TOM TOM
Chapter
Page 2


In Chapter 3, we have learnt that atoms and
molecules are the fundamental building
blocks of matter. The existence of different
kinds of matter is due to different atoms
constituting them. Now the questions arise:
(i) What makes the atom of one element
different from the atom of another element?
and (ii) Are atoms really indivisible, as
proposed by Dalton, or are there smaller
constituents inside the atom? We shall find
out the answers to these questions in this
chapter. We will learn about sub-atomic
particles and the various models that have
been proposed to explain how these particles
are arranged within the atom.
A major challenge before the scientists at
the end of the 19th century was to reveal the
structure of the atom as well as to explain its
important properties. The elucidation of the
structure of atoms is based on a series of
experiments.
One of the first indications that atoms are
not indivisible, comes from studying static
electricity and the condition under which
electricity is conducted by different
substances.
4.1 Charged Particles in Matter
For understanding the nature of charged
particles in matter, let us carry out the
following activities:
Activity ______________4.1
A. Comb dry hair. Does the comb then
attract small pieces of paper?
B. Rub a glass rod with a silk cloth and
bring the rod near an inflated balloon.
Observe what happens.
From these activities, can we conclude
that on rubbing two objects together, they
become electrically charged? Where does this
charge come from? This question can be
answered by knowing that an atom is divisible
and consists of charged particles.
Many scientists contributed in revealing
the presence of charged particles in an atom.
It was known by 1900 that the atom was
not a simple, indivisible particle but contained
at least one sub-atomic particle – the electron
identified by J.J. Thomson. Even before the
electron was identified, E. Goldstein in 1886
discovered the presence of new radiations  in
a gas discharge and called them canal rays.
These rays were positively charged radiations
which ultimately led to the discovery of
another sub-atomic particle. This sub-atomic
particle had a charge, equal in magnitude but
opposite in sign to that of the electron. Its
mass was approximately 2000 times as that
of the electron. It was given the name of
proton. In general, an electron is represented
as ‘e
–
’ and a proton as ‘p
+
’. 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.
It seemed highly likely that an atom was
composed of protons and electrons, mutually
balancing their charges. It also appeared that
the protons were in the interior of the atom,
for whereas electrons could easily be peeled
off but not protons. Now the big question was:
what sort of structure did  these particles of
the atom form? We will find the answer to
this question below.
4 4
4 4 4
S S S S STRUCTURE TRUCTURE TRUCTURE TRUCTURE TRUCTURE     OF OF OF OF OF     THE THE THE THE THE A A A A ATOM TOM TOM TOM TOM
Chapter
uestions
1. What are canal rays?
2. If an atom contains one electron
and one proton, will it carry any
charge or not?
4.2 The Structure of an Atom
We have learnt Dalton’s atomic theory in
Chapter 3, which suggested that the atom
was indivisible and indestructible. But the
discovery of two fundamental particles
(electrons and protons) inside the atom, led
to the failure of this aspect of Dalton’s atomic
theory. It was then considered necessary to
know how electrons and protons are arranged
within an atom. For explaining this, many
scientists proposed various atomic models.
J.J. Thomson was the first one to propose a
model for the structure of an atom.
4.2.1 THOMSON’S MODEL OF AN ATOM
Thomson proposed the model of an atom to
be similar to that of a Christmas pudding.
The electrons, in a sphere of positive charge,
were like currants (dry fruits) in a spherical
Christmas pudding. We can also think of a
watermelon, the positive charge in the atom
is spread all over like the red edible part of
the watermelon, while the electrons are
studded in the positively charged sphere, like
the seeds in the watermelon (Fig. 4.1).
Thomson proposed that:
(i) An atom consists of a positively
charged sphere and the electrons are
embedded in it.
(ii) The negative and positive charges are
equal in magnitude. So, the atom as a
whole is electrically neutral.
Although Thomson’s model explained that
atoms are electrically neutral, the results of
experiments carried out by other scientists
could not be explained by this model, as we
will see below.
4.2.2 RUTHERFORD’S MODEL OF AN ATOM
Ernest Rutherford was interested in knowing
how the electrons are arranged within an
atom. Rutherford designed an experiment for
this. In this experiment, fast moving alpha
(a)-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.
• a-particles are doubly-charged helium
ions. Since they have a mass of 4 u, the
fast-moving a-particles have a
considerable amount of energy.
• It was expected that a-particles would be
deflected by the sub-atomic particles in
the gold atoms. Since the a-particles were
much heavier than the protons, he did
not expect to see large deflections.
Q
Fig.4.1: Thomson’s model of an atom
J.J. Thomson (1856-
1940), a British
physicist, was born in
Cheetham Hill, a suburb
of Manchester, on
18 December 1856. He
was awarded the Nobel
prize in Physics in 1906
for his work on the
discovery of electrons.
He directed the Cavendish Laboratory at
Cambridge for 35 years and seven of his
research assistants subsequently won
Nobel prizes.
STRUCTURE OF THE ATOM 47
Page 3


In Chapter 3, we have learnt that atoms and
molecules are the fundamental building
blocks of matter. The existence of different
kinds of matter is due to different atoms
constituting them. Now the questions arise:
(i) What makes the atom of one element
different from the atom of another element?
and (ii) Are atoms really indivisible, as
proposed by Dalton, or are there smaller
constituents inside the atom? We shall find
out the answers to these questions in this
chapter. We will learn about sub-atomic
particles and the various models that have
been proposed to explain how these particles
are arranged within the atom.
A major challenge before the scientists at
the end of the 19th century was to reveal the
structure of the atom as well as to explain its
important properties. The elucidation of the
structure of atoms is based on a series of
experiments.
One of the first indications that atoms are
not indivisible, comes from studying static
electricity and the condition under which
electricity is conducted by different
substances.
4.1 Charged Particles in Matter
For understanding the nature of charged
particles in matter, let us carry out the
following activities:
Activity ______________4.1
A. Comb dry hair. Does the comb then
attract small pieces of paper?
B. Rub a glass rod with a silk cloth and
bring the rod near an inflated balloon.
Observe what happens.
From these activities, can we conclude
that on rubbing two objects together, they
become electrically charged? Where does this
charge come from? This question can be
answered by knowing that an atom is divisible
and consists of charged particles.
Many scientists contributed in revealing
the presence of charged particles in an atom.
It was known by 1900 that the atom was
not a simple, indivisible particle but contained
at least one sub-atomic particle – the electron
identified by J.J. Thomson. Even before the
electron was identified, E. Goldstein in 1886
discovered the presence of new radiations  in
a gas discharge and called them canal rays.
These rays were positively charged radiations
which ultimately led to the discovery of
another sub-atomic particle. This sub-atomic
particle had a charge, equal in magnitude but
opposite in sign to that of the electron. Its
mass was approximately 2000 times as that
of the electron. It was given the name of
proton. In general, an electron is represented
as ‘e
–
’ and a proton as ‘p
+
’. 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.
It seemed highly likely that an atom was
composed of protons and electrons, mutually
balancing their charges. It also appeared that
the protons were in the interior of the atom,
for whereas electrons could easily be peeled
off but not protons. Now the big question was:
what sort of structure did  these particles of
the atom form? We will find the answer to
this question below.
4 4
4 4 4
S S S S STRUCTURE TRUCTURE TRUCTURE TRUCTURE TRUCTURE     OF OF OF OF OF     THE THE THE THE THE A A A A ATOM TOM TOM TOM TOM
Chapter
uestions
1. What are canal rays?
2. If an atom contains one electron
and one proton, will it carry any
charge or not?
4.2 The Structure of an Atom
We have learnt Dalton’s atomic theory in
Chapter 3, which suggested that the atom
was indivisible and indestructible. But the
discovery of two fundamental particles
(electrons and protons) inside the atom, led
to the failure of this aspect of Dalton’s atomic
theory. It was then considered necessary to
know how electrons and protons are arranged
within an atom. For explaining this, many
scientists proposed various atomic models.
J.J. Thomson was the first one to propose a
model for the structure of an atom.
4.2.1 THOMSON’S MODEL OF AN ATOM
Thomson proposed the model of an atom to
be similar to that of a Christmas pudding.
The electrons, in a sphere of positive charge,
were like currants (dry fruits) in a spherical
Christmas pudding. We can also think of a
watermelon, the positive charge in the atom
is spread all over like the red edible part of
the watermelon, while the electrons are
studded in the positively charged sphere, like
the seeds in the watermelon (Fig. 4.1).
Thomson proposed that:
(i) An atom consists of a positively
charged sphere and the electrons are
embedded in it.
(ii) The negative and positive charges are
equal in magnitude. So, the atom as a
whole is electrically neutral.
Although Thomson’s model explained that
atoms are electrically neutral, the results of
experiments carried out by other scientists
could not be explained by this model, as we
will see below.
4.2.2 RUTHERFORD’S MODEL OF AN ATOM
Ernest Rutherford was interested in knowing
how the electrons are arranged within an
atom. Rutherford designed an experiment for
this. In this experiment, fast moving alpha
(a)-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.
• a-particles are doubly-charged helium
ions. Since they have a mass of 4 u, the
fast-moving a-particles have a
considerable amount of energy.
• It was expected that a-particles would be
deflected by the sub-atomic particles in
the gold atoms. Since the a-particles were
much heavier than the protons, he did
not expect to see large deflections.
Q
Fig.4.1: Thomson’s model of an atom
J.J. Thomson (1856-
1940), a British
physicist, was born in
Cheetham Hill, a suburb
of Manchester, on
18 December 1856. He
was awarded the Nobel
prize in Physics in 1906
for his work on the
discovery of electrons.
He directed the Cavendish Laboratory at
Cambridge for 35 years and seven of his
research assistants subsequently won
Nobel prizes.
STRUCTURE OF THE ATOM 47 SCIENCE 48
Fig. 4.2: Scattering of a-particles by a gold foil
But, the a-particle scattering experiment
gave totally unexpected results (Fig. 4.2).  The
following observations were made:
(i) Most of the fast moving a-particles
passed straight through the gold foil.
(ii) Some of the a-particles were deflected
by the foil by small angles.
(iii) 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”.
hear a sound when each stone strikes the
wall. If he repeats this ten times, he will hear
the sound ten times. But if a blind-folded
child were to throw stones at a barbed-wire
fence, most of the stones would not hit the
fencing and no sound would be heard. This
is because there are lots of gaps in the fence
which allow the stone to pass through them.
Following a similar reasoning, Rutherford
concluded from the a-particle scattering
experiment that–
(i) Most of the space inside the atom is
empty because most of the a-particles
passed through the gold foil without
getting deflected.
(ii) Very few particles were deflected from
their path, indicating that the positive
charge of the atom occupies very little
space.
(iii) A very small fraction of a-particles
were deflected by 180
0
,
 
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 10
5 
times less
than the radius of the atom.
On the basis of his experiment,
Rutherford put forward the nuclear model of
an atom, which had the following features:
(i) There is a positively charged centre in
an atom called the nucleus. Nearly all
the mass of an atom resides in the
nucleus.
(ii) The electrons revolve around the
nucleus in well-defined orbits.
(iii) The size of the nucleus is very small
as compared to the size of the atom.
Drawbacks of Rutherford’s model of
the atom
The orbital revolution of the electron 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.
E. Rutherford (1871-1937)
was born at Spring Grove
on 30 August 1871. He was
known as the ‘Father’ of
nuclear physics.  He is
famous for his work on
radioactivity and the
discovery of the nucleus of an atom with
the gold foil experiment. He got the Nobel
prize in chemistry in 1908.
Let us think of an activity in an open field
to understand the implications  of this
experiment. Let a child stand in front of a
wall with his eyes closed. Let him throw
stones at the wall from a distance. He will
Page 4


In Chapter 3, we have learnt that atoms and
molecules are the fundamental building
blocks of matter. The existence of different
kinds of matter is due to different atoms
constituting them. Now the questions arise:
(i) What makes the atom of one element
different from the atom of another element?
and (ii) Are atoms really indivisible, as
proposed by Dalton, or are there smaller
constituents inside the atom? We shall find
out the answers to these questions in this
chapter. We will learn about sub-atomic
particles and the various models that have
been proposed to explain how these particles
are arranged within the atom.
A major challenge before the scientists at
the end of the 19th century was to reveal the
structure of the atom as well as to explain its
important properties. The elucidation of the
structure of atoms is based on a series of
experiments.
One of the first indications that atoms are
not indivisible, comes from studying static
electricity and the condition under which
electricity is conducted by different
substances.
4.1 Charged Particles in Matter
For understanding the nature of charged
particles in matter, let us carry out the
following activities:
Activity ______________4.1
A. Comb dry hair. Does the comb then
attract small pieces of paper?
B. Rub a glass rod with a silk cloth and
bring the rod near an inflated balloon.
Observe what happens.
From these activities, can we conclude
that on rubbing two objects together, they
become electrically charged? Where does this
charge come from? This question can be
answered by knowing that an atom is divisible
and consists of charged particles.
Many scientists contributed in revealing
the presence of charged particles in an atom.
It was known by 1900 that the atom was
not a simple, indivisible particle but contained
at least one sub-atomic particle – the electron
identified by J.J. Thomson. Even before the
electron was identified, E. Goldstein in 1886
discovered the presence of new radiations  in
a gas discharge and called them canal rays.
These rays were positively charged radiations
which ultimately led to the discovery of
another sub-atomic particle. This sub-atomic
particle had a charge, equal in magnitude but
opposite in sign to that of the electron. Its
mass was approximately 2000 times as that
of the electron. It was given the name of
proton. In general, an electron is represented
as ‘e
–
’ and a proton as ‘p
+
’. 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.
It seemed highly likely that an atom was
composed of protons and electrons, mutually
balancing their charges. It also appeared that
the protons were in the interior of the atom,
for whereas electrons could easily be peeled
off but not protons. Now the big question was:
what sort of structure did  these particles of
the atom form? We will find the answer to
this question below.
4 4
4 4 4
S S S S STRUCTURE TRUCTURE TRUCTURE TRUCTURE TRUCTURE     OF OF OF OF OF     THE THE THE THE THE A A A A ATOM TOM TOM TOM TOM
Chapter
uestions
1. What are canal rays?
2. If an atom contains one electron
and one proton, will it carry any
charge or not?
4.2 The Structure of an Atom
We have learnt Dalton’s atomic theory in
Chapter 3, which suggested that the atom
was indivisible and indestructible. But the
discovery of two fundamental particles
(electrons and protons) inside the atom, led
to the failure of this aspect of Dalton’s atomic
theory. It was then considered necessary to
know how electrons and protons are arranged
within an atom. For explaining this, many
scientists proposed various atomic models.
J.J. Thomson was the first one to propose a
model for the structure of an atom.
4.2.1 THOMSON’S MODEL OF AN ATOM
Thomson proposed the model of an atom to
be similar to that of a Christmas pudding.
The electrons, in a sphere of positive charge,
were like currants (dry fruits) in a spherical
Christmas pudding. We can also think of a
watermelon, the positive charge in the atom
is spread all over like the red edible part of
the watermelon, while the electrons are
studded in the positively charged sphere, like
the seeds in the watermelon (Fig. 4.1).
Thomson proposed that:
(i) An atom consists of a positively
charged sphere and the electrons are
embedded in it.
(ii) The negative and positive charges are
equal in magnitude. So, the atom as a
whole is electrically neutral.
Although Thomson’s model explained that
atoms are electrically neutral, the results of
experiments carried out by other scientists
could not be explained by this model, as we
will see below.
4.2.2 RUTHERFORD’S MODEL OF AN ATOM
Ernest Rutherford was interested in knowing
how the electrons are arranged within an
atom. Rutherford designed an experiment for
this. In this experiment, fast moving alpha
(a)-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.
• a-particles are doubly-charged helium
ions. Since they have a mass of 4 u, the
fast-moving a-particles have a
considerable amount of energy.
• It was expected that a-particles would be
deflected by the sub-atomic particles in
the gold atoms. Since the a-particles were
much heavier than the protons, he did
not expect to see large deflections.
Q
Fig.4.1: Thomson’s model of an atom
J.J. Thomson (1856-
1940), a British
physicist, was born in
Cheetham Hill, a suburb
of Manchester, on
18 December 1856. He
was awarded the Nobel
prize in Physics in 1906
for his work on the
discovery of electrons.
He directed the Cavendish Laboratory at
Cambridge for 35 years and seven of his
research assistants subsequently won
Nobel prizes.
STRUCTURE OF THE ATOM 47 SCIENCE 48
Fig. 4.2: Scattering of a-particles by a gold foil
But, the a-particle scattering experiment
gave totally unexpected results (Fig. 4.2).  The
following observations were made:
(i) Most of the fast moving a-particles
passed straight through the gold foil.
(ii) Some of the a-particles were deflected
by the foil by small angles.
(iii) 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”.
hear a sound when each stone strikes the
wall. If he repeats this ten times, he will hear
the sound ten times. But if a blind-folded
child were to throw stones at a barbed-wire
fence, most of the stones would not hit the
fencing and no sound would be heard. This
is because there are lots of gaps in the fence
which allow the stone to pass through them.
Following a similar reasoning, Rutherford
concluded from the a-particle scattering
experiment that–
(i) Most of the space inside the atom is
empty because most of the a-particles
passed through the gold foil without
getting deflected.
(ii) Very few particles were deflected from
their path, indicating that the positive
charge of the atom occupies very little
space.
(iii) A very small fraction of a-particles
were deflected by 180
0
,
 
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 10
5 
times less
than the radius of the atom.
On the basis of his experiment,
Rutherford put forward the nuclear model of
an atom, which had the following features:
(i) There is a positively charged centre in
an atom called the nucleus. Nearly all
the mass of an atom resides in the
nucleus.
(ii) The electrons revolve around the
nucleus in well-defined orbits.
(iii) The size of the nucleus is very small
as compared to the size of the atom.
Drawbacks of Rutherford’s model of
the atom
The orbital revolution of the electron 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.
E. Rutherford (1871-1937)
was born at Spring Grove
on 30 August 1871. He was
known as the ‘Father’ of
nuclear physics.  He is
famous for his work on
radioactivity and the
discovery of the nucleus of an atom with
the gold foil experiment. He got the Nobel
prize in chemistry in 1908.
Let us think of an activity in an open field
to understand the implications  of this
experiment. Let a child stand in front of a
wall with his eyes closed. Let him throw
stones at the wall from a distance. He will
STRUCTURE OF THE ATOM 49
4.2.3 BOHR’S MODEL OF ATOM
In order to overcome the objections raised
against Rutherford’s model of the atom,
Neils Bohr put forward the following
postulates about the 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.
uestions
1. On the basis of Thomson’s model
of an atom, explain how the atom
is neutral as a whole.
2. On the basis of Rutherford’s
model of an atom, which sub-
atomic particle is present in the
nucleus of an atom?
3. Draw a sketch of Bohr’s model
of an atom with three shells.
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?
4.2.4 NEUTRONS
In 1932, J. Chadwick discovered another sub-
atomic particle which had no charge and a
mass nearly equal to that of a proton. It was
eventually named as neutron. Neutrons are
present in the nucleus of all atoms, except
hydrogen. In general, a neutron is
represented as ‘n’. The mass of an atom is
therefore given by the sum of the masses of
protons and neutrons present in the nucleus.
uestions
1. Name the three sub-atomic
particles of an atom.
2. Helium atom has an atomic mass
of 4 u and two protons in its
nucleus. How many neutrons
does it have?
4.3 How are Electrons Distributed
in Different Orbits (Shells)?
The distribution of electrons into different
orbits of an atom was suggested by Bohr and
Bury.
The following rules are 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
Neils Bohr (1885-1962)
was born in Copenhagen
on 7 October 1885. He was
appointed professor of
physics at Copenhagen
University in 1916. He got
the Nobel prize for his work
on the structure of atom in
1922. Among Professor
Bohr’s numerous writings, three appearing
as books are:
(i) The Theory of Spectra and Atomic
Constitution, (ii) Atomic Theory and,
(iii) The Description of Nature.
These orbits or shells are called energy
levels. Energy levels in an atom are shown in
Fig. 4.3.
Q
Fig. 4.3: A few energy levels in an atom
These orbits or shells are represented by
the letters K,L,M,N,… or the numbers,
n=1,2,3,4,….
Q
Page 5


In Chapter 3, we have learnt that atoms and
molecules are the fundamental building
blocks of matter. The existence of different
kinds of matter is due to different atoms
constituting them. Now the questions arise:
(i) What makes the atom of one element
different from the atom of another element?
and (ii) Are atoms really indivisible, as
proposed by Dalton, or are there smaller
constituents inside the atom? We shall find
out the answers to these questions in this
chapter. We will learn about sub-atomic
particles and the various models that have
been proposed to explain how these particles
are arranged within the atom.
A major challenge before the scientists at
the end of the 19th century was to reveal the
structure of the atom as well as to explain its
important properties. The elucidation of the
structure of atoms is based on a series of
experiments.
One of the first indications that atoms are
not indivisible, comes from studying static
electricity and the condition under which
electricity is conducted by different
substances.
4.1 Charged Particles in Matter
For understanding the nature of charged
particles in matter, let us carry out the
following activities:
Activity ______________4.1
A. Comb dry hair. Does the comb then
attract small pieces of paper?
B. Rub a glass rod with a silk cloth and
bring the rod near an inflated balloon.
Observe what happens.
From these activities, can we conclude
that on rubbing two objects together, they
become electrically charged? Where does this
charge come from? This question can be
answered by knowing that an atom is divisible
and consists of charged particles.
Many scientists contributed in revealing
the presence of charged particles in an atom.
It was known by 1900 that the atom was
not a simple, indivisible particle but contained
at least one sub-atomic particle – the electron
identified by J.J. Thomson. Even before the
electron was identified, E. Goldstein in 1886
discovered the presence of new radiations  in
a gas discharge and called them canal rays.
These rays were positively charged radiations
which ultimately led to the discovery of
another sub-atomic particle. This sub-atomic
particle had a charge, equal in magnitude but
opposite in sign to that of the electron. Its
mass was approximately 2000 times as that
of the electron. It was given the name of
proton. In general, an electron is represented
as ‘e
–
’ and a proton as ‘p
+
’. 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.
It seemed highly likely that an atom was
composed of protons and electrons, mutually
balancing their charges. It also appeared that
the protons were in the interior of the atom,
for whereas electrons could easily be peeled
off but not protons. Now the big question was:
what sort of structure did  these particles of
the atom form? We will find the answer to
this question below.
4 4
4 4 4
S S S S STRUCTURE TRUCTURE TRUCTURE TRUCTURE TRUCTURE     OF OF OF OF OF     THE THE THE THE THE A A A A ATOM TOM TOM TOM TOM
Chapter
uestions
1. What are canal rays?
2. If an atom contains one electron
and one proton, will it carry any
charge or not?
4.2 The Structure of an Atom
We have learnt Dalton’s atomic theory in
Chapter 3, which suggested that the atom
was indivisible and indestructible. But the
discovery of two fundamental particles
(electrons and protons) inside the atom, led
to the failure of this aspect of Dalton’s atomic
theory. It was then considered necessary to
know how electrons and protons are arranged
within an atom. For explaining this, many
scientists proposed various atomic models.
J.J. Thomson was the first one to propose a
model for the structure of an atom.
4.2.1 THOMSON’S MODEL OF AN ATOM
Thomson proposed the model of an atom to
be similar to that of a Christmas pudding.
The electrons, in a sphere of positive charge,
were like currants (dry fruits) in a spherical
Christmas pudding. We can also think of a
watermelon, the positive charge in the atom
is spread all over like the red edible part of
the watermelon, while the electrons are
studded in the positively charged sphere, like
the seeds in the watermelon (Fig. 4.1).
Thomson proposed that:
(i) An atom consists of a positively
charged sphere and the electrons are
embedded in it.
(ii) The negative and positive charges are
equal in magnitude. So, the atom as a
whole is electrically neutral.
Although Thomson’s model explained that
atoms are electrically neutral, the results of
experiments carried out by other scientists
could not be explained by this model, as we
will see below.
4.2.2 RUTHERFORD’S MODEL OF AN ATOM
Ernest Rutherford was interested in knowing
how the electrons are arranged within an
atom. Rutherford designed an experiment for
this. In this experiment, fast moving alpha
(a)-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.
• a-particles are doubly-charged helium
ions. Since they have a mass of 4 u, the
fast-moving a-particles have a
considerable amount of energy.
• It was expected that a-particles would be
deflected by the sub-atomic particles in
the gold atoms. Since the a-particles were
much heavier than the protons, he did
not expect to see large deflections.
Q
Fig.4.1: Thomson’s model of an atom
J.J. Thomson (1856-
1940), a British
physicist, was born in
Cheetham Hill, a suburb
of Manchester, on
18 December 1856. He
was awarded the Nobel
prize in Physics in 1906
for his work on the
discovery of electrons.
He directed the Cavendish Laboratory at
Cambridge for 35 years and seven of his
research assistants subsequently won
Nobel prizes.
STRUCTURE OF THE ATOM 47 SCIENCE 48
Fig. 4.2: Scattering of a-particles by a gold foil
But, the a-particle scattering experiment
gave totally unexpected results (Fig. 4.2).  The
following observations were made:
(i) Most of the fast moving a-particles
passed straight through the gold foil.
(ii) Some of the a-particles were deflected
by the foil by small angles.
(iii) 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”.
hear a sound when each stone strikes the
wall. If he repeats this ten times, he will hear
the sound ten times. But if a blind-folded
child were to throw stones at a barbed-wire
fence, most of the stones would not hit the
fencing and no sound would be heard. This
is because there are lots of gaps in the fence
which allow the stone to pass through them.
Following a similar reasoning, Rutherford
concluded from the a-particle scattering
experiment that–
(i) Most of the space inside the atom is
empty because most of the a-particles
passed through the gold foil without
getting deflected.
(ii) Very few particles were deflected from
their path, indicating that the positive
charge of the atom occupies very little
space.
(iii) A very small fraction of a-particles
were deflected by 180
0
,
 
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 10
5 
times less
than the radius of the atom.
On the basis of his experiment,
Rutherford put forward the nuclear model of
an atom, which had the following features:
(i) There is a positively charged centre in
an atom called the nucleus. Nearly all
the mass of an atom resides in the
nucleus.
(ii) The electrons revolve around the
nucleus in well-defined orbits.
(iii) The size of the nucleus is very small
as compared to the size of the atom.
Drawbacks of Rutherford’s model of
the atom
The orbital revolution of the electron 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.
E. Rutherford (1871-1937)
was born at Spring Grove
on 30 August 1871. He was
known as the ‘Father’ of
nuclear physics.  He is
famous for his work on
radioactivity and the
discovery of the nucleus of an atom with
the gold foil experiment. He got the Nobel
prize in chemistry in 1908.
Let us think of an activity in an open field
to understand the implications  of this
experiment. Let a child stand in front of a
wall with his eyes closed. Let him throw
stones at the wall from a distance. He will
STRUCTURE OF THE ATOM 49
4.2.3 BOHR’S MODEL OF ATOM
In order to overcome the objections raised
against Rutherford’s model of the atom,
Neils Bohr put forward the following
postulates about the 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.
uestions
1. On the basis of Thomson’s model
of an atom, explain how the atom
is neutral as a whole.
2. On the basis of Rutherford’s
model of an atom, which sub-
atomic particle is present in the
nucleus of an atom?
3. Draw a sketch of Bohr’s model
of an atom with three shells.
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?
4.2.4 NEUTRONS
In 1932, J. Chadwick discovered another sub-
atomic particle which had no charge and a
mass nearly equal to that of a proton. It was
eventually named as neutron. Neutrons are
present in the nucleus of all atoms, except
hydrogen. In general, a neutron is
represented as ‘n’. The mass of an atom is
therefore given by the sum of the masses of
protons and neutrons present in the nucleus.
uestions
1. Name the three sub-atomic
particles of an atom.
2. Helium atom has an atomic mass
of 4 u and two protons in its
nucleus. How many neutrons
does it have?
4.3 How are Electrons Distributed
in Different Orbits (Shells)?
The distribution of electrons into different
orbits of an atom was suggested by Bohr and
Bury.
The following rules are 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
Neils Bohr (1885-1962)
was born in Copenhagen
on 7 October 1885. He was
appointed professor of
physics at Copenhagen
University in 1916. He got
the Nobel prize for his work
on the structure of atom in
1922. Among Professor
Bohr’s numerous writings, three appearing
as books are:
(i) The Theory of Spectra and Atomic
Constitution, (ii) Atomic Theory and,
(iii) The Description of Nature.
These orbits or shells are called energy
levels. Energy levels in an atom are shown in
Fig. 4.3.
Q
Fig. 4.3: A few energy levels in an atom
These orbits or shells are represented by
the letters K,L,M,N,… or the numbers,
n=1,2,3,4,….
Q
SCIENCE 50
formula 2n
2
,
 
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 × 1
2
 = 2,
second orbit or L-shell will be = 2 × 2
2
= 8, third orbit or M-shell will be = 2 ×
3
2 
= 18, fourth orbit or N-shell will be
= 2 × 4
2
= 32, and so on.
(ii) The maximum number of electrons
that can be accommodated in the
outermost orbit is 8.
(iii) 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.
Atomic structure of the first eighteen
elements is shown schematically in Fig. 4.4.
• The composition of atoms of the first
eighteen elements is given in Table 4.1.
uestions
1. Write the distribution of electrons
in  carbon and sodium atoms.
2. If K and L shells of an atom are
full, then what would be the total
number of electrons in the atom?
4.4 Valency
We have learnt how the electrons in an atom
are arranged in different shells/orbits. The
electrons present in the outermost shell of
an atom are known as the valence electrons.
From the Bohr-Bury scheme, we also
know that the outermost shell of an atom can
Activity ______________4.2
• Make a static atomic model displaying
electronic configuration of the first
eighteen elements.
Fig.4.4: Schematic atomic structure of the first eighteen elements
Q
accommodate a maximum of 8 electrons. It
was observed that the atoms of elements,
having a completely filled outermost shell
show little chemical activity. In other words,
their combining capacity or valency is zero.
Of these inert elements, the helium atom has