NCERT Textbook: Structure of Atom | NCERT Textbooks (Class 6 to Class 12) - CTET & State TET PDF Download

 Page 1


26 CHEMISTRY
The rich diversity of chemical behaviour of different elements
can be traced to the differences in the internal structure of
atoms of these elements.
UNIT 2
STRUCTURE OF ATOM
After studying this unit you will be
able to
• • • • • know about the discovery of
electron, proton and neutron and
their characteristics;
• • • • • describe Thomson, Rutherford
and Bohr atomic models;
• • • • • understand the important
features of the quantum
mechanical model of atom;
• • • • •understand nature of
electromagnetic radiation and
Planck’s quantum theory;
• • • • • explain the photoelectric effect
and describe features of atomic
spectra;
• • • • • state the de Broglie relation and
Heisenberg uncertainty principle;
• • • • • define an atomic orbital in terms
of quantum numbers;
• • • • • state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity;
• • • • • write the electronic configurations
of atoms.
The existence of atoms has been proposed since the time
of early Indian and Greek philosophers (400 B.C.) who
were of the view that atoms are the fundamental building
blocks of matter. According to them, the continued
subdivisions of matter would ultimately yield atoms which
would not be further divisible. The word ‘atom’  has been
derived from the Greek word ‘a-tomio’ which means ‘uncut-
able’ or ‘non-divisible’. These earlier ideas were mere
speculations and there was no way to test them
experimentally. These ideas remained dormant  for a very
long time and were revived again by scientists in the
nineteenth century.
The atomic theory of matter was first proposed on a
firm scientific basis by John Dalton, a British school
teacher in 1808. His theory, called Dalton’s atomic
theory, regarded the atom as the ultimate particle of
matter (Unit 1).
In this unit we start with the experimental
observations made by scientists towards the end of
nineteenth and beginning of twentieth century. These
established that atoms can be further divided into sub-
atomic particles, i.e., electrons,  protons and neutrons—
a concept very different from that of Dalton. The major
problems before the scientists at that time were:
• • • • • to account for the stability of atom after the discovery
of sub-atomic particles,
• • • • • to compare the behaviour of one element from other
in terms of both physical and chemical properties,
2015-16
Page 2


26 CHEMISTRY
The rich diversity of chemical behaviour of different elements
can be traced to the differences in the internal structure of
atoms of these elements.
UNIT 2
STRUCTURE OF ATOM
After studying this unit you will be
able to
• • • • • know about the discovery of
electron, proton and neutron and
their characteristics;
• • • • • describe Thomson, Rutherford
and Bohr atomic models;
• • • • • understand the important
features of the quantum
mechanical model of atom;
• • • • •understand nature of
electromagnetic radiation and
Planck’s quantum theory;
• • • • • explain the photoelectric effect
and describe features of atomic
spectra;
• • • • • state the de Broglie relation and
Heisenberg uncertainty principle;
• • • • • define an atomic orbital in terms
of quantum numbers;
• • • • • state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity;
• • • • • write the electronic configurations
of atoms.
The existence of atoms has been proposed since the time
of early Indian and Greek philosophers (400 B.C.) who
were of the view that atoms are the fundamental building
blocks of matter. According to them, the continued
subdivisions of matter would ultimately yield atoms which
would not be further divisible. The word ‘atom’  has been
derived from the Greek word ‘a-tomio’ which means ‘uncut-
able’ or ‘non-divisible’. These earlier ideas were mere
speculations and there was no way to test them
experimentally. These ideas remained dormant  for a very
long time and were revived again by scientists in the
nineteenth century.
The atomic theory of matter was first proposed on a
firm scientific basis by John Dalton, a British school
teacher in 1808. His theory, called Dalton’s atomic
theory, regarded the atom as the ultimate particle of
matter (Unit 1).
In this unit we start with the experimental
observations made by scientists towards the end of
nineteenth and beginning of twentieth century. These
established that atoms can be further divided into sub-
atomic particles, i.e., electrons,  protons and neutrons—
a concept very different from that of Dalton. The major
problems before the scientists at that time were:
• • • • • to account for the stability of atom after the discovery
of sub-atomic particles,
• • • • • to compare the behaviour of one element from other
in terms of both physical and chemical properties,
2015-16
27 STRUCTURE OF ATOM
• • • • • to explain the formation of different kinds
of molecules by the combination of
different atoms and,
• • • • • to understand the origin and nature of the
characteristics of electromagnetic
radiation absorbed or emitted by atoms.
2.1 SUB-ATOMIC PARTICLES
Dalton’s atomic theory was able to explain
the law of conservation of mass, law of
constant composition and law of multiple
proportion very successfully. However, it failed
to explain the results of many experiments,
for example, it was known that substances
like glass or ebonite when rubbed with silk or
fur generate electricity. Many different kinds
of sub-atomic particles were discovered in the
twentieth century. However, in this section
we will talk about only two particles, namely
electron and proton.
2.1.1 Discovery of Electron
In 1830, Michael Faraday showed that if
electricity is passed through a solution of an
electrolyte, chemical reactions occurred at the
electrodes, which resulted in the liberation
and deposition of matter at the electrodes. He
formulated certain laws which you will study
in class XII. These results suggested the
particulate nature of electricity.
An insight into the structure of atom was
obtained from the experiments on electrical
discharge through gases. Before we discuss
these results we need to keep in mind a basic
rule regarding the behaviour of charged
particles : “Like charges repel each other and
unlike charges attract each other”.
In mid 1850s many scientists mainly
Faraday began to study electrical discharge
in partially evacuated tubes, known as
cathode ray discharge tubes. It is depicted
in Fig. 2.1. A cathode ray tube is made of glass
containing two thin pieces of metal, called
electrodes, sealed in it. The electrical
discharge through the gases could be
observed only at very low pressures and at
very high voltages. The pressure of different
gases could be adjusted by evacuation. When
sufficiently high voltage is applied across the
electrodes, current starts flowing through a
stream of particles moving in the tube from
the negative electrode (cathode) to the positive
electrode (anode). These were called cathode
rays or cathode ray particles. The flow of
current from cathode to anode was further
checked by making a hole in the anode and
coating the tube behind anode with
phosphorescent material zinc sulphide. When
these rays, after passing through anode, strike
the zinc sulphide coating, a bright spot on
the coating is developed(same thing happens
in a television set) [Fig. 2.1(b)].
Fig. 2.1(a)  A cathode ray discharge tube
Fig. 2.1(b) A cathode ray discharge tube with
perforated anode
The results of these experiments are
summarised below.
(i) The cathode rays start from cathode and
move towards the anode.
(ii) These rays themselves are not visible but
their behaviour can be observed with the
help of certain kind of materials
(fluorescent or phosphorescent) which
glow when hit by them. Television
picture tubes are cathode ray tubes and
television pictures result due to
fluorescence on the television screen
coated with certain fluorescent or
phosphorescent materials.
2015-16
Page 3


26 CHEMISTRY
The rich diversity of chemical behaviour of different elements
can be traced to the differences in the internal structure of
atoms of these elements.
UNIT 2
STRUCTURE OF ATOM
After studying this unit you will be
able to
• • • • • know about the discovery of
electron, proton and neutron and
their characteristics;
• • • • • describe Thomson, Rutherford
and Bohr atomic models;
• • • • • understand the important
features of the quantum
mechanical model of atom;
• • • • •understand nature of
electromagnetic radiation and
Planck’s quantum theory;
• • • • • explain the photoelectric effect
and describe features of atomic
spectra;
• • • • • state the de Broglie relation and
Heisenberg uncertainty principle;
• • • • • define an atomic orbital in terms
of quantum numbers;
• • • • • state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity;
• • • • • write the electronic configurations
of atoms.
The existence of atoms has been proposed since the time
of early Indian and Greek philosophers (400 B.C.) who
were of the view that atoms are the fundamental building
blocks of matter. According to them, the continued
subdivisions of matter would ultimately yield atoms which
would not be further divisible. The word ‘atom’  has been
derived from the Greek word ‘a-tomio’ which means ‘uncut-
able’ or ‘non-divisible’. These earlier ideas were mere
speculations and there was no way to test them
experimentally. These ideas remained dormant  for a very
long time and were revived again by scientists in the
nineteenth century.
The atomic theory of matter was first proposed on a
firm scientific basis by John Dalton, a British school
teacher in 1808. His theory, called Dalton’s atomic
theory, regarded the atom as the ultimate particle of
matter (Unit 1).
In this unit we start with the experimental
observations made by scientists towards the end of
nineteenth and beginning of twentieth century. These
established that atoms can be further divided into sub-
atomic particles, i.e., electrons,  protons and neutrons—
a concept very different from that of Dalton. The major
problems before the scientists at that time were:
• • • • • to account for the stability of atom after the discovery
of sub-atomic particles,
• • • • • to compare the behaviour of one element from other
in terms of both physical and chemical properties,
2015-16
27 STRUCTURE OF ATOM
• • • • • to explain the formation of different kinds
of molecules by the combination of
different atoms and,
• • • • • to understand the origin and nature of the
characteristics of electromagnetic
radiation absorbed or emitted by atoms.
2.1 SUB-ATOMIC PARTICLES
Dalton’s atomic theory was able to explain
the law of conservation of mass, law of
constant composition and law of multiple
proportion very successfully. However, it failed
to explain the results of many experiments,
for example, it was known that substances
like glass or ebonite when rubbed with silk or
fur generate electricity. Many different kinds
of sub-atomic particles were discovered in the
twentieth century. However, in this section
we will talk about only two particles, namely
electron and proton.
2.1.1 Discovery of Electron
In 1830, Michael Faraday showed that if
electricity is passed through a solution of an
electrolyte, chemical reactions occurred at the
electrodes, which resulted in the liberation
and deposition of matter at the electrodes. He
formulated certain laws which you will study
in class XII. These results suggested the
particulate nature of electricity.
An insight into the structure of atom was
obtained from the experiments on electrical
discharge through gases. Before we discuss
these results we need to keep in mind a basic
rule regarding the behaviour of charged
particles : “Like charges repel each other and
unlike charges attract each other”.
In mid 1850s many scientists mainly
Faraday began to study electrical discharge
in partially evacuated tubes, known as
cathode ray discharge tubes. It is depicted
in Fig. 2.1. A cathode ray tube is made of glass
containing two thin pieces of metal, called
electrodes, sealed in it. The electrical
discharge through the gases could be
observed only at very low pressures and at
very high voltages. The pressure of different
gases could be adjusted by evacuation. When
sufficiently high voltage is applied across the
electrodes, current starts flowing through a
stream of particles moving in the tube from
the negative electrode (cathode) to the positive
electrode (anode). These were called cathode
rays or cathode ray particles. The flow of
current from cathode to anode was further
checked by making a hole in the anode and
coating the tube behind anode with
phosphorescent material zinc sulphide. When
these rays, after passing through anode, strike
the zinc sulphide coating, a bright spot on
the coating is developed(same thing happens
in a television set) [Fig. 2.1(b)].
Fig. 2.1(a)  A cathode ray discharge tube
Fig. 2.1(b) A cathode ray discharge tube with
perforated anode
The results of these experiments are
summarised below.
(i) The cathode rays start from cathode and
move towards the anode.
(ii) These rays themselves are not visible but
their behaviour can be observed with the
help of certain kind of materials
(fluorescent or phosphorescent) which
glow when hit by them. Television
picture tubes are cathode ray tubes and
television pictures result due to
fluorescence on the television screen
coated with certain fluorescent or
phosphorescent materials.
2015-16
28 CHEMISTRY
(iii) In the absence of electrical or magnetic
field, these rays travel in straight lines
(Fig. 2.2).
(iv) In the presence of electrical or magnetic
field, the behaviour of cathode rays are
similar to that expected from negatively
charged particles, suggesting that the
cathode rays consist of negatively
charged particles, called electrons.
(v) The characteristics of cathode rays
(electrons) do not depend upon the
material of electrodes and the nature of
the gas present in the cathode ray tube.
Thus, we can conclude that electrons are
basic constituent of all the atoms.
2.1.2 Charge to Mass Ratio of Electron
In 1897, British physicist J.J. Thomson
measured the ratio of electrical charge (e) to
the mass  of electron (m
e 
) by using cathode
ray tube and applying electrical and magnetic
field perpendicular to each other as well as to
the path of electrons (Fig. 2.2). Thomson
argued that the amount of deviation of the
particles from their path in the presence of
electrical or magnetic field depends upon:
(i) the magnitude of the negative charge on
the particle, greater the magnitude of the
charge on the particle, greater is the
interaction with the electric or magnetic
field and thus greater is the deflection.
Fig. 2.2   The apparatus to determine the charge to the mass ratio of electron
(ii) the mass of the particle — lighter the
particle, greater the deflection.
(iii) the strength of the electrical or magnetic
field — the deflection of electrons from
its original path increases with the
increase in the voltage across the
electrodes, or the strength of the
magnetic field.
When only electric field is applied, the
electrons deviate from their path and hit the
cathode ray tube at point A. Similarly when
only magnetic field is applied, electron strikes
the cathode ray tube at point C. By carefully
balancing the electrical and magnetic field
strength, it is possible to bring back the
electron to the path followed as in the absence
of electric or magnetic field and they hit the
screen at point B. By carrying out accurate
measurements on the amount of deflections
observed by the electrons on the electric field
strength or magnetic field strength, Thomson
was able to determine the value of e/m
e
 as:
e
e
m
 = 1.758820 × 10
11
 C kg
–1
(2.1)
Where m
e
 is the mass of the electron in kg
and e is the magnitude of the charge on the
electron in coulomb (C). Since electrons
are negatively charged, the charge on electron
is –e.
C:\Chemistry XI\Unit-2\Unit-2(2)-Lay-3(reprint).pmd     27.7.6, 16.10.6  (Reprint)
2015-16
Page 4


26 CHEMISTRY
The rich diversity of chemical behaviour of different elements
can be traced to the differences in the internal structure of
atoms of these elements.
UNIT 2
STRUCTURE OF ATOM
After studying this unit you will be
able to
• • • • • know about the discovery of
electron, proton and neutron and
their characteristics;
• • • • • describe Thomson, Rutherford
and Bohr atomic models;
• • • • • understand the important
features of the quantum
mechanical model of atom;
• • • • •understand nature of
electromagnetic radiation and
Planck’s quantum theory;
• • • • • explain the photoelectric effect
and describe features of atomic
spectra;
• • • • • state the de Broglie relation and
Heisenberg uncertainty principle;
• • • • • define an atomic orbital in terms
of quantum numbers;
• • • • • state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity;
• • • • • write the electronic configurations
of atoms.
The existence of atoms has been proposed since the time
of early Indian and Greek philosophers (400 B.C.) who
were of the view that atoms are the fundamental building
blocks of matter. According to them, the continued
subdivisions of matter would ultimately yield atoms which
would not be further divisible. The word ‘atom’  has been
derived from the Greek word ‘a-tomio’ which means ‘uncut-
able’ or ‘non-divisible’. These earlier ideas were mere
speculations and there was no way to test them
experimentally. These ideas remained dormant  for a very
long time and were revived again by scientists in the
nineteenth century.
The atomic theory of matter was first proposed on a
firm scientific basis by John Dalton, a British school
teacher in 1808. His theory, called Dalton’s atomic
theory, regarded the atom as the ultimate particle of
matter (Unit 1).
In this unit we start with the experimental
observations made by scientists towards the end of
nineteenth and beginning of twentieth century. These
established that atoms can be further divided into sub-
atomic particles, i.e., electrons,  protons and neutrons—
a concept very different from that of Dalton. The major
problems before the scientists at that time were:
• • • • • to account for the stability of atom after the discovery
of sub-atomic particles,
• • • • • to compare the behaviour of one element from other
in terms of both physical and chemical properties,
2015-16
27 STRUCTURE OF ATOM
• • • • • to explain the formation of different kinds
of molecules by the combination of
different atoms and,
• • • • • to understand the origin and nature of the
characteristics of electromagnetic
radiation absorbed or emitted by atoms.
2.1 SUB-ATOMIC PARTICLES
Dalton’s atomic theory was able to explain
the law of conservation of mass, law of
constant composition and law of multiple
proportion very successfully. However, it failed
to explain the results of many experiments,
for example, it was known that substances
like glass or ebonite when rubbed with silk or
fur generate electricity. Many different kinds
of sub-atomic particles were discovered in the
twentieth century. However, in this section
we will talk about only two particles, namely
electron and proton.
2.1.1 Discovery of Electron
In 1830, Michael Faraday showed that if
electricity is passed through a solution of an
electrolyte, chemical reactions occurred at the
electrodes, which resulted in the liberation
and deposition of matter at the electrodes. He
formulated certain laws which you will study
in class XII. These results suggested the
particulate nature of electricity.
An insight into the structure of atom was
obtained from the experiments on electrical
discharge through gases. Before we discuss
these results we need to keep in mind a basic
rule regarding the behaviour of charged
particles : “Like charges repel each other and
unlike charges attract each other”.
In mid 1850s many scientists mainly
Faraday began to study electrical discharge
in partially evacuated tubes, known as
cathode ray discharge tubes. It is depicted
in Fig. 2.1. A cathode ray tube is made of glass
containing two thin pieces of metal, called
electrodes, sealed in it. The electrical
discharge through the gases could be
observed only at very low pressures and at
very high voltages. The pressure of different
gases could be adjusted by evacuation. When
sufficiently high voltage is applied across the
electrodes, current starts flowing through a
stream of particles moving in the tube from
the negative electrode (cathode) to the positive
electrode (anode). These were called cathode
rays or cathode ray particles. The flow of
current from cathode to anode was further
checked by making a hole in the anode and
coating the tube behind anode with
phosphorescent material zinc sulphide. When
these rays, after passing through anode, strike
the zinc sulphide coating, a bright spot on
the coating is developed(same thing happens
in a television set) [Fig. 2.1(b)].
Fig. 2.1(a)  A cathode ray discharge tube
Fig. 2.1(b) A cathode ray discharge tube with
perforated anode
The results of these experiments are
summarised below.
(i) The cathode rays start from cathode and
move towards the anode.
(ii) These rays themselves are not visible but
their behaviour can be observed with the
help of certain kind of materials
(fluorescent or phosphorescent) which
glow when hit by them. Television
picture tubes are cathode ray tubes and
television pictures result due to
fluorescence on the television screen
coated with certain fluorescent or
phosphorescent materials.
2015-16
28 CHEMISTRY
(iii) In the absence of electrical or magnetic
field, these rays travel in straight lines
(Fig. 2.2).
(iv) In the presence of electrical or magnetic
field, the behaviour of cathode rays are
similar to that expected from negatively
charged particles, suggesting that the
cathode rays consist of negatively
charged particles, called electrons.
(v) The characteristics of cathode rays
(electrons) do not depend upon the
material of electrodes and the nature of
the gas present in the cathode ray tube.
Thus, we can conclude that electrons are
basic constituent of all the atoms.
2.1.2 Charge to Mass Ratio of Electron
In 1897, British physicist J.J. Thomson
measured the ratio of electrical charge (e) to
the mass  of electron (m
e 
) by using cathode
ray tube and applying electrical and magnetic
field perpendicular to each other as well as to
the path of electrons (Fig. 2.2). Thomson
argued that the amount of deviation of the
particles from their path in the presence of
electrical or magnetic field depends upon:
(i) the magnitude of the negative charge on
the particle, greater the magnitude of the
charge on the particle, greater is the
interaction with the electric or magnetic
field and thus greater is the deflection.
Fig. 2.2   The apparatus to determine the charge to the mass ratio of electron
(ii) the mass of the particle — lighter the
particle, greater the deflection.
(iii) the strength of the electrical or magnetic
field — the deflection of electrons from
its original path increases with the
increase in the voltage across the
electrodes, or the strength of the
magnetic field.
When only electric field is applied, the
electrons deviate from their path and hit the
cathode ray tube at point A. Similarly when
only magnetic field is applied, electron strikes
the cathode ray tube at point C. By carefully
balancing the electrical and magnetic field
strength, it is possible to bring back the
electron to the path followed as in the absence
of electric or magnetic field and they hit the
screen at point B. By carrying out accurate
measurements on the amount of deflections
observed by the electrons on the electric field
strength or magnetic field strength, Thomson
was able to determine the value of e/m
e
 as:
e
e
m
 = 1.758820 × 10
11
 C kg
–1
(2.1)
Where m
e
 is the mass of the electron in kg
and e is the magnitude of the charge on the
electron in coulomb (C). Since electrons
are negatively charged, the charge on electron
is –e.
C:\Chemistry XI\Unit-2\Unit-2(2)-Lay-3(reprint).pmd     27.7.6, 16.10.6  (Reprint)
2015-16
29 STRUCTURE OF ATOM
2.1.3 Charge on the Electron
R.A. Millikan (1868-1953) devised a method
known as oil drop experiment (1906-14), to
determine the charge on the electrons. He
found that the charge on the electron to be
– 1.6 × 10
–19
 C. The present accepted value of
electrical charge is – 1.6022 × 10
–19 
C. The
mass of the  electron (m
e
) was determined by
combining these results with Thomson’s value
of e/m
e
 ratio.
–19
e 11 –1
e
1.6022 × 10 C
= =
/ 1.758820 × 10 C kg
e
m
e m
     = 9.1094×10
–31 
kg                             (2.2)
2.1.4 Discovery of Protons and Neutrons
Electrical discharge carried out in the
modified cathode ray tube led to the discovery
of particles carrying positive charge, also
known as canal rays. The characteristics of
these positively charged particles are listed
below.
(i) unlike cathode rays, the positively
charged particles depend upon the
nature of gas present in the cathode ray
tube. These are simply the positively
charged gaseous ions.
(ii) The charge to mass ratio of the particles
is found to depend on the gas from which
these originate.
(iii) Some of the positively charged particles
carry a multiple of the fundamental unit
of electrical charge.
(iv) The behaviour of these particles in the
magnetic or electrical field is opposite to
that observed for electron or cathode
rays.
The smallest and lightest positive ion was
obtained from hydrogen and was called
proton. This positively charged particle was
characterised in 1919. Later, a need was felt
for the presence of electrically neutral particle
as one of the constituent of atom. These
particles were discovered by Chadwick (1932)
by bombarding a thin sheet of beryllium by
a-particles. When electrically neutral particles
having a mass slightly greater than that of
the protons was emitted. He named these
particles as neutrons. The important
Millikan’s Oil Drop Method
In this method, oil droplets in the form of
mist, produced by the atomiser, were allowed
to enter through a tiny hole in the upper plate
of electrical condenser. The downward motion
of these droplets was viewed through the
telescope, equipped with a micrometer eye
piece. By measuring the rate of fall of these
droplets, Millikan was able to measure the
mass of oil droplets.The air inside the
chamber was ionized by passing a beam of
X-rays through it. The electrical charge on
these oil droplets was acquired by collisions
with gaseous ions. The fall of these charged
oil droplets can be retarded, accelerated or
made stationary depending upon the charge
on the droplets and the polarity and strength
of the voltage applied to the plate. By carefully
measuring the effects of electrical field
strength on the motion of oil droplets,
Millikan concluded that the magnitude of
electrical charge, q, on the droplets is always
an integral multiple of the electrical charge,
e, that is, q = n e, where n = 1, 2, 3... .
Fig. 2.3 The Millikan oil drop apparatus for
measuring charge ‘e’. In chamber, the
forces acting on oil drop are:
gravitational, electrostatic due to
electrical field and a viscous drag force
when the oil drop is moving.
properties of these fundamental particles are
given in Table 2.1.
2.2 ATOMIC MODELS
Observations obtained from the experiments
mentioned in the previous sections have
suggested that Dalton’s indivisible atom is
composed of sub-atomic particles carrying
positive and negative charges. Different
2015-16
Page 5


26 CHEMISTRY
The rich diversity of chemical behaviour of different elements
can be traced to the differences in the internal structure of
atoms of these elements.
UNIT 2
STRUCTURE OF ATOM
After studying this unit you will be
able to
• • • • • know about the discovery of
electron, proton and neutron and
their characteristics;
• • • • • describe Thomson, Rutherford
and Bohr atomic models;
• • • • • understand the important
features of the quantum
mechanical model of atom;
• • • • •understand nature of
electromagnetic radiation and
Planck’s quantum theory;
• • • • • explain the photoelectric effect
and describe features of atomic
spectra;
• • • • • state the de Broglie relation and
Heisenberg uncertainty principle;
• • • • • define an atomic orbital in terms
of quantum numbers;
• • • • • state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity;
• • • • • write the electronic configurations
of atoms.
The existence of atoms has been proposed since the time
of early Indian and Greek philosophers (400 B.C.) who
were of the view that atoms are the fundamental building
blocks of matter. According to them, the continued
subdivisions of matter would ultimately yield atoms which
would not be further divisible. The word ‘atom’  has been
derived from the Greek word ‘a-tomio’ which means ‘uncut-
able’ or ‘non-divisible’. These earlier ideas were mere
speculations and there was no way to test them
experimentally. These ideas remained dormant  for a very
long time and were revived again by scientists in the
nineteenth century.
The atomic theory of matter was first proposed on a
firm scientific basis by John Dalton, a British school
teacher in 1808. His theory, called Dalton’s atomic
theory, regarded the atom as the ultimate particle of
matter (Unit 1).
In this unit we start with the experimental
observations made by scientists towards the end of
nineteenth and beginning of twentieth century. These
established that atoms can be further divided into sub-
atomic particles, i.e., electrons,  protons and neutrons—
a concept very different from that of Dalton. The major
problems before the scientists at that time were:
• • • • • to account for the stability of atom after the discovery
of sub-atomic particles,
• • • • • to compare the behaviour of one element from other
in terms of both physical and chemical properties,
2015-16
27 STRUCTURE OF ATOM
• • • • • to explain the formation of different kinds
of molecules by the combination of
different atoms and,
• • • • • to understand the origin and nature of the
characteristics of electromagnetic
radiation absorbed or emitted by atoms.
2.1 SUB-ATOMIC PARTICLES
Dalton’s atomic theory was able to explain
the law of conservation of mass, law of
constant composition and law of multiple
proportion very successfully. However, it failed
to explain the results of many experiments,
for example, it was known that substances
like glass or ebonite when rubbed with silk or
fur generate electricity. Many different kinds
of sub-atomic particles were discovered in the
twentieth century. However, in this section
we will talk about only two particles, namely
electron and proton.
2.1.1 Discovery of Electron
In 1830, Michael Faraday showed that if
electricity is passed through a solution of an
electrolyte, chemical reactions occurred at the
electrodes, which resulted in the liberation
and deposition of matter at the electrodes. He
formulated certain laws which you will study
in class XII. These results suggested the
particulate nature of electricity.
An insight into the structure of atom was
obtained from the experiments on electrical
discharge through gases. Before we discuss
these results we need to keep in mind a basic
rule regarding the behaviour of charged
particles : “Like charges repel each other and
unlike charges attract each other”.
In mid 1850s many scientists mainly
Faraday began to study electrical discharge
in partially evacuated tubes, known as
cathode ray discharge tubes. It is depicted
in Fig. 2.1. A cathode ray tube is made of glass
containing two thin pieces of metal, called
electrodes, sealed in it. The electrical
discharge through the gases could be
observed only at very low pressures and at
very high voltages. The pressure of different
gases could be adjusted by evacuation. When
sufficiently high voltage is applied across the
electrodes, current starts flowing through a
stream of particles moving in the tube from
the negative electrode (cathode) to the positive
electrode (anode). These were called cathode
rays or cathode ray particles. The flow of
current from cathode to anode was further
checked by making a hole in the anode and
coating the tube behind anode with
phosphorescent material zinc sulphide. When
these rays, after passing through anode, strike
the zinc sulphide coating, a bright spot on
the coating is developed(same thing happens
in a television set) [Fig. 2.1(b)].
Fig. 2.1(a)  A cathode ray discharge tube
Fig. 2.1(b) A cathode ray discharge tube with
perforated anode
The results of these experiments are
summarised below.
(i) The cathode rays start from cathode and
move towards the anode.
(ii) These rays themselves are not visible but
their behaviour can be observed with the
help of certain kind of materials
(fluorescent or phosphorescent) which
glow when hit by them. Television
picture tubes are cathode ray tubes and
television pictures result due to
fluorescence on the television screen
coated with certain fluorescent or
phosphorescent materials.
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28 CHEMISTRY
(iii) In the absence of electrical or magnetic
field, these rays travel in straight lines
(Fig. 2.2).
(iv) In the presence of electrical or magnetic
field, the behaviour of cathode rays are
similar to that expected from negatively
charged particles, suggesting that the
cathode rays consist of negatively
charged particles, called electrons.
(v) The characteristics of cathode rays
(electrons) do not depend upon the
material of electrodes and the nature of
the gas present in the cathode ray tube.
Thus, we can conclude that electrons are
basic constituent of all the atoms.
2.1.2 Charge to Mass Ratio of Electron
In 1897, British physicist J.J. Thomson
measured the ratio of electrical charge (e) to
the mass  of electron (m
e 
) by using cathode
ray tube and applying electrical and magnetic
field perpendicular to each other as well as to
the path of electrons (Fig. 2.2). Thomson
argued that the amount of deviation of the
particles from their path in the presence of
electrical or magnetic field depends upon:
(i) the magnitude of the negative charge on
the particle, greater the magnitude of the
charge on the particle, greater is the
interaction with the electric or magnetic
field and thus greater is the deflection.
Fig. 2.2   The apparatus to determine the charge to the mass ratio of electron
(ii) the mass of the particle — lighter the
particle, greater the deflection.
(iii) the strength of the electrical or magnetic
field — the deflection of electrons from
its original path increases with the
increase in the voltage across the
electrodes, or the strength of the
magnetic field.
When only electric field is applied, the
electrons deviate from their path and hit the
cathode ray tube at point A. Similarly when
only magnetic field is applied, electron strikes
the cathode ray tube at point C. By carefully
balancing the electrical and magnetic field
strength, it is possible to bring back the
electron to the path followed as in the absence
of electric or magnetic field and they hit the
screen at point B. By carrying out accurate
measurements on the amount of deflections
observed by the electrons on the electric field
strength or magnetic field strength, Thomson
was able to determine the value of e/m
e
 as:
e
e
m
 = 1.758820 × 10
11
 C kg
–1
(2.1)
Where m
e
 is the mass of the electron in kg
and e is the magnitude of the charge on the
electron in coulomb (C). Since electrons
are negatively charged, the charge on electron
is –e.
C:\Chemistry XI\Unit-2\Unit-2(2)-Lay-3(reprint).pmd     27.7.6, 16.10.6  (Reprint)
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29 STRUCTURE OF ATOM
2.1.3 Charge on the Electron
R.A. Millikan (1868-1953) devised a method
known as oil drop experiment (1906-14), to
determine the charge on the electrons. He
found that the charge on the electron to be
– 1.6 × 10
–19
 C. The present accepted value of
electrical charge is – 1.6022 × 10
–19 
C. The
mass of the  electron (m
e
) was determined by
combining these results with Thomson’s value
of e/m
e
 ratio.
–19
e 11 –1
e
1.6022 × 10 C
= =
/ 1.758820 × 10 C kg
e
m
e m
     = 9.1094×10
–31 
kg                             (2.2)
2.1.4 Discovery of Protons and Neutrons
Electrical discharge carried out in the
modified cathode ray tube led to the discovery
of particles carrying positive charge, also
known as canal rays. The characteristics of
these positively charged particles are listed
below.
(i) unlike cathode rays, the positively
charged particles depend upon the
nature of gas present in the cathode ray
tube. These are simply the positively
charged gaseous ions.
(ii) The charge to mass ratio of the particles
is found to depend on the gas from which
these originate.
(iii) Some of the positively charged particles
carry a multiple of the fundamental unit
of electrical charge.
(iv) The behaviour of these particles in the
magnetic or electrical field is opposite to
that observed for electron or cathode
rays.
The smallest and lightest positive ion was
obtained from hydrogen and was called
proton. This positively charged particle was
characterised in 1919. Later, a need was felt
for the presence of electrically neutral particle
as one of the constituent of atom. These
particles were discovered by Chadwick (1932)
by bombarding a thin sheet of beryllium by
a-particles. When electrically neutral particles
having a mass slightly greater than that of
the protons was emitted. He named these
particles as neutrons. The important
Millikan’s Oil Drop Method
In this method, oil droplets in the form of
mist, produced by the atomiser, were allowed
to enter through a tiny hole in the upper plate
of electrical condenser. The downward motion
of these droplets was viewed through the
telescope, equipped with a micrometer eye
piece. By measuring the rate of fall of these
droplets, Millikan was able to measure the
mass of oil droplets.The air inside the
chamber was ionized by passing a beam of
X-rays through it. The electrical charge on
these oil droplets was acquired by collisions
with gaseous ions. The fall of these charged
oil droplets can be retarded, accelerated or
made stationary depending upon the charge
on the droplets and the polarity and strength
of the voltage applied to the plate. By carefully
measuring the effects of electrical field
strength on the motion of oil droplets,
Millikan concluded that the magnitude of
electrical charge, q, on the droplets is always
an integral multiple of the electrical charge,
e, that is, q = n e, where n = 1, 2, 3... .
Fig. 2.3 The Millikan oil drop apparatus for
measuring charge ‘e’. In chamber, the
forces acting on oil drop are:
gravitational, electrostatic due to
electrical field and a viscous drag force
when the oil drop is moving.
properties of these fundamental particles are
given in Table 2.1.
2.2 ATOMIC MODELS
Observations obtained from the experiments
mentioned in the previous sections have
suggested that Dalton’s indivisible atom is
composed of sub-atomic particles carrying
positive and negative charges. Different
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30 CHEMISTRY
Table 2.1  Properties of Fundamental Particles
atomic models were proposed to explain the
distributions of these charged particles in an
atom. Although some of these models were
not able to explain the stability of atoms, two
of these models, proposed by J. J. Thomson
and Ernest Rutherford are discussed below.
2.2.1 Thomson Model of Atom
J. J. Thomson, in 1898, proposed that an
atom possesses a spherical shape (radius
approximately 10
–10 
m) in which the positive
charge is uniformly distributed. The electrons
are embedded into it in such a manner as to
give the most stable electrostatic arrangement
(Fig. 2.4). Many different names are given to
this model, for example, plum pudding,
raisin pudding or watermelon. This model
In the later half of the nineteenth century
different kinds of rays were discovered,
besides those mentioned earlier. Wilhalm
Röentgen (1845-1923) in 1895 showed
that when electrons strike a material in
the cathode ray tubes, produce rays
which can cause fluorescence in the
fluorescent materials placed outside the
cathode ray tubes. Since Röentgen did
not know the nature of the radiation, he
named them X-rays and the name is still
carried on. It was noticed that X-rays are
produced effectively when electrons
strike the dense metal anode, called
targets. These are not deflected by the
electric and magnetic fields and have a
very high penetrating power through the
matter and that is the reason that these
rays are used to study the interior of the
objects. These rays are of very short
wavelengths (~0.1 nm) and possess
electro-magnetic character (Section
2.3.1).
Henri Becqueral (1852-1908)
observed that there are certain elements
which emit radiation on their own and
named this phenomenon as
radioactivity and the elements known
as radioactive elements. This field was
developed by Marie Curie, Piere Curie,
Rutherford and Fredrick Soddy. It was
observed that three kinds of rays i.e., a,
ß- and ?-rays are emitted. Rutherford
found that a-rays consists of high energy
particles carrying two units of positive
charge and four unit of atomic mass. He
Fig.2.4  Thomson model of atom
can be visualised as a pudding or watermelon
of positive charge with plums or seeds
(electrons) embedded into it. An important
feature of this model is that the mass of the
atom is assumed to be uniformly distributed
over the atom. Although this model was able
to explain the overall neutrality of the atom,
but was not consistent with the results of later
experiments. Thomson was awarded Nobel
Prize for physics in 1906, for his theoretical
and experimental investigations on the
conduction of electricity by gases.
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