NCERT Textbook: Structure of Atom | Science & Technology for UPSC CSE PDF Download

 Page 1


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
str Uct Ure 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;
• de??ne 	 an	 atomic 	 orbital 	 in	 terms
of quantum numbers;
• state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity; and
• write 	 th e 	 electron ic 	 con ?? gu ration s
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 	 ??rm 	 scienti??c 	 basis 	 by 	 John 	 Dal ton, 	 a 	 B ri tish	
school teacher in 1808. His theory, called Dalton’s 
atomic theory, regarded the atom as the ultimate 
particle	 of	 matter 	 (Unit	 1). 	 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 get electrically charged.
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 are made of sub-atomic particles, i.e., electrons,  
protons and neutrons — a concept very different from 
that 	 of	 Dalton.	
o bjectives
Unit 2.indd   29 9/9/2022   4:28:07 PM
Reprint 2026-27
Page 2


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
str Uct Ure 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;
• de??ne 	 an	 atomic 	 orbital 	 in	 terms
of quantum numbers;
• state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity; and
• write 	 th e 	 electron ic 	 con ?? gu ration s
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 	 ??rm 	 scienti??c 	 basis 	 by 	 John 	 Dal ton, 	 a 	 B ri tish	
school teacher in 1808. His theory, called Dalton’s 
atomic theory, regarded the atom as the ultimate 
particle	 of	 matter 	 (Unit	 1). 	 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 get electrically charged.
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 are made of sub-atomic particles, i.e., electrons,  
protons and neutrons — a concept very different from 
that 	 of	 Dalton.	
o bjectives
Unit 2.indd   29 9/9/2022   4:28:07 PM
Reprint 2026-27
30 CHEMISTRY
2.1 Discovery of sUb-atomic 
Particles
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”. 
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.
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 of the 
glass 	 tubes. 	 When 	 suf??ciently 	 high 	 voltage	
is applied across the electrodes, current 
starts 	 ??owing 	 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 	 ??ow 	 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 is developed 
on the coating [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 
(??uorescent 	 or	 phosphor escent) 	 which	
glow when hit by them. Television 
picture tubes are cathode ray tubes 
and television pictures result due to 
??uorescence 	 on 	 the 	 television 	 screen	
coated with certain fluorescent or 
phosphorescent materials.
(iii) In the absence of electrical or magnetic 
??eld, 	 these	 rays	 travel	 in 	 straight	 lines	
(Fig. 2.2).
(iv) In the presence of electrical or magnetic 
??eld, 	 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 
Unit 2.indd   30 9/9/2022   4:28:08 PM
Reprint 2026-27
Page 3


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
str Uct Ure 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;
• de??ne 	 an	 atomic 	 orbital 	 in	 terms
of quantum numbers;
• state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity; and
• write 	 th e 	 electron ic 	 con ?? gu ration s
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 	 ??rm 	 scienti??c 	 basis 	 by 	 John 	 Dal ton, 	 a 	 B ri tish	
school teacher in 1808. His theory, called Dalton’s 
atomic theory, regarded the atom as the ultimate 
particle	 of	 matter 	 (Unit	 1). 	 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 get electrically charged.
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 are made of sub-atomic particles, i.e., electrons,  
protons and neutrons — a concept very different from 
that 	 of	 Dalton.	
o bjectives
Unit 2.indd   29 9/9/2022   4:28:07 PM
Reprint 2026-27
30 CHEMISTRY
2.1 Discovery of sUb-atomic 
Particles
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”. 
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.
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 of the 
glass 	 tubes. 	 When 	 suf??ciently 	 high 	 voltage	
is applied across the electrodes, current 
starts 	 ??owing 	 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 	 ??ow 	 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 is developed 
on the coating [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 
(??uorescent 	 or	 phosphor escent) 	 which	
glow when hit by them. Television 
picture tubes are cathode ray tubes 
and television pictures result due to 
??uorescence 	 on 	 the 	 television 	 screen	
coated with certain fluorescent or 
phosphorescent materials.
(iii) In the absence of electrical or magnetic 
??eld, 	 these	 rays	 travel	 in 	 straight	 lines	
(Fig. 2.2).
(iv) In the presence of electrical or magnetic 
??eld, 	 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 
Unit 2.indd   30 9/9/2022   4:28:08 PM
Reprint 2026-27
31 STRUCTURE OF ATOM
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 r atio 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 
??eld 	 perpendicular	 to 	 each	 other 	 as 	 well 	 as 	
to the path of electrons (Fig. 2.2). When only 
electric 	 ??eld 	 is 	 applied,	 the	 electrons	 deviate	
from their path and hit the cathode ray tube 
at point A (Fig. 2.2). Similarly when only 
magnetic 	 ??eld 	 is 	 applied, 	 electron 	 strikes	
the cathode ray tube at point C. By carefully 
balancing 	 the 	 electrical 	 and 	 magnetic 	 ??eld 	
strength, it is possible to bring back the 
electron to the path which is followed in the 
absence 	 of	 electric	 or	 magnetic	 ??eld 	 and	 they	
hit the screen at point B. Thomson argued 
that the amount of deviation of the particles 
from their path in the presence of electrical 
or 	magnetic 	??eld 	 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 
??eld 	and	thus	greater	is 	the	de??ection.
(ii) the mass of the particle — lighter the 
particle,	greater	the 	de??ection.
(iii) the strength of the electrical or magnetic 
??eld 	 — 	 the 	 de??ection 	 of 	 electrons 	 from 	 its	
original path increases with the increase 
in the voltage across the electrodes, or 
the 	strength 	of	 the	magnetic	 ??eld.
By carrying out accurate measurements 
on 	 the 	 amoun t 	 of 	 de??ections 	 observed 	 by	
the	electrons	 on	the	electric	 ??eld 	strength	or	
magnet i c	 ??el d 	 st rengt h, 	 Thomson 	 w as 	 abl e 	 t o 	
determine the value of e/m
e
 as:
 = 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.
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 the charge on the electron to be  
– 1.6 × 10
–19
 C. The present accepted value of 
electrical charge is – 1.602176 × 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.
= 9.1094×10
–31 
kg                             (2.2)
Fig. 2.2 The apparatus to determine the charge to the mass ratio of electron
Unit 2.indd   31 9/9/2022   4:28:08 PM
Reprint 2026-27
Page 4


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
str Uct Ure 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;
• de??ne 	 an	 atomic 	 orbital 	 in	 terms
of quantum numbers;
• state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity; and
• write 	 th e 	 electron ic 	 con ?? gu ration s
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 	 ??rm 	 scienti??c 	 basis 	 by 	 John 	 Dal ton, 	 a 	 B ri tish	
school teacher in 1808. His theory, called Dalton’s 
atomic theory, regarded the atom as the ultimate 
particle	 of	 matter 	 (Unit	 1). 	 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 get electrically charged.
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 are made of sub-atomic particles, i.e., electrons,  
protons and neutrons — a concept very different from 
that 	 of	 Dalton.	
o bjectives
Unit 2.indd   29 9/9/2022   4:28:07 PM
Reprint 2026-27
30 CHEMISTRY
2.1 Discovery of sUb-atomic 
Particles
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”. 
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.
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 of the 
glass 	 tubes. 	 When 	 suf??ciently 	 high 	 voltage	
is applied across the electrodes, current 
starts 	 ??owing 	 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 	 ??ow 	 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 is developed 
on the coating [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 
(??uorescent 	 or	 phosphor escent) 	 which	
glow when hit by them. Television 
picture tubes are cathode ray tubes 
and television pictures result due to 
??uorescence 	 on 	 the 	 television 	 screen	
coated with certain fluorescent or 
phosphorescent materials.
(iii) In the absence of electrical or magnetic 
??eld, 	 these	 rays	 travel	 in 	 straight	 lines	
(Fig. 2.2).
(iv) In the presence of electrical or magnetic 
??eld, 	 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 
Unit 2.indd   30 9/9/2022   4:28:08 PM
Reprint 2026-27
31 STRUCTURE OF ATOM
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 r atio 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 
??eld 	 perpendicular	 to 	 each	 other 	 as 	 well 	 as 	
to the path of electrons (Fig. 2.2). When only 
electric 	 ??eld 	 is 	 applied,	 the	 electrons	 deviate	
from their path and hit the cathode ray tube 
at point A (Fig. 2.2). Similarly when only 
magnetic 	 ??eld 	 is 	 applied, 	 electron 	 strikes	
the cathode ray tube at point C. By carefully 
balancing 	 the 	 electrical 	 and 	 magnetic 	 ??eld 	
strength, it is possible to bring back the 
electron to the path which is followed in the 
absence 	 of	 electric	 or	 magnetic	 ??eld 	 and	 they	
hit the screen at point B. Thomson argued 
that the amount of deviation of the particles 
from their path in the presence of electrical 
or 	magnetic 	??eld 	 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 
??eld 	and	thus	greater	is 	the	de??ection.
(ii) the mass of the particle — lighter the 
particle,	greater	the 	de??ection.
(iii) the strength of the electrical or magnetic 
??eld 	 — 	 the 	 de??ection 	 of 	 electrons 	 from 	 its	
original path increases with the increase 
in the voltage across the electrodes, or 
the 	strength 	of	 the	magnetic	 ??eld.
By carrying out accurate measurements 
on 	 the 	 amoun t 	 of 	 de??ections 	 observed 	 by	
the	electrons	 on	the	electric	 ??eld 	strength	or	
magnet i c	 ??el d 	 st rengt h, 	 Thomson 	 w as 	 abl e 	 t o 	
determine the value of e/m
e
 as:
 = 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.
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 the charge on the electron to be  
– 1.6 × 10
–19
 C. The present accepted value of 
electrical charge is – 1.602176 × 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.
= 9.1094×10
–31 
kg                             (2.2)
Fig. 2.2 The apparatus to determine the charge to the mass ratio of electron
Unit 2.indd   31 9/9/2022   4:28:08 PM
Reprint 2026-27
32 CHEMISTRY
2.1.4 Discovery of Protons and neutrons
Electrical 	 discharge 	 carried 	 out 	 in 	 the 	 modi??ed 	
cathode ray tube led to the discovery of canal 
rays carrying positively charged particles. The 
characteristics of these positively charged 
particles are listed below.
(i) Unlike cathode rays, mass of positively 
charged particles depends 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 
depends 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 	 ??eld 	 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 protons were emitted. He named 
these particles as neutrons. The important 
properties of all these fundamental particles 
are given in Table 2.1.
2.2 atomic mo Dels
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. The major 
problems before the scientists after the 
discovery of sub-atomic particles were:
•	 to account for the stability of atom,
•	 to compare the behaviour of elements 
in terms of both physical and chemical 
properties,
millikan’s o il 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 
??eld 	 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... .
•	 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.
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 ??eld and a viscous drag 
force when the oil drop is moving.
Unit 2.indd   32 9/9/2022   4:28:08 PM
Reprint 2026-27
Page 5


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
str Uct Ure 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;
• de??ne 	 an	 atomic 	 orbital 	 in	 terms
of quantum numbers;
• state aufbau principle, Pauli
exclusion principle and Hund’s
rule of maximum multiplicity; and
• write 	 th e 	 electron ic 	 con ?? gu ration s
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 	 ??rm 	 scienti??c 	 basis 	 by 	 John 	 Dal ton, 	 a 	 B ri tish	
school teacher in 1808. His theory, called Dalton’s 
atomic theory, regarded the atom as the ultimate 
particle	 of	 matter 	 (Unit	 1). 	 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 get electrically charged.
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 are made of sub-atomic particles, i.e., electrons,  
protons and neutrons — a concept very different from 
that 	 of	 Dalton.	
o bjectives
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30 CHEMISTRY
2.1 Discovery of sUb-atomic 
Particles
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”. 
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.
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 of the 
glass 	 tubes. 	 When 	 suf??ciently 	 high 	 voltage	
is applied across the electrodes, current 
starts 	 ??owing 	 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 	 ??ow 	 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 is developed 
on the coating [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 
(??uorescent 	 or	 phosphor escent) 	 which	
glow when hit by them. Television 
picture tubes are cathode ray tubes 
and television pictures result due to 
??uorescence 	 on 	 the 	 television 	 screen	
coated with certain fluorescent or 
phosphorescent materials.
(iii) In the absence of electrical or magnetic 
??eld, 	 these	 rays	 travel	 in 	 straight	 lines	
(Fig. 2.2).
(iv) In the presence of electrical or magnetic 
??eld, 	 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 
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31 STRUCTURE OF ATOM
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 r atio 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 
??eld 	 perpendicular	 to 	 each	 other 	 as 	 well 	 as 	
to the path of electrons (Fig. 2.2). When only 
electric 	 ??eld 	 is 	 applied,	 the	 electrons	 deviate	
from their path and hit the cathode ray tube 
at point A (Fig. 2.2). Similarly when only 
magnetic 	 ??eld 	 is 	 applied, 	 electron 	 strikes	
the cathode ray tube at point C. By carefully 
balancing 	 the 	 electrical 	 and 	 magnetic 	 ??eld 	
strength, it is possible to bring back the 
electron to the path which is followed in the 
absence 	 of	 electric	 or	 magnetic	 ??eld 	 and	 they	
hit the screen at point B. Thomson argued 
that the amount of deviation of the particles 
from their path in the presence of electrical 
or 	magnetic 	??eld 	 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 
??eld 	and	thus	greater	is 	the	de??ection.
(ii) the mass of the particle — lighter the 
particle,	greater	the 	de??ection.
(iii) the strength of the electrical or magnetic 
??eld 	 — 	 the 	 de??ection 	 of 	 electrons 	 from 	 its	
original path increases with the increase 
in the voltage across the electrodes, or 
the 	strength 	of	 the	magnetic	 ??eld.
By carrying out accurate measurements 
on 	 the 	 amoun t 	 of 	 de??ections 	 observed 	 by	
the	electrons	 on	the	electric	 ??eld 	strength	or	
magnet i c	 ??el d 	 st rengt h, 	 Thomson 	 w as 	 abl e 	 t o 	
determine the value of e/m
e
 as:
 = 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.
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 the charge on the electron to be  
– 1.6 × 10
–19
 C. The present accepted value of 
electrical charge is – 1.602176 × 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.
= 9.1094×10
–31 
kg                             (2.2)
Fig. 2.2 The apparatus to determine the charge to the mass ratio of electron
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32 CHEMISTRY
2.1.4 Discovery of Protons and neutrons
Electrical 	 discharge 	 carried 	 out 	 in 	 the 	 modi??ed 	
cathode ray tube led to the discovery of canal 
rays carrying positively charged particles. The 
characteristics of these positively charged 
particles are listed below.
(i) Unlike cathode rays, mass of positively 
charged particles depends 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 
depends 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 	 ??eld 	 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 protons were emitted. He named 
these particles as neutrons. The important 
properties of all these fundamental particles 
are given in Table 2.1.
2.2 atomic mo Dels
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. The major 
problems before the scientists after the 
discovery of sub-atomic particles were:
•	 to account for the stability of atom,
•	 to compare the behaviour of elements 
in terms of both physical and chemical 
properties,
millikan’s o il 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 
??eld 	 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... .
•	 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.
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 ??eld and a viscous drag 
force when the oil drop is moving.
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33 STRUCTURE OF ATOM
D i fferent 	 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, one proposed 
by 	 J.J. 	 Thomson	 and	 the	 other 	 proposed	 by	
Ernest Rutherford are discussed below.
2.2.1 t homson 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 
ca n 	 ca u se 	 ??u orescen ce 	 in 	 th e 	 ??u orescen t 	
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 	 de??ected 	 by 	 the	
electric	 and	 magnetic 	 ??elds 	 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 	 ??eld 	 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 concluded 
that a- particles are helium nuclei as when a- 
particles combined with two electrons yielded 
helium gas. ß-rays are negatively charged
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.
t able 2.1  Properties of f undamental Particles 
name symbol absolute 
charge/c
r elative 
charge
mass/kg mass/u approx. 
mass/u
Electron
Proton
Neutron
e
p
n
– 1.602176×10
–19
+ 1.602176×10
–19
0
–1
+1
0
9.109382×10
–31
1.6726216×10
–27
1.674927×10
–27
0.00054
1.00727
1.00867
0
1
1
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