CH 2 Structure of Atom Class 11 Notes | EduRev

Class 11 : CH 2 Structure of Atom Class 11 Notes | EduRev

 Page 1


17 
 
CHAPTER 2 
 STRUCTURE OF ATOM 
 
• Atom is the smallest indivisible particle of the matter. Atom is made of electron, 
proton and    neutrons. 
PARTICLE ELECTRON PROTON  NEUTRON 
Discovery Sir. J. J. Thomson 
(1869) 
Goldstein (1886) Chadwick (1932) 
Nature of charge Negative Positive Neutral 
Amount of charge 1.6 x 10
-19
Coloumb 1.6 x 10
-19
Coloumb ?0 
Mass 9.11 x 10
?-31
kg 1.672614 x 10
?-27
kg 1.67492 x10
?-27
kg 
 Electrons were discovered using cathode ray discharge tube experiment. 
 Nucleus was discovered by Rutherford in 1911. 
 Cathode ray discharge tube experiment: A cathode ray discharge tube madeof 
glass is taken with two electrodes. At very low pressure and high 
voltage,current starts flowing through a stream of particles moving in the tube 
fromcathode to anode. These rays were called cathode rays. When a 
perforatedanode was taken, the cathode rays struck the other end of the glass 
tube atthe fluorescent coating and a bright spot on the coating was developed 
Results: 
a. Cathode rays consist of negatively charged electrons. 
b. Cathode rays themselves are not visible but their behavior can be 
observed with    
help of fluorescent or phosphorescent materials. 
c. In absence of electrical or magnetic field cathode rays travel in 
straight lines 
d. In presence of electrical or magnetic field, behaviour of cathode rays 
is similar to  that shown by electrons 
e. The characteristics of the cathode rays do not depend upon the 
material of the electrodes and the nature of the gas present in the cathode ray 
tube. 
 Charge to mass ratio of an electron was determined by Thomson. The chargeto 
mass ratio of an electron as 1.758820 x 10
11 
C kg
-1
 
 Charge on an electron was determined by R A Millikan by using an oil 
dropexperiment. The value of the charge on an electron is -1.6 x 10
-19
C. 
 The mass on an electron was determined by combining the results 
ofThomson’s experiment and Millikan’s oil drop experiment. The mass of 
anelectron was determined to be 9.1094 x 10
-31
kg. 
 Discovery of protons and canal rays: Modified cathode ray tube 
experimentwas carried out which led to the discovery of protons. 
 Characteristics of positively charged particles: 
a. Charge to mass ratio of particles depends on gas from which these originate 
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Page 2


17 
 
CHAPTER 2 
 STRUCTURE OF ATOM 
 
• Atom is the smallest indivisible particle of the matter. Atom is made of electron, 
proton and    neutrons. 
PARTICLE ELECTRON PROTON  NEUTRON 
Discovery Sir. J. J. Thomson 
(1869) 
Goldstein (1886) Chadwick (1932) 
Nature of charge Negative Positive Neutral 
Amount of charge 1.6 x 10
-19
Coloumb 1.6 x 10
-19
Coloumb ?0 
Mass 9.11 x 10
?-31
kg 1.672614 x 10
?-27
kg 1.67492 x10
?-27
kg 
 Electrons were discovered using cathode ray discharge tube experiment. 
 Nucleus was discovered by Rutherford in 1911. 
 Cathode ray discharge tube experiment: A cathode ray discharge tube madeof 
glass is taken with two electrodes. At very low pressure and high 
voltage,current starts flowing through a stream of particles moving in the tube 
fromcathode to anode. These rays were called cathode rays. When a 
perforatedanode was taken, the cathode rays struck the other end of the glass 
tube atthe fluorescent coating and a bright spot on the coating was developed 
Results: 
a. Cathode rays consist of negatively charged electrons. 
b. Cathode rays themselves are not visible but their behavior can be 
observed with    
help of fluorescent or phosphorescent materials. 
c. In absence of electrical or magnetic field cathode rays travel in 
straight lines 
d. In presence of electrical or magnetic field, behaviour of cathode rays 
is similar to  that shown by electrons 
e. The characteristics of the cathode rays do not depend upon the 
material of the electrodes and the nature of the gas present in the cathode ray 
tube. 
 Charge to mass ratio of an electron was determined by Thomson. The chargeto 
mass ratio of an electron as 1.758820 x 10
11 
C kg
-1
 
 Charge on an electron was determined by R A Millikan by using an oil 
dropexperiment. The value of the charge on an electron is -1.6 x 10
-19
C. 
 The mass on an electron was determined by combining the results 
ofThomson’s experiment and Millikan’s oil drop experiment. The mass of 
anelectron was determined to be 9.1094 x 10
-31
kg. 
 Discovery of protons and canal rays: Modified cathode ray tube 
experimentwas carried out which led to the discovery of protons. 
 Characteristics of positively charged particles: 
a. Charge to mass ratio of particles depends on gas from which these originate 
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Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
18 
 
b. The positively charged particles depend upon the nature of gas present in the 
cathode  ray discharge tube 
c. Some of the positively charged particles carry a multiple of fundamental of 
electrical  charge. 
d. Behaviour of positively charged particles in electrical or magnetic field is 
opposite to that observed for cathode rays 
 Neutrons were discovered by James Chadwick by bombarding a thin sheet of 
beryllium by a?- particles. They are electrically neutral particles having a mass 
slightly greater than that of the protons. 
 Atomic number (Z) : the number of protons present in the nucleus 
(Moseley1913). 
 Mass Number (A) :Sum of the number of protons and neutrons present in 
thenucleus. 
 Thomson model of an atom: This model proposed that atom is considered asa 
uniform positively charged sphere and electrons are embedded in it.An 
important feature of Thomson model of an atom was that mass of atom 
isconsidered to be evenly spread over the atom.Thomson model of atom is also 
called as Plum pudding, raisin pudding orwatermelon modelThomson model 
of atom was discarded because it could not explain certainexperimental results 
like the scattering of a?- particles by thin metal foils. 
 Observations from a- particles scattering experiment by Rutherford: 
a. Most of the a?- particles  passed through gold foil un deflected 
b. A small fraction of a?- particles got deflected through small angles 
c. Very few a?- particles did not pass through foil but suffered large deflection 
nearly180
o
 
 Conclusions Rutherford drew from a?- particles scattering experiment: 
a. Since most of the a?-particles passed through foil undeflected, it means most 
of the  space in atom is empty 
b. Since some of the a?-particles are deflected to certain angles, it means that 
there is positively mass present in atom 
c. Since only some of the a?-particles suffered large deflections, the positively 
charged mass must be occupying very small space 
d. Strong deflections or even bouncing back of a?-particles from metal foil 
were due to  direct collision with positively charged mass in atom 
 Rutherford’s model of atom: This model explained that atom consists 
ofnucleus which is concentrated in a very small volume. The nucleus 
comprisesof protons and neutrons. The electrons revolve around the nucleus in 
fixedorbits. Electrons and nucleus are held together by electrostatic forces 
ofattraction. 
 Drawbacks of Rutherford’s model of atom: 
a. According to Rutherford’s model of atom, electrons which are negatively 
charged particles revolve around the nucleus in fixed orbits. Thus,  
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Page 3


17 
 
CHAPTER 2 
 STRUCTURE OF ATOM 
 
• Atom is the smallest indivisible particle of the matter. Atom is made of electron, 
proton and    neutrons. 
PARTICLE ELECTRON PROTON  NEUTRON 
Discovery Sir. J. J. Thomson 
(1869) 
Goldstein (1886) Chadwick (1932) 
Nature of charge Negative Positive Neutral 
Amount of charge 1.6 x 10
-19
Coloumb 1.6 x 10
-19
Coloumb ?0 
Mass 9.11 x 10
?-31
kg 1.672614 x 10
?-27
kg 1.67492 x10
?-27
kg 
 Electrons were discovered using cathode ray discharge tube experiment. 
 Nucleus was discovered by Rutherford in 1911. 
 Cathode ray discharge tube experiment: A cathode ray discharge tube madeof 
glass is taken with two electrodes. At very low pressure and high 
voltage,current starts flowing through a stream of particles moving in the tube 
fromcathode to anode. These rays were called cathode rays. When a 
perforatedanode was taken, the cathode rays struck the other end of the glass 
tube atthe fluorescent coating and a bright spot on the coating was developed 
Results: 
a. Cathode rays consist of negatively charged electrons. 
b. Cathode rays themselves are not visible but their behavior can be 
observed with    
help of fluorescent or phosphorescent materials. 
c. In absence of electrical or magnetic field cathode rays travel in 
straight lines 
d. In presence of electrical or magnetic field, behaviour of cathode rays 
is similar to  that shown by electrons 
e. The characteristics of the cathode rays do not depend upon the 
material of the electrodes and the nature of the gas present in the cathode ray 
tube. 
 Charge to mass ratio of an electron was determined by Thomson. The chargeto 
mass ratio of an electron as 1.758820 x 10
11 
C kg
-1
 
 Charge on an electron was determined by R A Millikan by using an oil 
dropexperiment. The value of the charge on an electron is -1.6 x 10
-19
C. 
 The mass on an electron was determined by combining the results 
ofThomson’s experiment and Millikan’s oil drop experiment. The mass of 
anelectron was determined to be 9.1094 x 10
-31
kg. 
 Discovery of protons and canal rays: Modified cathode ray tube 
experimentwas carried out which led to the discovery of protons. 
 Characteristics of positively charged particles: 
a. Charge to mass ratio of particles depends on gas from which these originate 
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
18 
 
b. The positively charged particles depend upon the nature of gas present in the 
cathode  ray discharge tube 
c. Some of the positively charged particles carry a multiple of fundamental of 
electrical  charge. 
d. Behaviour of positively charged particles in electrical or magnetic field is 
opposite to that observed for cathode rays 
 Neutrons were discovered by James Chadwick by bombarding a thin sheet of 
beryllium by a?- particles. They are electrically neutral particles having a mass 
slightly greater than that of the protons. 
 Atomic number (Z) : the number of protons present in the nucleus 
(Moseley1913). 
 Mass Number (A) :Sum of the number of protons and neutrons present in 
thenucleus. 
 Thomson model of an atom: This model proposed that atom is considered asa 
uniform positively charged sphere and electrons are embedded in it.An 
important feature of Thomson model of an atom was that mass of atom 
isconsidered to be evenly spread over the atom.Thomson model of atom is also 
called as Plum pudding, raisin pudding orwatermelon modelThomson model 
of atom was discarded because it could not explain certainexperimental results 
like the scattering of a?- particles by thin metal foils. 
 Observations from a- particles scattering experiment by Rutherford: 
a. Most of the a?- particles  passed through gold foil un deflected 
b. A small fraction of a?- particles got deflected through small angles 
c. Very few a?- particles did not pass through foil but suffered large deflection 
nearly180
o
 
 Conclusions Rutherford drew from a?- particles scattering experiment: 
a. Since most of the a?-particles passed through foil undeflected, it means most 
of the  space in atom is empty 
b. Since some of the a?-particles are deflected to certain angles, it means that 
there is positively mass present in atom 
c. Since only some of the a?-particles suffered large deflections, the positively 
charged mass must be occupying very small space 
d. Strong deflections or even bouncing back of a?-particles from metal foil 
were due to  direct collision with positively charged mass in atom 
 Rutherford’s model of atom: This model explained that atom consists 
ofnucleus which is concentrated in a very small volume. The nucleus 
comprisesof protons and neutrons. The electrons revolve around the nucleus in 
fixedorbits. Electrons and nucleus are held together by electrostatic forces 
ofattraction. 
 Drawbacks of Rutherford’s model of atom: 
a. According to Rutherford’s model of atom, electrons which are negatively 
charged particles revolve around the nucleus in fixed orbits. Thus,  
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Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
19 
 
b. theelectrons undergo acceleration. According to electromagnetic theory of 
Maxwell, a charged particle undergoing acceleration should 
emitelectromagnetic radiation. Thus, an electron in an orbit should 
emitradiation. Thus, the orbit should shrink. But this does not happen. 
c. The model does not give any information about how electrons 
aredistributed around nucleus and what are energies of these electrons 
 Isotopes: These are the atoms of the same element having the same 
atomicnumber but different mass number.e g 
1
H
1
,
1
H
2
,
1
H
3
 
 Isobars: Isobars are the atoms of different elements having the same 
massnumber but different atomic number.e g 
18
Ar
40
 , 
20
Ca
40
 
 Isoelectronic species: These are those species which have the same numberof 
electrons. 
 Electromagnetic radiations: The radiations which are associated 
withelectrical and magnetic fields are called electromagnetic radiations. When 
anelectrically charged particle moves under acceleration, alternating 
electricaland magnetic fields are produced and transmitted. These fields 
aretransmitted in the form of waves. These waves are called 
electromagneticwaves or electromagnetic radiations. 
 Properties of electromagnetic radiations: 
a. Oscillating electric and magnetic field are produced by oscillating charged 
particles. These fields are perpendicular to each other and both 
areperpendicular to the direction of propagation of the wave. 
b. They do not need a medium to travel. That means they can even travel in 
vacuum. 
 Characteristics of electromagnetic radiations: 
a. Wavelength: It may be defined as the distance between two 
neighbouring crests or troughs of wave as shown. It is denoted by ?. 
b. Frequency ( ?): It may be defined as the number of waves which 
passthrough a particular point in one second. 
c. Velocity (v): It is defined as the distance travelled by a wave in 
onesecond. In vacuum all types of electromagnetic radiations travel with 
thesame velocity. Its value is 3 X10
8
m sec
-1.
 It is denoted by v 
d. Wave number: Wave number is defined as the number of wavelengths   
per unit length. 
 Velocity = frequency x wavelength     c = ?? 
 Planck's Quantum Theory- 
o The radiant energy is emitted or absorbed not continuously but 
discontinuously in the form of small discrete packets of energy called 
‘quantum’. In case of light , the quantum of energy is called a ‘photon’ 
o The energy of each quantum is directly proportional to the frequency of 
the radiation, i.e. E a ?     or E= h?      where h= Planck’s constant = 
6.626 x 10
-27
 Js 
o Energy is always emitted or absorbed as integral multiple of this 
quantum. E=nh? Where n=1,2,3,4,..... 
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
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Page 4


17 
 
CHAPTER 2 
 STRUCTURE OF ATOM 
 
• Atom is the smallest indivisible particle of the matter. Atom is made of electron, 
proton and    neutrons. 
PARTICLE ELECTRON PROTON  NEUTRON 
Discovery Sir. J. J. Thomson 
(1869) 
Goldstein (1886) Chadwick (1932) 
Nature of charge Negative Positive Neutral 
Amount of charge 1.6 x 10
-19
Coloumb 1.6 x 10
-19
Coloumb ?0 
Mass 9.11 x 10
?-31
kg 1.672614 x 10
?-27
kg 1.67492 x10
?-27
kg 
 Electrons were discovered using cathode ray discharge tube experiment. 
 Nucleus was discovered by Rutherford in 1911. 
 Cathode ray discharge tube experiment: A cathode ray discharge tube madeof 
glass is taken with two electrodes. At very low pressure and high 
voltage,current starts flowing through a stream of particles moving in the tube 
fromcathode to anode. These rays were called cathode rays. When a 
perforatedanode was taken, the cathode rays struck the other end of the glass 
tube atthe fluorescent coating and a bright spot on the coating was developed 
Results: 
a. Cathode rays consist of negatively charged electrons. 
b. Cathode rays themselves are not visible but their behavior can be 
observed with    
help of fluorescent or phosphorescent materials. 
c. In absence of electrical or magnetic field cathode rays travel in 
straight lines 
d. In presence of electrical or magnetic field, behaviour of cathode rays 
is similar to  that shown by electrons 
e. The characteristics of the cathode rays do not depend upon the 
material of the electrodes and the nature of the gas present in the cathode ray 
tube. 
 Charge to mass ratio of an electron was determined by Thomson. The chargeto 
mass ratio of an electron as 1.758820 x 10
11 
C kg
-1
 
 Charge on an electron was determined by R A Millikan by using an oil 
dropexperiment. The value of the charge on an electron is -1.6 x 10
-19
C. 
 The mass on an electron was determined by combining the results 
ofThomson’s experiment and Millikan’s oil drop experiment. The mass of 
anelectron was determined to be 9.1094 x 10
-31
kg. 
 Discovery of protons and canal rays: Modified cathode ray tube 
experimentwas carried out which led to the discovery of protons. 
 Characteristics of positively charged particles: 
a. Charge to mass ratio of particles depends on gas from which these originate 
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
18 
 
b. The positively charged particles depend upon the nature of gas present in the 
cathode  ray discharge tube 
c. Some of the positively charged particles carry a multiple of fundamental of 
electrical  charge. 
d. Behaviour of positively charged particles in electrical or magnetic field is 
opposite to that observed for cathode rays 
 Neutrons were discovered by James Chadwick by bombarding a thin sheet of 
beryllium by a?- particles. They are electrically neutral particles having a mass 
slightly greater than that of the protons. 
 Atomic number (Z) : the number of protons present in the nucleus 
(Moseley1913). 
 Mass Number (A) :Sum of the number of protons and neutrons present in 
thenucleus. 
 Thomson model of an atom: This model proposed that atom is considered asa 
uniform positively charged sphere and electrons are embedded in it.An 
important feature of Thomson model of an atom was that mass of atom 
isconsidered to be evenly spread over the atom.Thomson model of atom is also 
called as Plum pudding, raisin pudding orwatermelon modelThomson model 
of atom was discarded because it could not explain certainexperimental results 
like the scattering of a?- particles by thin metal foils. 
 Observations from a- particles scattering experiment by Rutherford: 
a. Most of the a?- particles  passed through gold foil un deflected 
b. A small fraction of a?- particles got deflected through small angles 
c. Very few a?- particles did not pass through foil but suffered large deflection 
nearly180
o
 
 Conclusions Rutherford drew from a?- particles scattering experiment: 
a. Since most of the a?-particles passed through foil undeflected, it means most 
of the  space in atom is empty 
b. Since some of the a?-particles are deflected to certain angles, it means that 
there is positively mass present in atom 
c. Since only some of the a?-particles suffered large deflections, the positively 
charged mass must be occupying very small space 
d. Strong deflections or even bouncing back of a?-particles from metal foil 
were due to  direct collision with positively charged mass in atom 
 Rutherford’s model of atom: This model explained that atom consists 
ofnucleus which is concentrated in a very small volume. The nucleus 
comprisesof protons and neutrons. The electrons revolve around the nucleus in 
fixedorbits. Electrons and nucleus are held together by electrostatic forces 
ofattraction. 
 Drawbacks of Rutherford’s model of atom: 
a. According to Rutherford’s model of atom, electrons which are negatively 
charged particles revolve around the nucleus in fixed orbits. Thus,  
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
19 
 
b. theelectrons undergo acceleration. According to electromagnetic theory of 
Maxwell, a charged particle undergoing acceleration should 
emitelectromagnetic radiation. Thus, an electron in an orbit should 
emitradiation. Thus, the orbit should shrink. But this does not happen. 
c. The model does not give any information about how electrons 
aredistributed around nucleus and what are energies of these electrons 
 Isotopes: These are the atoms of the same element having the same 
atomicnumber but different mass number.e g 
1
H
1
,
1
H
2
,
1
H
3
 
 Isobars: Isobars are the atoms of different elements having the same 
massnumber but different atomic number.e g 
18
Ar
40
 , 
20
Ca
40
 
 Isoelectronic species: These are those species which have the same numberof 
electrons. 
 Electromagnetic radiations: The radiations which are associated 
withelectrical and magnetic fields are called electromagnetic radiations. When 
anelectrically charged particle moves under acceleration, alternating 
electricaland magnetic fields are produced and transmitted. These fields 
aretransmitted in the form of waves. These waves are called 
electromagneticwaves or electromagnetic radiations. 
 Properties of electromagnetic radiations: 
a. Oscillating electric and magnetic field are produced by oscillating charged 
particles. These fields are perpendicular to each other and both 
areperpendicular to the direction of propagation of the wave. 
b. They do not need a medium to travel. That means they can even travel in 
vacuum. 
 Characteristics of electromagnetic radiations: 
a. Wavelength: It may be defined as the distance between two 
neighbouring crests or troughs of wave as shown. It is denoted by ?. 
b. Frequency ( ?): It may be defined as the number of waves which 
passthrough a particular point in one second. 
c. Velocity (v): It is defined as the distance travelled by a wave in 
onesecond. In vacuum all types of electromagnetic radiations travel with 
thesame velocity. Its value is 3 X10
8
m sec
-1.
 It is denoted by v 
d. Wave number: Wave number is defined as the number of wavelengths   
per unit length. 
 Velocity = frequency x wavelength     c = ?? 
 Planck's Quantum Theory- 
o The radiant energy is emitted or absorbed not continuously but 
discontinuously in the form of small discrete packets of energy called 
‘quantum’. In case of light , the quantum of energy is called a ‘photon’ 
o The energy of each quantum is directly proportional to the frequency of 
the radiation, i.e. E a ?     or E= h?      where h= Planck’s constant = 
6.626 x 10
-27
 Js 
o Energy is always emitted or absorbed as integral multiple of this 
quantum. E=nh? Where n=1,2,3,4,..... 
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20 
 
 Black body: An ideal body, which emits and absorbs all frequencies, is calleda 
black body. The radiation emitted by such a body is called black 
bodyradiation. 
 Photoelectric effect: The phenomenon of ejection of electrons from thesurface 
of metal when light of suitable frequency strikes it is calledphotoelectric effect. 
The ejected electrons are called photoelectrons. 
 Experimental results observed for the experiment of Photoelectric effect- 
o When beam of light falls on a metal surface electrons are 
ejectedimmediately. 
o Number of electrons ejected is proportional to intensity or brightness of 
light 
o Threshold frequency (vo): For each metal there is a 
characteristicminimum frequency below which photoelectric effect is 
not observed. Thisis called threshold frequency. 
o If frequency of light is less than the threshold frequency there is 
noejection of electrons no matter how long it falls on surface or how 
high isits intensity. 
 Photoelectric work function (Wo): The minimum energy required to 
ejectelectrons is called photoelectric work function.Wo= hvo 
 Energy of the ejected electrons :   
 
 Dual behavior of electromagnetic radiation- The light possesses both particle 
and wave like properties, i.e., light has dual behavior . whenever radiation 
interacts with matter, it displays particle like properties.(Black body radiation 
and photoelectric effect) Wave like properties are exhibited when it 
propagates(interference an diffraction) 
 When a white light is passed through a prism, it splits into a series ofcoloured 
bands known as spectrum. 
 Spectrum is of two types: continuous and line spectrum 
a. The spectrum which consists of all the wavelengths is called 
continuous  
spectrum. 
b. A spectrum in which only specific wavelengths are present is known 
as a line  
spectrum. It has bright lines with dark spaces between them. 
 Electromagnetic spectrum is a continuous spectrum. It consists of a range 
ofelectromagnetic radiations arranged in the order of increasing wavelengths 
ordecreasing frequencies. It extends from radio waves to gamma rays. 
 Spectrum is also classified as emission and line spectrum. 
o  Emission spectrum: The spectrum of radiationemitted by a substance 
that has absorbed energy is called an emissionspectrum. 
o Absorption spectrum is the spectrum obtained when radiation is 
passedthrough a sample of material. The sample absorbs radiation of 
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Page 5


17 
 
CHAPTER 2 
 STRUCTURE OF ATOM 
 
• Atom is the smallest indivisible particle of the matter. Atom is made of electron, 
proton and    neutrons. 
PARTICLE ELECTRON PROTON  NEUTRON 
Discovery Sir. J. J. Thomson 
(1869) 
Goldstein (1886) Chadwick (1932) 
Nature of charge Negative Positive Neutral 
Amount of charge 1.6 x 10
-19
Coloumb 1.6 x 10
-19
Coloumb ?0 
Mass 9.11 x 10
?-31
kg 1.672614 x 10
?-27
kg 1.67492 x10
?-27
kg 
 Electrons were discovered using cathode ray discharge tube experiment. 
 Nucleus was discovered by Rutherford in 1911. 
 Cathode ray discharge tube experiment: A cathode ray discharge tube madeof 
glass is taken with two electrodes. At very low pressure and high 
voltage,current starts flowing through a stream of particles moving in the tube 
fromcathode to anode. These rays were called cathode rays. When a 
perforatedanode was taken, the cathode rays struck the other end of the glass 
tube atthe fluorescent coating and a bright spot on the coating was developed 
Results: 
a. Cathode rays consist of negatively charged electrons. 
b. Cathode rays themselves are not visible but their behavior can be 
observed with    
help of fluorescent or phosphorescent materials. 
c. In absence of electrical or magnetic field cathode rays travel in 
straight lines 
d. In presence of electrical or magnetic field, behaviour of cathode rays 
is similar to  that shown by electrons 
e. The characteristics of the cathode rays do not depend upon the 
material of the electrodes and the nature of the gas present in the cathode ray 
tube. 
 Charge to mass ratio of an electron was determined by Thomson. The chargeto 
mass ratio of an electron as 1.758820 x 10
11 
C kg
-1
 
 Charge on an electron was determined by R A Millikan by using an oil 
dropexperiment. The value of the charge on an electron is -1.6 x 10
-19
C. 
 The mass on an electron was determined by combining the results 
ofThomson’s experiment and Millikan’s oil drop experiment. The mass of 
anelectron was determined to be 9.1094 x 10
-31
kg. 
 Discovery of protons and canal rays: Modified cathode ray tube 
experimentwas carried out which led to the discovery of protons. 
 Characteristics of positively charged particles: 
a. Charge to mass ratio of particles depends on gas from which these originate 
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CBSE sample papers, Question papers, Notes for Class 6 to 12 
Please Visit www.ncerthelp.com For Video lectures of all subjects Class 9 to 12 
18 
 
b. The positively charged particles depend upon the nature of gas present in the 
cathode  ray discharge tube 
c. Some of the positively charged particles carry a multiple of fundamental of 
electrical  charge. 
d. Behaviour of positively charged particles in electrical or magnetic field is 
opposite to that observed for cathode rays 
 Neutrons were discovered by James Chadwick by bombarding a thin sheet of 
beryllium by a?- particles. They are electrically neutral particles having a mass 
slightly greater than that of the protons. 
 Atomic number (Z) : the number of protons present in the nucleus 
(Moseley1913). 
 Mass Number (A) :Sum of the number of protons and neutrons present in 
thenucleus. 
 Thomson model of an atom: This model proposed that atom is considered asa 
uniform positively charged sphere and electrons are embedded in it.An 
important feature of Thomson model of an atom was that mass of atom 
isconsidered to be evenly spread over the atom.Thomson model of atom is also 
called as Plum pudding, raisin pudding orwatermelon modelThomson model 
of atom was discarded because it could not explain certainexperimental results 
like the scattering of a?- particles by thin metal foils. 
 Observations from a- particles scattering experiment by Rutherford: 
a. Most of the a?- particles  passed through gold foil un deflected 
b. A small fraction of a?- particles got deflected through small angles 
c. Very few a?- particles did not pass through foil but suffered large deflection 
nearly180
o
 
 Conclusions Rutherford drew from a?- particles scattering experiment: 
a. Since most of the a?-particles passed through foil undeflected, it means most 
of the  space in atom is empty 
b. Since some of the a?-particles are deflected to certain angles, it means that 
there is positively mass present in atom 
c. Since only some of the a?-particles suffered large deflections, the positively 
charged mass must be occupying very small space 
d. Strong deflections or even bouncing back of a?-particles from metal foil 
were due to  direct collision with positively charged mass in atom 
 Rutherford’s model of atom: This model explained that atom consists 
ofnucleus which is concentrated in a very small volume. The nucleus 
comprisesof protons and neutrons. The electrons revolve around the nucleus in 
fixedorbits. Electrons and nucleus are held together by electrostatic forces 
ofattraction. 
 Drawbacks of Rutherford’s model of atom: 
a. According to Rutherford’s model of atom, electrons which are negatively 
charged particles revolve around the nucleus in fixed orbits. Thus,  
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b. theelectrons undergo acceleration. According to electromagnetic theory of 
Maxwell, a charged particle undergoing acceleration should 
emitelectromagnetic radiation. Thus, an electron in an orbit should 
emitradiation. Thus, the orbit should shrink. But this does not happen. 
c. The model does not give any information about how electrons 
aredistributed around nucleus and what are energies of these electrons 
 Isotopes: These are the atoms of the same element having the same 
atomicnumber but different mass number.e g 
1
H
1
,
1
H
2
,
1
H
3
 
 Isobars: Isobars are the atoms of different elements having the same 
massnumber but different atomic number.e g 
18
Ar
40
 , 
20
Ca
40
 
 Isoelectronic species: These are those species which have the same numberof 
electrons. 
 Electromagnetic radiations: The radiations which are associated 
withelectrical and magnetic fields are called electromagnetic radiations. When 
anelectrically charged particle moves under acceleration, alternating 
electricaland magnetic fields are produced and transmitted. These fields 
aretransmitted in the form of waves. These waves are called 
electromagneticwaves or electromagnetic radiations. 
 Properties of electromagnetic radiations: 
a. Oscillating electric and magnetic field are produced by oscillating charged 
particles. These fields are perpendicular to each other and both 
areperpendicular to the direction of propagation of the wave. 
b. They do not need a medium to travel. That means they can even travel in 
vacuum. 
 Characteristics of electromagnetic radiations: 
a. Wavelength: It may be defined as the distance between two 
neighbouring crests or troughs of wave as shown. It is denoted by ?. 
b. Frequency ( ?): It may be defined as the number of waves which 
passthrough a particular point in one second. 
c. Velocity (v): It is defined as the distance travelled by a wave in 
onesecond. In vacuum all types of electromagnetic radiations travel with 
thesame velocity. Its value is 3 X10
8
m sec
-1.
 It is denoted by v 
d. Wave number: Wave number is defined as the number of wavelengths   
per unit length. 
 Velocity = frequency x wavelength     c = ?? 
 Planck's Quantum Theory- 
o The radiant energy is emitted or absorbed not continuously but 
discontinuously in the form of small discrete packets of energy called 
‘quantum’. In case of light , the quantum of energy is called a ‘photon’ 
o The energy of each quantum is directly proportional to the frequency of 
the radiation, i.e. E a ?     or E= h?      where h= Planck’s constant = 
6.626 x 10
-27
 Js 
o Energy is always emitted or absorbed as integral multiple of this 
quantum. E=nh? Where n=1,2,3,4,..... 
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 Black body: An ideal body, which emits and absorbs all frequencies, is calleda 
black body. The radiation emitted by such a body is called black 
bodyradiation. 
 Photoelectric effect: The phenomenon of ejection of electrons from thesurface 
of metal when light of suitable frequency strikes it is calledphotoelectric effect. 
The ejected electrons are called photoelectrons. 
 Experimental results observed for the experiment of Photoelectric effect- 
o When beam of light falls on a metal surface electrons are 
ejectedimmediately. 
o Number of electrons ejected is proportional to intensity or brightness of 
light 
o Threshold frequency (vo): For each metal there is a 
characteristicminimum frequency below which photoelectric effect is 
not observed. Thisis called threshold frequency. 
o If frequency of light is less than the threshold frequency there is 
noejection of electrons no matter how long it falls on surface or how 
high isits intensity. 
 Photoelectric work function (Wo): The minimum energy required to 
ejectelectrons is called photoelectric work function.Wo= hvo 
 Energy of the ejected electrons :   
 
 Dual behavior of electromagnetic radiation- The light possesses both particle 
and wave like properties, i.e., light has dual behavior . whenever radiation 
interacts with matter, it displays particle like properties.(Black body radiation 
and photoelectric effect) Wave like properties are exhibited when it 
propagates(interference an diffraction) 
 When a white light is passed through a prism, it splits into a series ofcoloured 
bands known as spectrum. 
 Spectrum is of two types: continuous and line spectrum 
a. The spectrum which consists of all the wavelengths is called 
continuous  
spectrum. 
b. A spectrum in which only specific wavelengths are present is known 
as a line  
spectrum. It has bright lines with dark spaces between them. 
 Electromagnetic spectrum is a continuous spectrum. It consists of a range 
ofelectromagnetic radiations arranged in the order of increasing wavelengths 
ordecreasing frequencies. It extends from radio waves to gamma rays. 
 Spectrum is also classified as emission and line spectrum. 
o  Emission spectrum: The spectrum of radiationemitted by a substance 
that has absorbed energy is called an emissionspectrum. 
o Absorption spectrum is the spectrum obtained when radiation is 
passedthrough a sample of material. The sample absorbs radiation of 
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certainwavelengths. The wavelengths which are absorbed are missing 
and comeas dark lines. 
 The study of emission or absorption spectra is referred as spectroscopy. 
 Spectral Lines for atomic hydrogen: 
 
 Rydberg equation 
R = Rydberg’s constant = 109677 cm
-1
 
 Bohr’s model for hydrogen atom: 
a. An electron in the hydrogen atom can move around the nucleus in a circular 
path of fixed  radius and energy. These paths are called orbits orenergy 
levels. These orbits are arranged concentrically around thenucleus. 
b. As long as an electron remains in a particular orbit, it does not lose or gain 
energy and its energy remains constant. 
c. When transition occurs between two stationary states that differ inenergy, 
the frequency of the radiation absorbed or emitted can becalculated 
 
d. An electron can move only in those orbits for which its angularmomentum 
is an integral multiple of h/2p 
 
 The radius of the nth orbit is given byr
n
 =52.9 pm x n
2
 
                             Z 
 energy of electron in nth orbit is : 
 
 Limitations of Bohr’s model of atom: 
a. Bohr’s model failed to account for the finer details of the hydrogen 
spectrum.  
b. Bohr’s model was also unable to explain spectrum of atoms containing 
more than one electron. 
 Dual behavior of matter: de Broglie proposed that matter exhibits 
dualbehavior i.e. matter shows both particle and wave nature. de Broglie’s 
relation is  
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