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 Goldstein (1886) Chadwick (1932)
(1869)
Nature of charge Negative Positive Neutral
-19 -19
Amount of charge 1.6 x 10 Coloumb 1.6 x 10 Coloumb 0
Mass 9.11 x 10-31kg 1.672614 x 10-27kg 1.67492 x10-27kg
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 1011 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 10-19C.
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 10-31kg.
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
17
, 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 α- 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 α- particles by thin metal foils.
Observations from α- particles scattering experiment by Rutherford:
a. Most of the α- particles passed through gold foil un deflected
b. A small fraction of α- particles got deflected through small angles
c. Very few α- particles did not pass through foil but suffered large deflection
nearly180o
Conclusions Rutherford drew from α•- particles scattering experiment:
a. Since most of the α-particles passed through foil undeflected, it means most
of the space in atom is empty
b. Since some of the α-particles are deflected to certain angles, it means that
there is positively mass present in atom
c. Since only some of the α-particles suffered large deflections, the positively
charged mass must be occupying very small space
d. Strong deflections or even bouncing back of α-particles from metal foil
were due to direct collision with positively charged mass in atom
5XWKHUIRUG¶VPRGHORIDWRP : 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.
'UDZEDFNVRI5XWKHUIRUG¶VPRGHORIDWRP :
a. According to Rutherford’s model of atom, electrons which are negatively
charged particles revolve around the nucleus in fixed orbits. Thus,
18
, 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 1H1,1H2,1H3
Isobars: Isobars are the atoms of different elements having the same
massnumber but different atomic number.e g 18Ar40 , 20Ca40
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 λ.
b. Frequency (ν): 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 X108m 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 = νλ
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 α υ or E= hυ where h= Planck’s constant =
-27
6.626 x 10 Js
o Energy is always emitted or absorbed as integral multiple of this
quantum. E=nhυ Where n=1,2,3,4,.....
19
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 Goldstein (1886) Chadwick (1932)
(1869)
Nature of charge Negative Positive Neutral
-19 -19
Amount of charge 1.6 x 10 Coloumb 1.6 x 10 Coloumb 0
Mass 9.11 x 10-31kg 1.672614 x 10-27kg 1.67492 x10-27kg
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 1011 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 10-19C.
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 10-31kg.
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
17
, 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 α- 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 α- particles by thin metal foils.
Observations from α- particles scattering experiment by Rutherford:
a. Most of the α- particles passed through gold foil un deflected
b. A small fraction of α- particles got deflected through small angles
c. Very few α- particles did not pass through foil but suffered large deflection
nearly180o
Conclusions Rutherford drew from α•- particles scattering experiment:
a. Since most of the α-particles passed through foil undeflected, it means most
of the space in atom is empty
b. Since some of the α-particles are deflected to certain angles, it means that
there is positively mass present in atom
c. Since only some of the α-particles suffered large deflections, the positively
charged mass must be occupying very small space
d. Strong deflections or even bouncing back of α-particles from metal foil
were due to direct collision with positively charged mass in atom
5XWKHUIRUG¶VPRGHORIDWRP : 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.
'UDZEDFNVRI5XWKHUIRUG¶VPRGHORIDWRP :
a. According to Rutherford’s model of atom, electrons which are negatively
charged particles revolve around the nucleus in fixed orbits. Thus,
18
, 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 1H1,1H2,1H3
Isobars: Isobars are the atoms of different elements having the same
massnumber but different atomic number.e g 18Ar40 , 20Ca40
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 λ.
b. Frequency (ν): 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 X108m 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 = νλ
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 α υ or E= hυ where h= Planck’s constant =
-27
6.626 x 10 Js
o Energy is always emitted or absorbed as integral multiple of this
quantum. E=nhυ Where n=1,2,3,4,.....
19
