2.STRUCTUREOF ATOM
1. Discovery of Electron by cathode ray discharge tubes
In 1879, William Crooks studied the conduction of electricity through gases at low pressure. 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. When
sufficiently high voltage is applied across the electrodes, current starts flowing through a stream of
particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These
were called cathode rays or cathode ray particles.
Properties of cathode rays:
(i) The cathode rays start from cathode and move towards the anode.
(ii) These rays themselves are not visible but they produce fluorescence on ZnS screen.
(iii) In the absence of electrical or magnetic field, these rays travel in straight lines
(iv) In the presence of electrical or magnetic field, the behavior of cathode rays are similar to that expected
from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles,
called electrons.
(v) The characteristics of cathode rays(electrons) do not depend upon the material of electrodes and the
nature of the gas present in the cathode ray tube.
Charge to Mass Ratio of Electron
J.J. Thomson measured the ratio of electrical charge (e) to the mass of electron (me ) by using cathode ray
tube and applying electrical and magnetic field perpendicular to each other as well as to the path of
electrons .
e/me = 1.758820 x 1011 C kg-1
where me = Mass of the electron in kg
e = magnitude of charge on the electron in coulomb (C).
Discovery of proton anode ray
In 1886, Goldstein modified the discharge tube by using a perforated cathode. On reducing the pressure, he
observed a new type of luminous rays passing through the holes or perforations of the cathode and moving
in a direction opposite to the cathode rays. These rays were named as positive rays or anode rays or as
canal rays. Anode rays are not emitted from the anode but from a space between anode and cathode.
Properties of Anode Rays
(i) The value of positive charge on the particles constituting anode rays depends upon the nature of the gas
in the discharge tube.
(ii) The charge to mass ratio of the particles is found to depend on the gas from which these originate.
(iii) Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge.
(iv)The behavior of these particles in the magnetic or electric field is opposite to that observed for electron
or cathode rays.
Proton: The smallest and lightest positive ion was obtained from hydrogen and was called proton. Mass of
proton = 1.676 x 10-27 kg
Charge on a proton = (+) 1.602 x 10-19 C
Neutron: It is a neutral particle. It was discovered by Chadwick (1932).
By the bombardment of thin sheets of beryllium with fast moving a-particles he observed • that highly
penetrating rays consist of neutral particles which were named neutrons.
16
,Thomson Model of Atom: 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
An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over
the atom.
Drawback of Thomson Model of Atom
This model was able to explain the overall neutrality of the atom, it could not satisfactorily, explain the
results of scattering experiments carried out by Rutherford in 1911.
Rutherford’s Nuclear Model of Atom
Rutherford in 1911, performed some scattering experiments in which he bombarded thin foils of metals
like gold, silver, platinum or copper with a beam of fast moving a-particles. The thin gold foil had a
circular fluorescent zinc sulphide screen around it. Whenever a-particles struck the screen, a tiny flash of
light was produced at that point.
The important observations are:
(i) Most of the a-particles passed through the foil without undergoing any deflection,
(ii) A few a-particles underwent deflection through small angles.
(iii) Very few mere deflected back i.e., through an angle of nearly 180°.
Conclusions:
(i) Since most of the a-particles passed through the foil without undergoing any deflection, there must be
sufficient empty space within the atom.
(ii) A small fraction of a-particles was deflected by small angles. The positive charge has to be
concentrated in a very small volume that repelled and deflected a few positively charged a-particles. This
very small portion of the atom was called nucleus.
(iii) The volume of nucleus is very small as compared to total volume of atom..
Rutherford’s Nuclear Model of an Atom
(i) The positive charge and most of the mass of the atom was densely concentrated in an extremely small
region. This very small portion of the atom was called nucleus by Rutherford.
(ii) The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular
paths called orbits.
(iii) Electrons and nucleus are held together by electrostatic forces of attraction
17
, Drawbacks of Rutherford Model
(i) Rutherford’s model cannot explain the stability of atom if the motion of electrons is described on the
basis of classical mechanics and electromagnetic theory.
(ii) Rutherford’s model does not give any idea about distribution of electrons around the nucleus and about
their energies.
Atomic Number: The number of protons present in the nucleus is equal to the atomic number (z). In
order to keep the electrical neutrality, the number of electrons in an atom is equal to the number of protons
(atomic number, z)
Atomic Number (z) = Number of protons in the nucleus of an atom.= Number of electrons in a neutral
atom
Mass Number :Number of protons and neutrons present in the nucleus are collectively known as nucleons.
The total number of nucleons is termed as mass number (A) of the atom.
Mass Number (A) = Number of protons (p) + Number of neutrons (n).
Isotopes :Atoms with identical atomic number but different atomic mass number are known as Isotopes.
Isotopes of Hydrogen: protium, deuterium and tritium.
Isobars :Isobars are the atoms with same mass number but different atomic number for Example
Developments Leading to the Bohr’s Model of Atom -Two developments played a major role in the
formulation of Bohr’s model of atom. These were:
(i) Dual character of the electromagnetic radiation which means that radiations possess both wave like and
particle like properties.
(ii) Experimental results regarding atomic spectra which can be explained only by assuming quantized
electronic energy levels in atoms.
Nature of Electromagnetic Radiation (Electromagnetic Wave Theory)
This theory was put forward by James Clark Maxwell in 1864. The main points of this theory are as
follows:
(i) The energy is emitted from any source (like the heated rod or the filament of a bulb through which
electric current is passed) continuously in the form of radiations and is called the radiant energy.
(ii) The radiations consist of electric and magnetic fields oscillating perpendicular to each other and both
perpendicular to the direction of propagation of the radiation.
(iii) The radiations possess wave character and travel with the velocity of light 3 x 108 m/sec.
(iv) These waves do not require any material medium for propagation. For example, rays from the sun
reach us through space which is a non-material medium.
Characteristics of a Wave –
(i)Wavelength: It is defined as the distance between any two consecutive crests or troughs. It is
represented by X and its S.I. unit is metre.
(ii)Frequency: Frequency of a wave is defined as the number of waves passing through a point in one
second. It is represented by v (nu) and is expressed in Hertz (Hz).
1 Hz = 1 cycle/sec. Velocity:
(iii) Velocity of a wave is defined as the linear distance travelled by the wave in one second.
It is represented by c and is expressed in cm/sec or m/sec.
(iv)Amplitude: Amplitude of a wave is the height of the crest or the depth of the through. It is represented
by V and is expressed in the units of length.
(v)Wave Number: It is defined as the number of waves present in 1 metre length. Evidently it will be
equal to the reciprocal of the wavelength. It is represented by bar v (read as nu bar).
Electromagnetic Spectrum: When electromagnetic radiations are arranged in order of their increasing
wavelengths or decreasing frequencies, the complete spectrum obtained is called electromagnetic
spectrum.
Limitations of Electromagnetic Wave Theory
Electromagnetic wave theory was successful in explaining properties of light such as interference,
18
1. Discovery of Electron by cathode ray discharge tubes
In 1879, William Crooks studied the conduction of electricity through gases at low pressure. 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. When
sufficiently high voltage is applied across the electrodes, current starts flowing through a stream of
particles moving in the tube from the negative electrode (cathode) to the positive electrode (anode). These
were called cathode rays or cathode ray particles.
Properties of cathode rays:
(i) The cathode rays start from cathode and move towards the anode.
(ii) These rays themselves are not visible but they produce fluorescence on ZnS screen.
(iii) In the absence of electrical or magnetic field, these rays travel in straight lines
(iv) In the presence of electrical or magnetic field, the behavior of cathode rays are similar to that expected
from negatively charged particles, suggesting that the cathode rays consist of negatively charged particles,
called electrons.
(v) The characteristics of cathode rays(electrons) do not depend upon the material of electrodes and the
nature of the gas present in the cathode ray tube.
Charge to Mass Ratio of Electron
J.J. Thomson measured the ratio of electrical charge (e) to the mass of electron (me ) by using cathode ray
tube and applying electrical and magnetic field perpendicular to each other as well as to the path of
electrons .
e/me = 1.758820 x 1011 C kg-1
where me = Mass of the electron in kg
e = magnitude of charge on the electron in coulomb (C).
Discovery of proton anode ray
In 1886, Goldstein modified the discharge tube by using a perforated cathode. On reducing the pressure, he
observed a new type of luminous rays passing through the holes or perforations of the cathode and moving
in a direction opposite to the cathode rays. These rays were named as positive rays or anode rays or as
canal rays. Anode rays are not emitted from the anode but from a space between anode and cathode.
Properties of Anode Rays
(i) The value of positive charge on the particles constituting anode rays depends upon the nature of the gas
in the discharge tube.
(ii) The charge to mass ratio of the particles is found to depend on the gas from which these originate.
(iii) Some of the positively charged particles carry a multiple of the fundamental unit of electrical charge.
(iv)The behavior of these particles in the magnetic or electric field is opposite to that observed for electron
or cathode rays.
Proton: The smallest and lightest positive ion was obtained from hydrogen and was called proton. Mass of
proton = 1.676 x 10-27 kg
Charge on a proton = (+) 1.602 x 10-19 C
Neutron: It is a neutral particle. It was discovered by Chadwick (1932).
By the bombardment of thin sheets of beryllium with fast moving a-particles he observed • that highly
penetrating rays consist of neutral particles which were named neutrons.
16
,Thomson Model of Atom: 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
An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over
the atom.
Drawback of Thomson Model of Atom
This model was able to explain the overall neutrality of the atom, it could not satisfactorily, explain the
results of scattering experiments carried out by Rutherford in 1911.
Rutherford’s Nuclear Model of Atom
Rutherford in 1911, performed some scattering experiments in which he bombarded thin foils of metals
like gold, silver, platinum or copper with a beam of fast moving a-particles. The thin gold foil had a
circular fluorescent zinc sulphide screen around it. Whenever a-particles struck the screen, a tiny flash of
light was produced at that point.
The important observations are:
(i) Most of the a-particles passed through the foil without undergoing any deflection,
(ii) A few a-particles underwent deflection through small angles.
(iii) Very few mere deflected back i.e., through an angle of nearly 180°.
Conclusions:
(i) Since most of the a-particles passed through the foil without undergoing any deflection, there must be
sufficient empty space within the atom.
(ii) A small fraction of a-particles was deflected by small angles. The positive charge has to be
concentrated in a very small volume that repelled and deflected a few positively charged a-particles. This
very small portion of the atom was called nucleus.
(iii) The volume of nucleus is very small as compared to total volume of atom..
Rutherford’s Nuclear Model of an Atom
(i) The positive charge and most of the mass of the atom was densely concentrated in an extremely small
region. This very small portion of the atom was called nucleus by Rutherford.
(ii) The nucleus is surrounded by electrons that move around the nucleus with a very high speed in circular
paths called orbits.
(iii) Electrons and nucleus are held together by electrostatic forces of attraction
17
, Drawbacks of Rutherford Model
(i) Rutherford’s model cannot explain the stability of atom if the motion of electrons is described on the
basis of classical mechanics and electromagnetic theory.
(ii) Rutherford’s model does not give any idea about distribution of electrons around the nucleus and about
their energies.
Atomic Number: The number of protons present in the nucleus is equal to the atomic number (z). In
order to keep the electrical neutrality, the number of electrons in an atom is equal to the number of protons
(atomic number, z)
Atomic Number (z) = Number of protons in the nucleus of an atom.= Number of electrons in a neutral
atom
Mass Number :Number of protons and neutrons present in the nucleus are collectively known as nucleons.
The total number of nucleons is termed as mass number (A) of the atom.
Mass Number (A) = Number of protons (p) + Number of neutrons (n).
Isotopes :Atoms with identical atomic number but different atomic mass number are known as Isotopes.
Isotopes of Hydrogen: protium, deuterium and tritium.
Isobars :Isobars are the atoms with same mass number but different atomic number for Example
Developments Leading to the Bohr’s Model of Atom -Two developments played a major role in the
formulation of Bohr’s model of atom. These were:
(i) Dual character of the electromagnetic radiation which means that radiations possess both wave like and
particle like properties.
(ii) Experimental results regarding atomic spectra which can be explained only by assuming quantized
electronic energy levels in atoms.
Nature of Electromagnetic Radiation (Electromagnetic Wave Theory)
This theory was put forward by James Clark Maxwell in 1864. The main points of this theory are as
follows:
(i) The energy is emitted from any source (like the heated rod or the filament of a bulb through which
electric current is passed) continuously in the form of radiations and is called the radiant energy.
(ii) The radiations consist of electric and magnetic fields oscillating perpendicular to each other and both
perpendicular to the direction of propagation of the radiation.
(iii) The radiations possess wave character and travel with the velocity of light 3 x 108 m/sec.
(iv) These waves do not require any material medium for propagation. For example, rays from the sun
reach us through space which is a non-material medium.
Characteristics of a Wave –
(i)Wavelength: It is defined as the distance between any two consecutive crests or troughs. It is
represented by X and its S.I. unit is metre.
(ii)Frequency: Frequency of a wave is defined as the number of waves passing through a point in one
second. It is represented by v (nu) and is expressed in Hertz (Hz).
1 Hz = 1 cycle/sec. Velocity:
(iii) Velocity of a wave is defined as the linear distance travelled by the wave in one second.
It is represented by c and is expressed in cm/sec or m/sec.
(iv)Amplitude: Amplitude of a wave is the height of the crest or the depth of the through. It is represented
by V and is expressed in the units of length.
(v)Wave Number: It is defined as the number of waves present in 1 metre length. Evidently it will be
equal to the reciprocal of the wavelength. It is represented by bar v (read as nu bar).
Electromagnetic Spectrum: When electromagnetic radiations are arranged in order of their increasing
wavelengths or decreasing frequencies, the complete spectrum obtained is called electromagnetic
spectrum.
Limitations of Electromagnetic Wave Theory
Electromagnetic wave theory was successful in explaining properties of light such as interference,
18
