STRUCTURE OF ATOM
SUB ATOMIC PARTICLES
DISCOVERY OF SUB – ATOMIC
PARTICLES
CATHODE RAY EXPERIMENTS AND ITS FINDINGS,
CHARGE TO MASS RATIO OF ELECTRON- THOMSON’S
EXPERIMENT,
CHARGE ON THE ELECTRONS ON THE BASIS OF MILLIKAN’S
OIL DROP METHOD,
DISCOVERY OF PROTONS AND NEUTRONS
ATOMIC NUMBER AND MASS NUMBER
• INTRODUCTION - It is equal to the no. of protons present in the
nucleus of atoms. It is also equal to the no. of electrons in an atom.
• ISOTOPES AND ISOBARS - The atoms with same atomic no. but
different mass no. are known as isotopes, whereas the reverse is
true for isobar.
• RUTHERFORD’S MODEL AND ITS DRAWBACKS• Scattering experiment done with gold leaf with thickness 100 nm
• Bombarding with fast moving alpha – particles and discovery of
nucleus o atom.
• Drawbacks – could not explain the stabilioty of an atom .
Wave nature of electron
. Maxwell suggested that when
electrically charged particle moves under
acceleration, alternating electrical and
magnetic fields are produced and
transmitted. These fields are transmitted in
the forms of waves called electromagnetic
waves or electromagnetic radiation.
Wave nature of light
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Light is the form of radiation known from
early days and speculation about its nature
dates back to remote ancient times. In earlier
days (Newton) light was supposed to be made
of particles (corpuscules).
Particle nature of light
(i) the nature of emission of radiation from
hot bodies (black -body radiation)
(ii) ejection of electrons from metal surface
when radiation strikes it (photoelectric
effect)
(iii) variation of heat capacity of solids as a
function of temperature
(iv) line spectra of atoms with special
reference to hydrogen.
PHOTO ELECTRIC EFFECT
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In 1887, H. Hertz performed a very interesting
experiment in which electrons (or electric
current) were ejected when certain metals (for
example potassium, rubidium, caesium etc.)
were exposed to a beam of light as shown in
The phenomenon is called Photo electric
effect.
DUAL BEHAVIOUR OF LIGHT
LIGHT
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possesses both particle and wave-like
properties, i.e., light has dual behaviour.
Depending on the experiment, we find that
light behaves either as a wave or as a stream
of particles. Whenever radiation interacts with
matter, it displays particle like properties in
contrast to the wavelike properties
(interference and diffraction), which it
HYDROGEN SPECTRUM
• The hydrogen spectrum consists
of several series of lines named after their
discoverers. Balmer showed in 1885 on the
basis of experimental observations that if
spectral lines are expressed in terms of
wavenumber (n ).
De Broglie equation
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as the photon has momentum as well as
wavelength, electrons should also have
momentum as well as wavelength, de Broglie,
from this analogy, gave the following relation
between wavelength (l) and momentum (p) of
a material particle.
HEIGENBERG’S UNCERTAINTY
PRINCIPLE
It states that it is impossible to
determine the exact position and
momentum of electron
simultaneously.
QUANTUM NUMBERS
• It determines the complete address (identity )
of an electron .
• Principle Quantum Number ,
• Azimuthal Quantum Number,
• Magnetic Quantum Number
• Spin Quantum Number
Shapes of Atomic Orbitals
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‘s’ orbital is spherical,
‘p’ orbital is dumb-bell shaped,
‘d’ orbital is double dumb-bell shaped,
‘f’ orbital is highly complex.
Pauli’s Exclusion Principle
• No two electrons in an atom can have the
same set of four quantum numbers .
• Only two electrons may exist in the same
orbital and these electrons must have
opposite spin.
Hund’s Rule of Maximum Multiplicity
• Pairing of electrons in the orbitals belonging
to the same subshell (p,d,f) does not take
place until each orbital belonging to that
subshell has got one electron each.
• Half- filled or fully- filled orbitals are more
stable than the incompletely filled orbitals.
Reason for this is its symmetry and exchange
energy .
Aufbau Principle
The lower the value of (n+l) for an orbital, the lower
is its energy . If two orbitals have the same value
of (n+l) , the orbital with lower value of n will have
the lower energy and in the ground state of the
Atoms, the orbitals are filled in order of their
increasing energies :
1s, 2s, 2p , 3s , 3p , 4s, 3d, 4p, 5s,4d, --