2. Contents:
Structure of an atom
Rutherford’s atomic model
Bohr’s atomic model
Electrons and energy levels
Nuclear elements and nuclear stability
Radioactivity
Decay process
Interaction of radiation with matter
6. Drawbacks:
According to Larmor theory electron
loses energy while orbiting nucleus.
Hence atom will collapse in 16
picoseconds
7. Bohr’s atomic model
According to Neil Bohr’s theory electron can have
only classical motions:
◦ Electrons in atoms orbit the nucleus.
◦ The electrons can only orbit stably, without
radiating, in certain orbits at a certain discrete set
of distances from the nucleus.
◦ Electrons can only gain and lose energy by
jumping from one allowed orbit to another,
absorbing or emitting electromagnetic radiation
with a frequency ν determined by the energy
difference of the levels according to the Planck
relation:
8. Electrons and energy levels
Mass of an electron: 9.1*10-28 g.
Charge: 1.59*10-19 coulombs.
A particle with same mass but a
positive charge is called positron.
No. of electrons in a shell is given by:
2n2 .
Binding energy of a single electron of
Hydrogen atom: E
b=-13.6/n2 eV
9. Nuclear elements
Fundamental constituents: neutrons
and protons.
Mass of proton: 1.672*10-24 g.
Neutron:
◦ No charge, mass ~ proton.
◦ Limited life in free state. Later
disintegrates into electron and proton.
Symbol:
10. Nuclear stability
Stability depends on n/p ratio.
Extra energy is released by the
nucleus by expelling particles or
photons.
11. Radioactivity
The process of transmutation of an
unstable element to another element
through the emission of radiation.
Unit: curie (Ci) (3.7*1010
disintegration/sec).
Radioactive decay law:
Half life of an element:
18. Decay Process
Isotopes: Z same, A different
e.g 1H1,1H2, 1H3
Isobars: A same, Z different
e.g. 40S, 40Cl
Isotones: Same number of neutrons.
e.g. Cl-37, K-39
Isomeres: They are different excited
states of the same type of nucleus.
Electron capture:
19. Isomeric and Isobaric
Transition
A nuclear process in which a nucleus
following the emission of an alpha
particle or a beta particle emits energy
without changing its number of protons
or neutrons.
21. Compton Effect
The Compton effect (Compton scattering)
is the result of a high-energy photon
colliding with a target, which releases
loosely bound electrons from the outer
shell of the atom or molecule. The
scattered radiation experiences a
wavelength shift.
22. Pair Production
Pair production is the creation of
an elementary particle and
its antiparticle, for example
an electron and the positron.
occurs when a photon interacts with a
nucleus.