Main postulates of Bohr’s atomic theory are:
1. Electrons revolve around the nucleus in circular path, which are known as "ORBITS" or "ENERGY LEVEL".
2. Energy of an electron in one of its allowed orbits is fixed.As long as an electron remains in one of its allowed orbit, it can not absorb or radiate energy.
3. If an electron jumps from lower energy level to a higher energy level, it absorbs a definite amount of energy.
4. If an electron jumps form higher energy level to a lower energy level, it radiates a definite amount of energy.
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5. Energy released or absorbed by an electron is equal to the difference of energy of two energy levels.
Let an electron jumps from a higher energy level E2 to a lower energy level E1.The energy is emitted in the form of light .Amount of energy released is given by:
E = E2 - E1
E2 - E1= h
h = Planck's consrant ( 6.6256 x 10-34 j.s)
= Frequency of radiant light
6. Spectrum of light emitted from an electron is a "LINE SPECTRUM".
7. Angular momentum of an electron is given by:

m v r = nh /2

Where n =1, 2, 3, ………..

m = mass of electron

V = velocity of electron

r = radius of orbit