In 1913, Neil Bohr proposed a new model of the atom which not only explained the drawbacks of Rutherford’s model but also the emission spectrum of hydrogen. Bohr’s theory was based on Plank’s quantum theory and was built on the following postulates;

1. Electrons revolve round the nucleus only in certain circular orbits. These orbits are associated with definite energies and are called Energy Shells or Energy Levels.

2. Only those orbits are permitted in which angular momentum (mvr) of electron is an integral multiple of h/2π i.e.

mvr = nh/2π

where, m = mass of electron, n = number of orbit in which electron is present, v = velocity of the electron, r = radius of the orbit, h = Planck’s constant.

3. As long as the electron remains in a particular orbit, it does not lose or gain energy. This means that energy of an electron in a particular orbit remains constant. That is why, these orbits are also called as Stationary States. This orbital rotation without emitting energy follows the Newtonian law; i.e. the force of attraction between the nucleus and electron is equal to the centrifugal force of moving electron.

4. When energy some external source is supplied to the electron, it may jump to some higher energy level by absorbing a definite amount of energy. When electron jumps back to the lower energy level it radiates same amount of energy in the form of light radiations.

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