Neils Bohr, a Danish physicist, in 1913 proposed model of atom which rectified the problems left by Rutherford’s Model. He proposed that

Electrons revolve round the nucleus in a fixed orbit.

He called these orbits as ‘stationary orbit’.

Each stationary orbit is associated with fixed amount of energy, thus electrons do not radiate energy as long as they keep on revolving around the nucleus in fixed orbit.

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The circular path around the nucleus is called orbit, energy level or shell. Energy level are represented by letter – K, L, M, N, …. and so on.

Therefore,

1^{st} orbit is denoted by – K

2^{nd }orbit is denoted by – L

3^{rd }orbit is denoted by – M, and so on.

The orbits are denoted by 1, 2, 3, …. and so on.

Distribution of Electrons in a Orbit or Shell:

The distribution of electrons in an orbit can be obtained by using formulae 2n^{ 2}

where ‘n’ is number of orbit.

Number of electrons in K-shell i.e. in 1^{st} orbit.

Here n = 1

Therefore,

`2n^2 = 2 xx 1^2 = 2`

Thus, maximum number of electrons in K-shell i.e. 1^{st }shell = 2

Number of electrons in L-shell, i.e. in 2^{nd}orbit

Here n = 2, therefore,

`2n^2= 2 xx 2^2= 8`

Thus, maximum number of electrons in L-shell = 8

Number of electrons in M-shell, i.e. in 3^{rd }orbit

Here n = 3, therefore,

`2n^2= 2 xx 3^2= 18`

Thus, maximum number of electrons in M-shell = 18

Number of electrons in N-shell, i.e. in 4^{th} shell

Here n = 4, therefore,

`2n^2= 2 xx 4^2= 32`

Thus, maximum number of electrons in N-shell = 32

Number of electrons in O-shell, i.e. in 5^{th} shell

Here n = 5, therefore,

`2n^2= 2 xx 5^2= 50`

Thus, maximum number of electrons in O-shell = 50

Number of electrons in P-shell, i.e. in 6^{th} shell

Here n = 6, therefore,

`2n^2= 2 xx 6^2 = 72`

Thus, maximum number of electrons in P-shell = 72

In similar way maximum number of electrons in any shell can be calculated.

A higher number of n means the orbit is farther from the nucleus. The orbit which is farther from the nuclues has higher level of energy. The orbit which is nearer the nucleus has lower level of energy.

When an electron continues to move in a definite orbite, it does not lose energy.

When an atom gets energy from any external source then an electron jumps to a higher energy level.

When an electron loses some energy, it jumps down to a lower energy level.