The distribution of electrons into different orbits of an atom was suggested by Bohr and Bury.

The following rules are followed for writing the number of electrons in different energy levels or shells:

(i) **The maximum number of electrons present in a shell is given by the formula 2n ^{2}** , where ‘n’ is the orbit number or energy level index, 1,2,3,….

Hence the maximum number of electrons in different shells are as follows:

first orbit or K-shell will be = 2 × 1

^{2}= 2,

second orbit or L-shell will be = 2 × 2

^{2}= 8,

third orbit or M-shell will be = 2 × 3

^{2}= 18,

fourth orbit or N-shell will be = 2 × 4

^{2}= 32, and so on.

(ii**) The maximum number of electrons that can be accommodated in the outermost orbit is 8.**

(iii) Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.

Atomic structure of the first eighteen elements