The rules for writing of the distribution of electrons in various shells for the first eighteen elements are given below:

(i) The maximum number of electrons that a shell can accommodate is given by the formula '2n^{2} ', where 'n' is the orbit number or energy level index (n = 1, 2, 3...).

The maximum number of electrons present in an orbit of n = 1 given by

2n^{2} = 2 × 1 2 = 2

Similarly, for second orbit, it is 2n^{2} = 2 × 2 2 = 8

For third orbit, it is 2n^{2} = 2 × 3 2 = 18

And so on .....

(ii) The outermost orbit can be accommodated by a maximum number of 8 electrons.

(iii) Shells are filled with electrons in a step wise manner i.e., the outer shell is not occupied with electrons unless the inner shells are completely filled with electrons.