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 '2n2 ', 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
2n2 = 2 × 1 2 = 2
Similarly, for second orbit, it is 2n2 = 2 × 2 2 = 8
For third orbit, it is 2n2 = 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.