Summarise the rules for writing the distribution of electrons in various shells for the first eighteen elements.

The following rules are followed for writing the distribution of electrons in different energy levels or shells for the first eighteen elements:
(i) The maximum number of electrons present in a shell is given by the formula 2n², 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 × 1² = 2,
second orbit or L-shell will be = 2 × 2² = 8,
third orbit or M-shell will be = 2 × 3²= 18,
fourth orbit or N-shell will be = 2 × 4²= 32,
(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.