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

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 (n = 1) or K-shell will be = 2 × 12 = 2,
second orbit (n = 2) or L-shell will be = 2 × 22 = 8,
third orbit (n = 3) or M-shell will be = 2 × 32 = 18,
fourth orbit (n = 4) or N-shell will be = 2 × 42 = 32
and so on.

(ii) The maximum number of electrons that can be accommodated in the outermost orbit is 8 for all shells except for K-shell which can accommodate only 2 electrons .

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