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,
(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.