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 × 1² = 2 Similarly, for second orbit, it is 2n² = 2 × 2² = 8 For third orbit, it is 2n² = 2 × 3² = 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.

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