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

Explanation

The various rules for writing of distribution of electrons in various shells of an atom are as follows :
(i) The maximum number of electrons which can be accommodated in any energy level of the atom of an element is given by `2n^(2)` (where n is the number of that energy level). By using this relation we will find that :
(a) the maximum number of electrons which can be accommodated in 1st energy level (n = 1) or K shell is `2 xx (1)^(2)=2xx1=2`
(b) the maximum number of electrons which can be accommodated in 2nd energy level (n = 2) or L shell is `2 xx (2)^(2) = 2xx 4 = 8`
(c) the maximum number of electrons which can be accommodated in 3rd energy level (n = 3) or M shell is `2xx (3)^(2) = 2 xx 9 = 18`
(d) the maximum number of electrons which can be accommodated in 4th energy level (n = 4) or N shell is `2xx (4)^(2) = 2xx 16 = 32`
(ii) The outermost shell of an atom cannot accomodate more than 8 electrons, even if it has the capacity to accommodate more electrons (If, however, the outermost shell of an atom is the first shell or K shell, then it cannot accommodate more than 2 electrons)
(iii) The electrons in an atom do not occupy a new shell unless all the inner shells are completely filled with electrons. This means that first all the electrons fill K shell, then L shell, then M shell, then N shell, and so on.