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.
