The rules for writing of the distribution of electrons in various shells for the first eighteen elements are given below.

$\rightarrow$ If $n$ gives the number of orbit or energy level, then $2 n^{2}$ gives the maximum number of electrons possible in a given orbit or energy level.

Thus,

First orbit or $K-$ shell will have $2$ electrons,

Second orbit or $L-$ shell will have $8$ electrons,

Third orbit or $M-$ shell will have $18$ electrons.

$\rightarrow$ If it is the outermost orbit, then it should have not more than $8$ electrons.

$\rightarrow$ There should be step-wise filling of electrons in different orbits, i.e., electrons are not accompanied in a given orbit if the earlier orbits or shells are incompletely filled.