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

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.