**Question.**

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

**Solution:**

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 $' 2 n^{2}$ ', where ' $n$ ' is the orbit number or energy level index $(n=1,2,3 \ldots)$

The maximum number of electrons present in an orbit of $n=1$ is given by $2 n^{2}=2 \times 1^{2}=2$

Similarly, for second orbit, it is $2 n^{2}=2 \times 2^{2}=8$

For third orbit, it is $2 \mathrm{n}^{2}=2 \times 3^{2}=18$

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 stepwise manner i.e., the outer shell is not occupied with electrons unless the inner shells are completely filled with electrons.

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