Question

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

Answer 1

The distribution of electrons in an atom is based on the following rules :
1. The maximum number of electrons which a shell can have is represented by $2n^2$, where $n$ is the quantum number of that particular energy shell. Thus, the maximum number of electrons in the first four shells are :

1st (K) shell : 2 x $1^2=2$
2nd (L) shell: $2\times 2^2=8$

12=2

2. The outermost shell, which is also called the valence shell, can have a maximum of 8 electrons.

3. The shell next to (or inner to) the outermost shell, which is called the penultimate shell, can accommodate a maximum of 18 electrons, (if permitted by rule 1).
4. Electrons are not accommodated in a given shell unless the inner shells are filled, i.e., the shells are filled in a step-wise manner.