Question

The total number of electrons in a nitrogen atom and a chlorine atom are $7$ and $17$ respectively. Find the number of valence electrons in them.
(A) $3,1$
(B) $5,8$
(C) $4,1$
(D) $5,7$

Answer 1

As we already know that the half filled and completely filled valence shells are considered to be stable. So in order to attain stability the electrons try to bond . and the electrons present in the last shell are considered as valence electrons.

Complete step by step solution:
We will firstly discuss the electronic configuration of the nitrogen and the chlorine atom. So as we know that the nitrogen is having the atomic number seven and its electronic distribution is as follows in which two electrons are there in $1s$ and then two electrons in $2s$ orbital and then three electrons in the $2p$ orbital as there are three electrons in the p orbital as it is half filled and also here the p orbital is considered as the valence shells and it is considered to be stable. The electronic configuration is shown below:
$1{s^2}2{s^2}2{p^3}$
As we already know that the half filled and completely filled valence shells are considered to be stable. Also from this discussion we can conclude that the only three electrons are involved in the bond formation.
We will now discuss the electronic configuration of chlorine . as we know that the atomic number of chlorine is seventeen. Further its electronic configuration is as follows:
$\left[ {Ne} \right]3{s^2}3{p^5}$
In this the neon being the ideal gas has the electronic configuration with complete first and second shell and then the complete $3s$ orbital and finally comes the $3p$ in which five electrons are present which is neither completely filled nor half filled but there is a lack of one electron to get completely filled hence, it is not stable.
From this we can conclude that the option A is correct .

Note:
The criteria of each electron to attain stability as the process for attaining stability marks the existence of the valence electrons so this should be kept in mind. Also, when an electron loses energy then it moves to an inner shell which is not fully occupied.