Find the number of electrons present in the ${\mathrm{Br}}^{-}$ion which are having the $(+1)$ value of magnetic quantum number 

The electronic configuration of ${\mathrm{Br}}^{-}$ion is 

${\mathrm{Br}}^{-}\Rightarrow 1s^{2}2s^2{\underset{ }{\mathit{2}p}}^{6}3s^2{\underset{ }{\mathit{3}p}}^{6}4s^2{\underset{ }{\mathit{3}d}}^{10}{\underset{ }{\mathit{4}p}}^6$

The number of electrons present in the subshell 2p,3p,3d,4p are 2,2,2,2 respectively.

The sum of the electrons are,

n=2+2+2+2

n=8

Hence, the number of electrons present in the ${\mathrm{Br}}^{-}$ion which are having the $(+1)$ value of magnetic quantum number is 8.