Which of the following should be the number of electrons present in $${X^{2 + }}$$ on the basis of electronic configuration, if the ion $${X^{3 - }}$$ has 14 protons ?

Solution :
$${X^{3 - }}$$  has 14 protons, i.e., $$X$$ also has 14 protons and therefore 14 electrons
$$\eqalign{ & X = 14 = 1{s^2},2{s^2}2{p^6},3{s^2}3{p^2} \cr & {X^{2 + }} = 1{s^2},2{s^2}2{p^6},3{s^2} = 12\,{\text{electrons}} \cr} $$