Question

Which of the following species is not isoelectronic with the others?

• A. $N{a}^{+}$
• B. $M{g}^{2+}$
• C. $O^{2-}$
• D. $C{l}^{-}$

Answer 1

The correct option is D $C{l}^{-}$

Isoelectronic species are those which have the same number of electrons.
Electrons in $N{a}^{+}=10$
Electrons in $M{g}^{2+}=10$
Electrons in $O^{2-}=10$
Electrons in $C{l}^{-}=18$
So, $C{l}^{-}$ ion is not isoelectronic with the other options.