Question

Which one of the following group of atoms or ions is not isoelectronic?

• A. He, H^-, Li⁺
• B. Na+, Mg²⁺, Al³⁺
• C. F^-, O^2-, N^3-
• D. K⁺, Ca²⁺, Ne

Answer 1

The correct option is D

K⁺, Ca²⁺, Ne

Do you remember the definition of isoelectronic? As the name suggests two entities are said to be isoelectronic if they have the same number of electrons.

If we calculate the number of electrons of all the options, we find that

$K^{+}-18electrons,C{a}^{2+}-18electrons,Ne-10electrons$

$He-2electrons,H^{-}-2electrons,L{i}^{+}-2electrons$

$N{a}^{+}-10electrons,M{g}^{2+}-10electrons,A{l}^{3+}-10electrons$

$F^{-}-10electrons,O^{2-}-10electrons,N^{3-}-10electrons$

Now we can see that $K^{+}$, $C{a}^{2+}$ and $Ne$ are not isoelectronic because the number of electrons is not the same.