Question

Which of the following are isoelectronic species, i.e., those having the same number of electrons?

Na⁺, K⁺, Mg²⁺, Ca²⁺, S^2-, Ar.

Answer 1

Species	Atomic number	No. of electrons on the species
Na+	11	10
K+	19	18
Mg2+	12	10
Ca2+	20	18
S2-	16	18
Ar	18	18

Thus, Na⁺ and Mg²⁺ are isoelectronic and K⁺, Ca²⁺, S^2- and Ar are also isoelectronic.