Question

Which of the following are isoelectronic species i.e., those having the same number of electrons?
$Na^+, K^+, Mg^{2+}, Ca^{2+}, S^{2–}, Ar$

Answer 1

Isoelectronic species have the same number of electrons.
Number of electrons in sodium (Na) = 11
Number of electrons in Na+ = 10
A positive charge denotes the loss of an electron.
Similarly,
Number of electrons in K+ = 18
Number of electrons in Mg2+ = 10
Number of electrons in Ca2+ = 18
A negative charge denotes the gain of an electron by a species.
Number of electrons in sulphur (S) = 16
∴ Number of electrons in S2- = 18
Number of electrons in argon (Ar) = 18

Hence, the following are isoelectronic species:

1. Na+ and Mg2+ (10 electrons each)
2. K+, Ca2+, S2- and Ar (18 electrons each)