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

Na⁺, K⁺, Mg²⁺, Ca²⁺, S^2–, Ar.

Isoelectronic species are defined as those species which belong to different atoms or ions which possess same number of electrons but different magnitude of nuclear charge.

A positive charged ion denotes the loss of an electron & A negative charged ion represents the gain of an electron by a species.

1] Number of electrons in sodium [Na] = 11

Therefore, Number of electrons in sodium ion [Na⁺] = 10

2] Number of electrons in potassium ion [K⁺] = 18

3] Number of electrons in magnesium ion [Mg²⁺] = 10

4] Number of electrons in calcium ion [Ca²⁺] = 18

5] Number of electrons in sulphur [S] = 16

∴ Number of electrons in sulphur ion [S^2-] = 18

6] Number of electrons in argon [Ar] = 18

Hence, the following ions are isoelectronic species:

1] Na⁺ and Mg²⁺ [10 electrons each]

2] K⁺, Ca²⁺, S^2- and Ar [18 electrons each]