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

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 S^2- = 18

Number of electrons in argon (Ar) = 18

Hence, the following are isoelectronic species:

1) Na⁺ and Mg²⁺ (10 electrons each)

2) K⁺, Ca²⁺, S^2– and Ar (18 electrons each)