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

Na+, K+, Mg2+, Ca2+, S2–, 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 [S2-] = 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+, S2- and Ar [18 electrons each]


22