In which of the following pairs are the ions isoelectronic?

Isoelectronic species are atoms or ions that possess the same number of electrons and identical electronic configurations. To determine isoelectronic pairs, one must calculate the total electrons by adjusting the atomic number for the ionic charge. Sodium (Na) has an atomic number of 11; the Na+ ion loses one electron, leaving it with 10 electrons [1]. Oxygen (O) has an atomic number of 8; the O2- ion gains two electrons, also resulting in 10 electrons [1]. Thus, Na+ and O2- are isoelectronic, both sharing the configuration of the noble gas Neon (Ne). In contrast, Mg2+ (10 electrons) and Ar (18 electrons) are not isoelectronic [1]. Similarly, Al3+ (10 electrons) and Cl- (18 electrons) differ, as do K+ (18 electrons) and Ne (10 electrons) [1].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > 3.3 HOW DO METALS AND NON-METALS REA ALS REACT? > p. 47