Which of the following pairs represents isoelectronic ions?

Isoelectronic species are defined as atoms, ions, or molecules that possess the same number of electrons and identical electronic configurations [1]. To identify the correct pair, we calculate the electron count for each ion. Calcium (Ca) has an atomic number of 20; losing two electrons to form Ca2+ results in 18 electrons. Sulfur (S) has an atomic number of 16; gaining two electrons to form S2- results in 18 electrons. Since both Ca2+ and S2- have 18 electrons, they are isoelectronic. Other options are incorrect: Na+ (10e-) and K+ (18e-) differ, as do Mg2+ (10e-) and Ca2+ (18e-). Potassium (K) as an atom has 19 electrons, while Mg2+ has 10, making them non-isoelectronic.

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