Which of the following species are considered isoelectronic?

The concept of isoelectronic species refers to atoms, ions, or molecules that have the same number of electrons, even though they may differ in nuclear charge or overall structure. To determine if a set of species is isoelectronic, we count the total number of electrons for each pair.

For the pair of species given in the correct answer, Na⁺ has lost one electron, making its electron configuration akin to that of neon, which contains ten electrons. F⁻, having gained one electron, also achieves an electron configuration equivalent to neon’s. Therefore, both Na⁺ and F⁻ have ten electrons, making them isoelectronic.

In the other options, CO₃²⁻ and ClO₃⁻ do not constitute isoelectronic species since CO₃²⁻ has a total of 26 electrons, while ClO₃⁻ contains a different total. Ar and Ca²⁺ also do not align as isoelectronic because Ar has 18 electrons, but Ca²⁺ has 20 electrons due to the loss of two, making them not share the same electron count. H₂O and H₂S have different numbers of electrons as well, with