Final answer:
Cl⁻ and Ca²⁺ are isoelectronic as they both have the same number of electrons (18), matching the electron configuration of argon ([Ne]3s²3p⁶).
Step-by-step explanation:
To determine which species make an isoelectronic pair, you need to compare their electron configurations. Isoelectronic species are atoms or ions that have the same number of electrons and hence the same electron configuration. Looking at the provided options: Cl⁻ has 18 electrons, O² has 10 electrons, F has 9 electrons, Ca²⁺ has 18 electrons, and Fe³⁺ has 23 electrons. Therefore, Cl⁻ (which is chlorine with an extra electron, total 18) and Ca²⁺ (calcium missing two electrons, total 18) are isoelectronic because they both have the electron configuration of argon, [Ne]3s²3p⁶.
It is important to note that while they are isoelectronic, the nuclear charge differs, which means Ca²⁺ will have a smaller ionic radius than Cl⁻ due to the greater attraction of electrons towards the larger number of protons in calcium.