Which set of ions contains isoelectronic species?

Isoelectronic species are ions or atoms that have the same number of electrons. To determine which set of ions contains isoelectronic species, we examine the electron configurations of the ions in each choice.

In the case of the set mentioned, calcium ion (Ca²⁺) has lost two electrons, bringing it to a total of 18 electrons, corresponding to the electron configuration of Argon (Ar), which also has 18 electrons. The chloride ion (Cl⁻) has gained one electron, resulting in a total of 18 electrons as well. Therefore, Ca²⁺, Cl⁻, and Ar all share the same number of electrons, making them isoelectronic.

The presence of three ions that all have 18 electrons confirms that this answer is correct, as they share a common electron configuration, even though they belong to different elements. This comparison illustrates the nature of isoelectronic species and reinforces the concept of electron configurations in determining whether species are isoelectronic.