Which pair of atoms is most likely to form an ionic bond?

The pairing of calcium and chlorine is most likely to form an ionic bond due to the differing electronegativities of the two elements. Calcium is an alkaline earth metal with a tendency to lose two electrons, thereby acquiring a positive charge. Chlorine, on the other hand, is a halogen with a strong tendency to gain an electron to achieve a full outer electron shell, resulting in a negative charge.

When calcium loses its two valence electrons, it becomes a positively charged ion (Ca²⁺). Chlorine, by gaining an electron, becomes a negatively charged ion (Cl⁻). The opposite charges of these ions create a strong electrostatic attraction, which is the essence of an ionic bond.

In contrast, the other options involve pairs of atoms that are either less likely to engage in the transfer of electrons necessary for ionic bonding or are more suited for covalent bonding due to their electronegativities and atomic characteristics. Thus, the combination of calcium and chlorine stands out as the pair most likely to result in the formation of an ionic bond.