Which type of bond occurs when electrons are not shared equally between two atoms?

A polar covalent bond occurs when electrons are not shared equally between two atoms. This typically happens when two different nonmetals with differing electronegativities form a bond. Electronegativity is the tendency of an atom to attract electrons; when one atom is more electronegative, it pulls the shared electrons closer to itself, creating a slight charge imbalance. This results in a molecule with a positive end and a negative end, leading to the term "polar."

In contrast, a nonpolar covalent bond involves equal sharing of electrons between two identical or similar atoms, resulting in no charge separation. Ionic bonds occur when electrons are transferred from one atom to another rather than shared, resulting in charged ions. Metallic bonds involve a 'sea of electrons' shared among a lattice of metal atoms, which differs fundamentally from the nature of covalent bonding. Therefore, the correct identification of a polar covalent bond highlights the unequal sharing of electrons, which is the key characteristic distinguishing it from the other types of bonds mentioned.