When a pair of electrons is shared by two atoms, what type of bond exists between them?

When a pair of electrons is shared between two atoms, this creates a covalent bond. Covalent bonding typically occurs between nonmetals and involves the mutual sharing of one or more pairs of electrons to achieve a full outer electron shell for each atom, thus stabilizing both. This sharing allows each atom to gain the electron configuration similar to that of noble gases, which is a more stable state.

Covalent bonds can vary in strength and length depending on the number of shared electron pairs. For example, a single covalent bond involves one shared pair of electrons, a double bond involves two pairs, and a triple bond involves three pairs. This characteristic distinguishes covalent bonds from other types of bonds, such as ionic, which involve the transfer of electrons from one atom to another rather than sharing, and metallic, which involve a 'sea of electrons' that are free to move around metal atoms.

Hydrogen bonds, while a significant intermolecular force, are not true bonds in the strict sense; they arise from the attraction between a hydrogen atom covalently bonded to a highly electronegative atom (like oxygen or nitrogen) and another electronegative atom. This underlines the unique nature of covalent bonding as the direct sharing of electrons.