covalency

Covalency is a term used in chemistry to describe the tendency of an atom to form covalent bonds by sharing electron pairs with other atoms. In practical terms, covalency is often described as the number of covalent bonds an atom can form, or equivalently, the number of electrons it can contribute to shared electron pairs.

For main-group elements, covalency typically aligns with the need to complete an outer shell. Carbon commonly forms four covalent bonds, nitrogen three, oxygen two, and halogens one. However, covalency is not a fixed property. Some elements can exceed the octet by using additional orbitals, enabling hypervalent compounds. Examples include sulfur in sulfur hexafluoride (SF6), which demonstrates six covalent bonds, and phosphorus in phosphorus pentachloride (PCl5) with five covalent bonds.

In chemical bonding, covalency is influenced by electronegativity differences between atoms. Smaller differences generally favor covalent character, while larger differences tend toward ionic character. Covalency is thus a spectrum, with bonds ranging from highly covalent to more ionic, depending on the atoms involved and the molecular context.

Modern descriptions of covalency rely on theories such as valence-bond theory and molecular orbital theory. These frameworks describe covalent bonds in terms of sharing electron density and bond order rather than a single fixed count. Consequently, covalency is best viewed as a property describing the potential for electron sharing in a molecule, varying with chemical environment and bonding situation.

Covalency is a foundational concept across chemistry, informing the understanding of organic molecules, inorganic compounds, and coordination chemistry.