lonepair

A lone pair, or nonbonding pair, is a pair of valence electrons localized on a single atom and not shared in covalent bonds. In Lewis structures they appear as two dots on the atom. Lone pairs arise from the octet (or expanded octet) of electrons and influence the atom’s chemistry but do not themselves form bonds.

In VSEPR theory, lone pairs occupy more space than bonding electron pairs, affecting molecular geometry and bond angles. For example, water has two lone pairs on oxygen, giving a bent shape with a bond angle of about 104.5 degrees; ammonia has one lone pair on nitrogen and adopts a roughly trigonal pyramidal shape with bond angles near 107 degrees. Molecules with no lone pairs on the central atom, like carbon dioxide or methane, tend toward linear or tetrahedral geometries as predicted by their bonding arrangement.

Lone pairs can act as Lewis bases, donating electron density to electron-deficient species to form coordinate covalent bonds. They also participate in resonance, influencing the distribution of electron density in molecules such as carbonate, nitro, and amide systems. The presence of lone pairs affects molecular polarity, acidity, basicity, and reactivity patterns.

In practice, chemists identify the number of lone pairs by counting valence electrons and subtracting shared electrons in bonds. Special cases include hypervalent species and transition-metal complexes where d-orbital participation alters simple counting.