A lone pair refers to a pair of valence electrons that are not shared with another atom and remain localized on a single atom. These electrons are often found in the outermost shell of an atom and can influence the atom's chemical behavior, including bond angles and molecular geometry. Lone pairs are important in the formation of molecular shapes, as they can repel other electron pairs (both bonding and lone pairs) according to the principles of VSEPR (Valence Shell Electron Pair Repulsion) theory.
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A lone pair is an electron pair assigned to one chemical atom without a connecting line to another chemical atom in a Lewis structure. It is usually shown as two dots. Water has two lone pairs on oxygen, accounting for four valence electrons; ammonia has one lone pair on nitrogen, accounting for two. In , each oxygen has two lone pairs, and carbon has none.
A lone pair can be reactive. In protonation of ammonia, the nitrogen lone pair becomes a new bond pair to a proton:The proton supplies no electron, so the eight valence electrons are retained: three bond pairs plus one lone pair become four bond pairs. The resulting nitrogen has formal charge ; each hydrogen has formal charge zero. This is why identifying an available lone pair helps explain the chemical base behaviour of ammonia, rather than merely decorating its formula.
Lewis already used the ammonia to example in The Atom and the Molecule (1916), pp. 777–778. In modern terms the dots identify an allocation of electrons, not two stationary particles or an isolated patch unaffected by the rest of the molecule. A molecular orbital description can spread electronic amplitude over several chemical atoms while a localized Lewis structure remains useful for acid-base chemistry.