Lone Pair
A pair of valence electrons localised on one atom and not involved in bonding.
Unit
Dimensionless
In depth
Lone pairs occupy more angular space than bonding pairs because they are held by only one nucleus, so they compress adjacent bond angles and distort molecular shape. They also make a molecule a Lewis base or a nucleophile, and are the electrons donated in every coordinate bond.
Examples in the real world
Ammonia's single lone pair reduces its H–N–H angle to 107°; water's two lone pairs reduce it further to 104.5°.