A formula unit in chemistry is the empirical formula of an ionic or covalent network solid compound used as an independent entity for stoichiometric calculations. It is the lowest whole number ratio of ions represented in an ionic compound. Examples include ionic NaCl and K2O and covalent networks such as SiO2 and C (as diamond or graphite).
Ionic compounds do not exist as individual molecules; a formula unit thus indicates the lowest reduced ratio of ions in the compound.
A formula unit shows the kinds and numbers of atoms in the smallest representative unit of a substance.