Formula Unit
The simplest whole-number ratio of ions represented in the empirical formula of an ionic or network compound.
Unit
Dimensionless
In depth
Ionic solids are continuous three-dimensional lattices with no discrete molecules, so the formula unit is a bookkeeping device rather than a real particle. Its mass, the formula mass, plays exactly the role that molecular mass plays for covalent substances in mole calculations.
Examples in the real world
NaCl is one Na⁺ and one Cl⁻ (formula mass 58.44); CaF₂ is one Ca²⁺ and two F⁻ ions.