What is a formula unit?

A formula unit is the chemical formula representing an ionic compound in its simplest and most reduced terms. It is related to the concept of an empirical formula, which is the simplest chemical formula expression for a non-ionic chemical compound. Empirical formulas reduce molecules to their simplest ratio of atoms.

For example, sodium chloride, more commonly known as table salt, can be expressed by the formula unit NaCl. Regardless of how many ions of sodium chloride are being used in a chemical equation, the formula unit for sodium chloride is the simplified expression of one sodium atom to one chlorine atom.