What is a formula unit?

A formula unit is the simplest ratio of the ions represented in an ionic compound. It is the lowest whole number ratio of ions in an ionic compound that maintains electrical neutrality. It is written using the chemical symbols and subscripts of the elements present in the compound.

For example, the formula unit for sodium chloride (table salt) is NaCl. This means that for every one sodium ion (Na+) in the compound, there is one chloride ion (Cl-) present. The ratio is 1:1, and this is represented by the formula unit NaCl.

Formula units are used to denote the composition of ionic compounds and are different from molecular formulas, which represent the actual number of atoms in a molecule of a covalent compound. Formula units are commonly used in discussing and writing chemical equations, reactions, and formulas for ionic compounds.