What is a formula unit?

A formula unit is the lowest whole number ratio of ions in an ionic compound. It's essentially the empirical formula for an ionic compound. Unlike molecules, which are discrete units formed by covalent bonds, ionic compounds exist as a lattice structure. Therefore, we don't refer to an "ionic molecule." Instead, we use the term formula unit to represent the simplest ratio of ions in that lattice.

For example, consider sodium chloride (NaCl). Sodium chloride consists of a lattice structure where each Na+ ion is surrounded by Cl- ions and vice-versa. The formula unit is NaCl, showing a 1:1 ratio of sodium and chloride ions.

Key aspects to understand about formula units:

• They apply specifically to ionic compounds.
• It is not a molecule.
• It represents the simplest ratio of ions.
• It's analogous to an empirical formula.
• It's crucial for calculations involving molar mass and stoichiometry.