What is the term for the simplest whole number ratio of atoms in an ionic compound?

The term that best describes the simplest whole number ratio of atoms in an ionic compound is the formula unit. This term is used specifically for ionic compounds, where the lowest whole number ratio of the constituent ions is represented. In an ionic compound, atoms are arranged in a lattice structure, and the formula unit conveys the ratio of the ions in that lattice. For instance, in sodium chloride (NaCl), the formula unit indicates that there is one sodium ion for every chloride ion, reflecting the simplest ratio.

The other terms listed do not apply in this context. The unit cell refers to the smallest repeating unit in a crystal lattice, the molecular formula specifies the actual number of atoms of each element in a molecule (which is not applicable to ionic compounds), and the empirical formula represents the simplest whole number ratio of elements in a compound, but it is more broadly used for covalent compounds and is not specific to ionic structures. In contrast, the formula unit specifically addresses the unique characteristics of ionic compounds.