Which term describes the lowest whole number ratio of ions in an ionic solid?

The term that describes the lowest whole number ratio of ions in an ionic solid is known as the formula unit. In ionic compounds, the formula unit represents the simplest ratio of the different ions that compose the compound. For example, in sodium chloride (NaCl), the formula unit indicates that there are equal numbers of sodium ions and chloride ions in the compound, creating a balanced charge.

While the empirical formula also denotes the simplest whole number ratio of elements in a compound, it is typically used more broadly for molecular compounds. The molecular formula specifies the actual number of atoms of each element in a molecule, which does not apply to ionic solids because they consist of lattice structures rather than distinct molecules. A chemical equation, on the other hand, represents a reaction using reactants and products rather than ratios of ions in a solid.

Thus, the formula unit effectively captures the lowest whole number ratio of ions in an ionic crystal structure, making it the correct term in this context.