Which term describes the smallest unit of an ionic compound?

• A. Ion
• B. Atom
• C. Formula Unit
• D. Molecule

Answer: (C)

In ionic compounds, the structure is a crystal lattice of alternating cations and anions rather than a discrete covalently bonded molecule. The smallest unit that represents the compound’s composition in this lattice is the formula unit—the simplest whole-number ratio of ions that yields a neutral assembly. For example, in sodium chloride the simplest ratio is one sodium ion for every chloride ion, so the formula unit is NaCl. This differs from a molecule, which would be a discrete, covalently bonded group of atoms. An atom is a single element, and an ion is a charged particle; the smallest unit that captures the composition of an ionic compound in the solid state is the formula unit.