Which term describes the building block that defines the empirical formula of an ionic compound in a fixed ratio of ions?

• A. Formula Unit
• B. Lewis Structure
• C. Polyatomic Ion
• D. Resonance

Answer: (A)

In ionic compounds, the structure extends in a lattice, so the smallest repeating unit that captures the fixed ratio of cations to anions is called the formula unit. This unit represents the simplest whole-number ratio of ions in the crystal and serves as the empirical formula for the compound. For example, NaCl has a 1:1 ratio, so its formula unit is NaCl.

This differs from a Lewis structure, which shows how electrons are arranged in covalent bonds within molecules rather than the lattice composition. A polyatomic ion is a charged cluster of atoms that can act as a unit inside compounds, but it isn’t the building block that defines the fixed lattice ratio. Resonance describes delocalized electrons and alternate valid Lewis structures for a molecule or ion, not the fixed ionic ratio.