What term refers to the collection of atoms corresponding to an ionic compound's formula such that the molar mass of the compound is the same as the mass of 1 mol of formula units?

• A. Ionic Compound
• B. Formula Unit
• C. Polyatomic Ion
• D. Lattice Energy

Answer: (B)

In ionic compounds, the building block you use for mass and composition is the formula unit. Unlike molecular compounds, ionic substances form a crystal lattice, so there aren’t discrete molecules. The formula unit is the smallest collection of ions that represents the overall formula and remains electrically balanced. Because of that, the mass of one mole of formula units—the molar mass of the compound—matches the mass of 1 mole of those repeating units.

So the term that fits is formula unit. The other terms don’t describe this mass-counting unit: an ionic compound is the substance as a whole, a polyatomic ion is a specific charged group of atoms that can be part of a formula unit but isn’t the unit itself, and lattice energy is the energy associated with forming the solid from gaseous ions.