Formula units refer to the simplest collection of atoms from which an ionic compound’s formula can be established. It represents the ratio between cations (positively charged ions) and anions (negatively charged ions) in an ionic compound.
This is the simplest positive integer ratio of atoms present in a compound.
Ionic Compound: A chemical compound composed of ions held together by electrostatic forces termed ionic bonding. The compound is neutral overall, but consists of positively charged ions called cations and negatively charged ions called anions.