Final answer:
The formula unit is the representative particle for ionic compounds, while the molecule is the representative particle for covalent compounds.
Step-by-step explanation:
The formula unit is the representative particle for ionic compounds. In chemistry, a formula unit is the simplest ratio of ions in the compound. For example, in sodium chloride (NaCl), the formula unit is one sodium ion (Na+) and one chloride ion (Cl-). The formula unit is used to describe the relative number of different ions in an ionic compound.
For covalent compounds, the representative particle is the molecule. The molecular formula is used to express the exact number of each type of atom in the molecule. For example, in water (H2O), the representative particle is one water molecule composed of two hydrogen atoms (H) and one oxygen atom (O).