Final answer:
The representative unit of a molecular compound is a discrete molecule held together by covalent bonds, while the representative unit of an ionic compound is a crystal lattice structure held together by electrostatic attractions.
Step-by-step explanation:
Molecular compounds are formed between nonmetals and nonmetals, while ionic compounds are formed between metals and nonmetals. The representative unit of a molecular compound is a discrete molecule made up of atoms held together by covalent bonds. In contrast, the representative unit of an ionic compound is a crystal lattice structure made up of cations and anions held together by electrostatic attractions.
For example, water (H2O) is a molecular compound where individual water molecules are held together by covalent bonds. Sodium chloride (NaCl) is an ionic compound where sodium cations (Na+) and chloride anions (Cl-) are arranged in a crystal lattice structure.
The physical properties of molecular compounds, such as their melting and boiling points, are generally lower than those of ionic compounds. Molecular compounds also do not conduct electricity well, while ionic compounds conduct electricity when dissolved in water or a molten state.