What unit is commonly described as the basic unit that makes up compounds?

• A. Atoms.
• B. Elements.
• C. Ions.
• D. Molecules.

Answer: (D)

Think of compounds as substances formed when atoms bond together. The smallest piece that still has the characteristics of the compound is a molecule, which captures the specific arrangement of atoms that defines the compound’s properties. For many common compounds you encounter—like water, carbon dioxide, or methane—this smallest unit is precisely a molecule, making it the best descriptor of the basic building block of a compound.

(Note: some ionic compounds are described by a formula unit rather than a discrete molecule in the solid state, but the everyday teaching convention uses molecules as the basic unit of a compound.)