What term describes the smallest unit of a substance that retains its properties?

The term that describes the smallest unit of a substance that retains its properties is a molecule. A molecule is formed when two or more atoms bond together, and it can represent the smallest portion of a compound or an element that still exhibits the chemical properties of that substance. For instance, a water molecule (H₂O) is the smallest unit that has the characteristics and chemical behavior of water.

While an atom is indeed the smallest unit of an element, it does not reflect the properties of compounds, which require more than one atom bonded together. Similarly, elements and compounds are terms that describe types of substances rather than the smallest units that retain the properties of those substances. An element consists of only one type of atom, while a compound consists of two or more different elements bonded together. Thus, the most accurate description of the smallest functional unit that maintains the properties of a substance is a molecule.