A molecule is the smallest unit of matter that retains the chemical properties of a compound. It is composed of two or more atoms that are held together by covalent bonds. The atoms in a molecule share electrons in order to achieve a stable electron configuration.

Molecules can be diatomic, triatomic, or polyatomic. Diatomic molecules consist of two atoms, such as H2O. Triatomic molecules consist of three atoms, such as CO2. Polyatomic molecules consist of four or more atoms, such as C6H12O6 (glucose).

The properties of a molecule are determined by the number and arrangement of atoms within it. The number of atoms in a molecule determines its molecular weight. The arrangement of atoms within a molecule determines its molecular structure. The molecular structure of a molecule determines its physical and chemical properties.