What are molecules defined as?

Molecules are defined as the smallest part of a compound that retains the chemical properties of that compound. They are formed when two or more atoms are chemically bonded together, either through covalent bonds, where atoms share electrons, or through other chemical interactions. This definition emphasizes that molecules can consist of one type of atom, as seen in elemental molecules like O₂, or multiple types, as in water (H₂O), which is a compound composed of hydrogen and oxygen atoms.

Understanding that a molecule is the fundamental unit of a chemical substance is essential for grasping concepts in chemistry, such as reactions, bonding, and the properties of materials. The other options do not accurately capture the definition of molecules. For instance, describing molecules solely as atoms combined in ionic bonds overlooks the broader range of chemical bonding that can occur, and defining them in terms of their state (particles in solid state) or their relative size in mixtures misrepresents their fundamental characteristics.