A molecule is defined as the smallest particle of an element or a compound that possesses the properties of the original substance. Some materials, such as helium, neon, and argon, tend to form monoatomic molecules. Most molecules, however, are made of two or more atoms. Many gases, such as oxygen, nitrogen, chlorine, and carbon monoxide, are diatomic (meaning they are composed of only two atoms). Triatomic molecules (which are composed of three atoms) include ozone (O₃) and carbon dioxide (CO ₂). In complex molecules, such as proteins, the number of atoms can range up to hundreds or thousands. Molecules vary in size from less than 1 millimicron to more than 500 millimicrons and in weight from 4 units (for helium) to 40 million units (for the tobacco mosaic virus).

Molecules differ from chemical compounds because compounds consist of different elements. Combinations of the same atoms form molecules, not compounds. For example, when two...