Molecule
A molecule is the smallest particle of a pure substance that still retains its chemical composition and properties. Molecules can be either polyatomic (composed of several atoms) or monoatomic (as in noble gases which are composed of single-atom molecules). Polyatomic molecules are electrically neutral clusters of two or more atoms joined by shared pairs of electrons (covalent bonds) that behave as a single particle. A substance that is made up of molecules is called a molecular substance. Many familiar substances are made of molecules (e.g. table sugar, water, most gases) while many other equally familiar substances are not molecular in their structure (e.g. salts, metals).
Most molecules are much too small to be seen with the naked eye, but there are exceptions. An example of a macroscopically-sized molecule is DNA, a macromolecule.
A property of molecules is the integer ratio of the elements that constitute the compound, the empirical formula. For example, in their pure forms, water is always composed of a 2:1 ratio of hydrogen to oxygen, and ethyl alcohol or ethanol is always composed of carbon, hydrogen, and oxygen in a 2:6:1 ratio. However, this does not determine the kind of molecule uniquely - dimethyl ether has the same ratio as ethanol, for instance. Molecules with the same atoms in different arrangements are called isomers.
Chemical formula on the other hand reflects the exact number of atoms that compose a molecule. The molecular mass is calculated from the chemical formula and is expressed in conventional units equal to 1/12 from the mass of a 12C isotope atom.
Molecules have fixed equilibrium geometries--bond lengths and angles--that are dictated by the laws of quantum mechanics. A pure substance is composed of molecules with the same geometrical structure. The chemical formula and the structure of a molecule are the two important factors that determine its properties, particularly its reactivity. Isomers share a chemical formula but normally have very different properties because of their different structures. Stereoisomers, a particular type of isomers, may have very similar physico-chemical properties and at the same time very different biochemical activities.
See also: Polar and nonpolar molecules.
