Molecule

A molecule is an electrically neutral cluster of two or more atoms joined by shared pairs of electrons (covalent bonds) that behaves 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, and the noble gases).

(left and center) and 2D (right) representations of the terpenoid, atisane. In the 3D model on the left, carbon atoms are represented by gray spheres, white spheres represent the hydrogen atoms and the cylinders represent the bonds. The model is enveloped in a "mesh" representation of the molecular surface, colored by areas of positive (red) and negative (blue) electric charge. In the 3D model (center), the light-blue spheres represent carbon atoms, the white spheres are hydrogen atoms, and the cylinders in between the atoms correspond to single-bonds.]]

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.

See also: Polar and nonpolar molecules.