In chemistry, a molecule is defined as a sufficiently stable electrically neutral group of at least two atoms in a definite arrangement held together by strong chemical bonds. In organic chemistry and biochemistry, the term molecule is used less strictly and also is applied to charged organic molecules and biomolecules. Molecules are distinguished from polyatomic ions in the strict sense.
This definition has evolved as knowledge of the structure of molecules has increased. Earlier definitions were less precise defining molecules as the smallest particles of pure chemical substances that still retain their composition and chemical properties. This definition often breaks down since many substances in ordinary experience, such as rocks, salts, and metals, are composed of atoms or ions, but are not made of molecules.
In the kinetic theory of gases the term molecule is often used for any gaseous particle regardless of their composition. According to this definition noble gases would also be considered molecules despite the fact that they are composed of a single non-bonded atom.