The sections below describe some of the important processes by which atoms become stable. The processes that result in the filling of the outer electron shells result in the formation of chemical bonds. In some cases, this involves the formation of molecules. Molecules are two or more atoms held together by the sharing of electrons (described below). Molecules composed of more than one type of element can also be called compounds. Hence, H₂ (same element) is a molecule, and H₂O (different elements) is both a molecule and a compound.