A molecule is an electroneutral (having no net electric charge) group of atoms held together by chemical bonds. (It is often said that the noble gases (He, Ne, Ar, Kr, Xe, and Rn) consist of monoatomic molecules
.) A molecule is the smallest unit of a pure substance that retains the composition and most chemical properties of that substance. I say "most"
rather than "all" because in some cases more than one molecule of a given substance is needed to exhibit a particular chemical property. As an example, the decomposition of nitric acid according to the equation, 4 HNO3
= 2 H2
O + 4 NO2
, requires at least four molecules of HNO3
to take place. Likewise, at least two molecules of NO2
are needed for the dimerization reaction of nitrogen dioxide, 2 NO2