In trying to correlate Dalton's atomic theory with Gay Lussac's law of gaseous volume, Berzelius put forward the hypothesis that equal volumes of all gases contain equal number of atoms under similar conditions of temperature. However, it was found that even a fraction of an atom could be involved in certain chemical reactions. This went against Dalton's atomic theory. Therefore, Avogadro postulated the existence of molecules along with atoms as two kinds of ultimate particles. According to Avogadro's postulates,
The smallest particle of an element, which may or may not have independent existence and takes part in a chemical reaction is an atom.
The smallest particle of a substance (element or compound) capable of independent existence is called molecule.