Avogadro's number- the unit mole come around the 1811, he first to realize that the volume of a gas is proportional to the number of atoms or molecules.

Amedeo avogro- 1776-1856; best known for hypothesis that equals volumes of different gases contain an equal nuke of molecules; same temperature and pressure hypothesis; rejected by other scientist. First scientist to realize that elements could exist in the form of molecules rather than individual atoms.