The word molecule was first used in a scientific sense by Avogadro in 1811 and was brought into English in 1794 when it meant "small particle" of any type. However, it was used for more than a century before that in French, and wasn't too different in Latin as molecula. Now we get semantic variation; molecula is a diminutive of moles, which meant "mass" (so a molecule is a "tiny mass") and is sort of related to Avogadro's mole, through German. Moles also had a secondary definition of "barrier", and that's important for explaining the Proto-Indo-European reconstruction: apparently it derives from the root mo, meaning "exertion", because it requires effort to overcome barriers. Usage of the word molecule in literature over time has been slowly decreasing since a peak in the 1960s.
Adam Aleksic, a rising sophomore studying linguistics and government at Harvard University, has been described as the internet's sixth most famous etymologist. He also has disturbing interests in politics, vexillology, geography, board games, conlanging, and law.