My professor often expresses frustration because, inconveniently, there is no word that groups together "atoms" and "molecules". "Microscopic particles" is totally insufficient.
I now find myself experiencing this frustration.
Chemists/Physicists often find themselves saying, especially in Statistical Mechanics, e.g. "Of a set of N atoms and molecules, we select one at random." If we say "particle", it excludes molecules. If we say "molecules", it excludes atoms.
As a bonus, is there perhaps such a word in another language? e.g. German or French?