The idea of atoms dates back to?
the Greeks. Atoms were thought to be the last indivisible units, but we've learnt how to split atoms.
Democritus and/or his mentor Leucippus came up with the idea, but for them it was a really lucky guess; they had no evidence for this (and they were completely wrong about what the atoms were actually like). The modern concept of atoms dates back to John Dalton, who unlike them had an actual reason for suspecting this was true.