Democritus of Abdera was the first person to put forward the idea of ατομος (atoms) he was greek and lived between 470 and 380 BC. A very interesting philosopher, he even believed that the Milky Way was a vast collection of stars, and democracy is named after him. His theory of atoms as being the basis of matter wasn't believed at the time, and wasn't rekindled until John Dalton 21 centuries later.