The word “atom” comes from the Greek a-tomos, which means “indivisible”. The Greek philosophers were the first to propose that all matter was composed of particles so small that they cannot be discerned.
The first models rested on intuitions as it was strictly impossible to prove the existence of these particles of matter by observation.
The discovery of the electron by J. J. Thomson in 1897 constituted the first experimental proof of the existence of elementary particles. The proton (1919) and the neutron (1932) were to follow.
Particle Physics is still very active today, seeking to find the secrets of matter.