One of the first atomic theorists was Democritus, a Greek philosopher in the 5th Century.  

B.C. He observed that if a stone was divided in half, the separate pieces would have the same properties as a whole. Based on his observation, he reasoned that if the stone were to be cut into smaller pieces many times the pieces would at some point be invisible. He names these pieces “atomos”, the Greek word for invisible. 

Democritus theorized that atoms were specific to the material they composed, differed in size and shape, were in constant motion in a void, collided with each other. He also theorized that during these collisions, atoms could either stick together or rebound. And therefore, changes in matter were a result of the dissociations or combinations of atoms as they moved through a void. His model of the atom is a solid, hard, idustructible and impressable building block.