Atomic theory is the description of atoms, the smallest units of elements. The scientific evidence for the existence of atoms and its even smaller constituents is so vast that most people now consider the existence of atoms to be a fact and not just a theory.

History

Beginning in about 600 B.C., many Greek philosophers struggled to understand the nature of matter. Some said everything was made of water, which comes in three forms (solid ice, liquid water, and gaseous steam). Others believed that matter was made entirely of fire in ever-changing forms. Still others believed that whatever comprised matter, it must be something that could not be destroyed but only recombined into new forms. If they could see small enough things, they would find that the same "building blocks" they started with were still there. One of these philosophers was named Democritus . He imagined starting...