The ancient Greek philosopher Democritus was of the first to develop an atomic theory, and in fact believed that everything is made of atoms which cannot be physically divided and are always in motion.
Aristotle discovered the idea that all made is able to be cut in half to acquire a smaller piece of that matter. Air, earth, water, and, fire were four elements he strongly believed in.
English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms. He formulated the first atomic theory since the “death of chemistry” that occurred during the prior 2000 years. Dalton theorized that all matter is made of atoms.
English scientist named J.J. Thomson provided the first hint that an atom is made of even smaller particles. He discovered the presence of a negative particle in the atom – the electron. He proposed a model of the atom that is sometimes called the “Plum Pudding” model.
Rutherford and Geiger devised a counter for alpha particles, work that led to Rutherford's nuclear theory of the atom, for which he won the 1908 Nobel Prize in chemistry. ... Bothe investigated alpha scattering, and Chadwick counted beta particles.
Niels Bohr proposed a theory for the hydrogen atom based on quantum theory that energy is transferred only in certain well defined quantities. Electrons should move around the nucleus but only in prescribed orbits.
Chemists began using the term in connection with the growing number of irreducible chemical elements.