Democritus believed atoms were small, hard particles that were made of the same material but different shapes and sizes, always moving and capable of joining together. He had no experimental evidence to support his theory.
Aristotle 384-322 BC
Aristotle did not believe in the atomic theory. He believed that all materials on the earth were made of the four elements: earth, fire, water, and air. He had no experimental evidence to support his belief but because he was respected people overlooked the work of Democritus for about 2000 years.
John Dalton 1766- 1844
"Dalton's theory proposed a number of basic ideas: elements are made of extremely small particles called atoms. Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties. Atoms can not be subdivided, created, or destroyed."
J.J. Thomson 1856-1940
He understood electrons to be negatively charged particles in atoms and reasoned that atoms of all elements must contain electrons. In 1897 he proposed that each atom was composed of smaller particles and because they have no electric charge they must also contain positive charges. His new model, referred to as the "Plum Pudding Model", depicts a positive sphere scattered with negative electrons.
He experimented with electric currents in glass tubes called cathode ray tubes. He was able to cause non-radioactive atoms to produce streams of negatively charged particles later named electrons.