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Democritus (460-370 BC)
First proposed the existence of an ultimate particle. Used the word "atomos" to describe this particle. -
Aristotle (384-322 BC)
He believed in the four elements of air, earth, water and fire. Aristotle felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter. This view held sway for 2000 years primarily because Aristotle was the tutor of Alexander the Great. -
John Dalton (1776-1844)
proposed the Law of Multiple Proportions. This law led directly to the proposal of the Atomic Theory in 1803. He also developed the concept of the mole and proposed a system of symbols to represent atoms of different elements. Dalton recognized the existence of atoms of elements and that compounds formed from the union of these atoms. -
James Chadwick (1891-1974)
He discovered the neutron in 1932. Chadwick was a collaborator of Rutherford's. Interestingly, the discovery of the neutron led directly to the discovery of ultimately to the atomic bomb. -
Dimitri Mendeleev (1834-1907)
He proposed the periodic law and developed the first periodic table in 1869. Medeleev's table was arranged according to increasing atomic weight and left holes for elements that were yet to be discovered.