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Important People in the Development of Atomic Theory Exam Review Democritus • The theory of Democritus held that everything is composed of "atoms", which are physically, but not geometrically, indivisible; • 440 BC Aristotle • Disagreed with Democritus • Only 4 elements existfire, water, air and earth • 350 BC • John Dalton Father of Modern Atomic Theory (1800) Five main points of Dalton's atomic theory • 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 cannot be subdivided, created, or destroyed. • Atoms of different elements combine in simple whole-number ratios to form chemical compounds. • In chemical reactions, atoms are combined, separated, or rearranged. Ludwig Boltzman • Developed the Kinetic Theory of Matter originally only gasses. • As pointed out by Albert Einstein in 1905, this experimental evidence for kinetic theory is generally seen as having confirmed the existence of atoms and molecules. • 1866 Dmitri Mendeleev • Authored 1st Periodic Table of the Elements, it contained all the elements known at the time. • 1869 James Chadwick • In 1931, Chadwick discovered the neutron. • It allowed researchers to make elements heavier than uranium. • For Chadwick's discovery of the neutron in 1932, he was awarded the Hughes Medal of the Royal Society in 1932 and the Nobel Prize in Physics in 1935.