History of the Atom

History of the atom: In the 5th cent. B.C. the Greek philosophers Democritus and Leucippus proposed that matter was made up of tiny, indivisible particles in constant motion. Aristotle, however, did not accept the theory, and it was ignored for centuries. Modern atomic theory began with the publication in 1808 by John Dalton of his experimental conclusions that all atoms of an element have same size and weight, and that atoms of elements unite chemically in simple numerical ratios (as molecules) to form compounds. In 1911 Ernest Rutherford explained an atom's structure in terms of a positively charged nucleus surrounded by negatively charged electrons orbiting around it. In 1913 Niels Bohr used quantum theory to explain why electrons could remain in certain allowed orbits without radiating energy. The development of quantum mechanics during the 1920s resulted in a satisfactory explanation of all phenomena related to the role of electrons in atoms.

--http://www.neutron.anl.gov/hyper-physics/histatom.html