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Democritus
According to Democritus' theory, atoms cannot be destroyed and they exist in a vacuum or void, which corresponds to the space between atoms. Atoms of a liquid are smooth and round; atoms of a solid are jagged and catch on to each other.
Atoms differ only in shape, position, and arrangement -
Newtown
Proposed a mechanical universe with small solid masses in motion. -
Dalton
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. -
G.J. Stoney
Proposed that electricity was made of discrete negative particles he called "electrons" -
J.J. Thomson
Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 108 coulombs/gram. -
Planck
Used the idea of quanta (discrete units of energy) to explain hot glowing matter. -
Nagaoka
Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle. -
Milikan
He discovered that the weight of an electron is 1836 times smaller than that of a hydrogen atom (atomis mass of 1) -
E. Rutherford
He established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus. -
H.G.J. Mosely
he sorted the chemical elements of the Periodic Table of the Elements in a quite logical order based on their physics. -
Bohr
Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of sucessive orbital shells of electrons. -
James Chadwick
Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron.