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460
Democritus
Greece
He hypothesized that all matter (including space and time) is composed of tiny indestructible units, called atoms. He in fact created the first atomic theory which is remarkably similar to modern science's understanding of atomic structure, and avoided many of the errors of their contemporaries. -
Newtown
England
Proposed a mechanical universe with small solid masses in motion. -
Dalton
England
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. He also suggested that the sizes of the particles making up gases must be different. -
G.J. Stoney
Irish Midlands
Proposed that electricity was made of discrete negative particles he called "electrons". -
J.J. Thomson
England
Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 108 coulombs/gram.
He also discovered the electron. Thomson showed that cathode rays were particles with a negative electric charge and much smaller than an atom. He also thought all atoms contained them. -
Planck
Germany
Used the idea of quanta (discrete units of energy) to explain hot glowing matter. He found the law of heat radiation. -
Nagaoka
Japan
Used the idea of quanta (discrete units of energy) to explain hot glowing matter. -
Millikan
U.S.A
Oil drop experiment determined the charge (e=1.602 x 10-19 coulomb) and the mass (m = 9.11 x 10-28 gram) of an electron. -
E. Rutherford
New Zeland
Using alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil. 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. Moseley
England
Using x-ray tubes, determined the charges on the nuclei of most atoms. He wrote “The atomic number of an element is equal to the number of protons in the nucleus". This work was used to reorganize the periodic table based upon atomic number instead of atomic mass. -
Bohr
Denmark
Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons. -
James Chadwick
England
Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. His discoveries made it possible to create elements heavier than uranium in the lab.
