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400 BCE
Democritus
The universe is composed of two elements: the atoms and the void in which they exist and move -
Isaac Newton
Proposed a mechanical universe with small solid masses in motion. -
John Dalton
Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. -
Dmitri Mendeleev
Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law. -
J.J. Thomson
Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 10 8 coulombs/gram.
Studied "canal rays" and found they were associated with the proton H + . -
Marie Curie
Studied uranium and thorium and called their spontaneous decay process "radioactivity". She and her husband Pierre also discovered the radioactive elements polonium and radium. -
Hans Geiger
Developed an electrical device to "click" when hit with alpha particles. -
R.A. Millikan
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. -
Ernest Rutherford
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. Moseley
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. -
Niels 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 electron -
Heisenberg
Described atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminancy - you can not know both the position and velocity of a particle. -
James Chadwick
Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. -
Lise Meitner
Conducted experiments verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction
