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Period: 400 to
Atomic Theory
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460
Democritus
Came up with the Atomic Theory concept -
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. -
Eugene Goldstein
Used a CRT to study "canal rays" which had electrical and magnetic properties opposite of an electron. -
J.J. Thompson
Studied "canal rays" and found they were associated with the proton H + . -
Robert 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. -
Hemry Mosely
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 electrons. -
Erwin Schrodinger
Viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom. -
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. -
Ernest Rutherford
Studied radiations emitted from uranium and thorium and named them alpha and beta. -
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.