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His mentor, Leucippus, originally came up with the atomic theory, but it was then adopted by Democritus. The atomic theory stated that “The universe is composed of two elements: the atoms and the void in which they exist and move.” According to Democritus atoms were miniscule quantities of matter.
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Isaac Newton theorized a mechanical universe with small, solid masses in motion.
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Dalton's atomic theory remains valid in modern chemical thought and his ideas were that all matter is made of atoms, atoms are indivisible and indestructible, and compounds are formed by a combination of two or more different kinds of atoms.
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Dmitri Mendeleev published his periodic table of elements and it arranged the known elements according to their chemical properties and in order of their relative atomic mass.
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J.J. Thomson discovered the electron by experimenting with a Crookes, or cathode ray, tube.
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Marie Curie was a pioneer in researching radioactivity, winning the Nobel Prize in Physics and in Chemistry.
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Hans Geiger was known for the Marsden experiment which discovered the atomic nucleus.
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Robert Millikan and his oil drop experiment helped to quantify the charge of an electron, which contributed greatly to our understanding of the structure of the atom and atomic theory.
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Ernest Rutherford established that the nucleus was: very dense,very small and positively charged. He also assumed that the electrons were located outside the nucleus.
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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".
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Developed an explanation of atomic structure that underlies regularities of the periodic table of elements.
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Heisenberg contributed to the atomic theory by including quantum mechanics, the branch of mechanics, based on quantum theory, used for interpreting the behavior of elementary particles and atoms.
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Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron.
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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.
