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Democritus in 400 BCE theorized that all material bodies are made up of indivisibly small “atoms.” -
In 1803, John Dalton theorized that Elements consist of indivisible small particles. He also theorized that all atoms of the same element are identical and that different elements have different types of atoms. -
In the year 1869, Mendeleev created the first official periodic table by jotting down the symbols for the chemical elements, putting them in order according to their atomic weights and inventing the periodic table -
Goldstein, in 1886 discovered positive particles by using a tube filled with hydrogen gas. This proved what the proton was and how it worked. -
Planck proved that energy is emitted in a similar manner. He proposed that only certain amounts of energy could be emitted -
Thompson after many experiments with cathode-ray tubes in 1904 theorized that all atoms contain tiny negatively charged subatomic particles or electrons. -
In the year 1911, Ernest Rothorford theorized about the nuclear structure of the atom, he learned that when alpha particles are fired into gas atoms, a few are violently deflected, which implies a dense, positively charged central region containing most of the atomic mass -
Robert learned by comparing applied electric force with changes in the motion of the oil drops, he was able to determine the electric charge on each drop. He found that all of the drops had charges that were simple multiples of a single number, the fundamental charge of the electron -
Niels Bohr's theory for the hydrogen atom, based on quantum theory that some physical quantities only take discrete values. Electrons move around a nucleus, but only in prescribed orbits, and If electrons jump to a lower-energy orbit, the difference is sent out as radiation. -
Erwin Schrodinger proved that the quantization of the hydrogen atom's energy levels that appeared in Niels Bohr's atomic model could be calculated from the Schrödinger equation, which describes how the wave function of a quantum mechanical system (in this case, a hydrogen atom's electron) evolves. -
Heisenberg, contributed to atomic theory through formulating quantum mechanics in terms of matrices and in discovering the uncertainty principle -
In 1932, James Chadwick discovered the neutron. He did this by conducting an experiment in which he bombarded Beryllium with alpha particles from the natural radioactive decay of Polonium.