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Democritus thought that if you take a piece of matter and continue to divide until you cannot anymore, which is known as an atom.
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Aristotle disagreed with Democritus and denied the existence of atoms. He believed that four elements made up all matter.
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Lavoisier was one of the first to use an analytical balance to monitor chemical reactions. He studied the thermal decomposition of mercury.
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He was the first to adapt Democritus’ theory into the first modern atomic model and developed many new ideas in his book, "New System of Chemical Philosophy."
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He showed that electrons have a negative electric charge and come from ordinary matter. For matter to be electrically neutral, there must also be positive charges lurking somewhere.
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Millikin concluded that the mass of an electron must be very small for such a large charge.
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He conducted an experiment intended to determine the angles through which a beam of alpha particles (helium nuclei) would scatter after passing through a thin foil of gold. He stated that electrons orbit the nucleus similar to the orbit of planets about the sun.
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He created a new model in which there are particular orbits in which the electrons do not lose energy and therefore do not spiral into the nucleus. This model was the beginning of quantum mechanics, which successfully explains many properties of atoms.
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De Broglie applied his theory of wave, the particle duality to the Bohr model to explain why only certain orbits are allowed for the electron. He argued that only certain orbits allow the electron to satisfy both its particle and wave properties at the same time.
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Heisenberg showed that it is impossible to take any measurement of an object without disturbing the object.
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Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position.
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Chadwick bombarded beryllium atoms with alpha particles and that eventually led him to the discovery of the neutron.
