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Antoine Lavoisier established the law of conservation of matter which stated that matter, like energy, is neither created or destroyed in a chemical process.
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Joseph Louis Proust established the law of constant compositon which stated that a given compound always contains the same elements in the same proportions by mass.
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John Dalton developed the first atomic theory.
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Michael Faraday was the first man to state that atoms contain particles that have an electrical charge.
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Henri Becquerel discovered that Uranium exhibits radioactivity.
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Joseph John Thomson discovered the electron and the charge of an electron per unit of mass.
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Marie Curie and her husband created a new element called Polonium. On December 28 of that same year her and her husband also announced the existence of a second element which they named Radium.
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Earnest Rutherford discovered the nucleus of an atom by accident while performing his "Gold Foil Experiment."
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Robert Andrewa Millikan discovered the charge and mass of an electron during his infamous "Oil Drop Experiment."