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Many theorised and proposed ideas were made before discoveries actually were, including:
440BC - Democritus and Leucippus hypothesise that all things are made of tiny bits of matter, atoms
360BC - Plato puts forth the word "stoicheia", which means elements in Ancient Greek
330BC - Aristotle theorises the four elements; earth, wind, water and fire -
The scientist and philosopher Sir Francis Bacon writes "The Proficience and Advancement of Learning", which outlines the Scientific Method
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Robert Boyle publishes the book "The Sceptical Chymist" which denotes the differences between Chemistry and Alchemy, and mentioned atoms, molecules and chemical reactions. This is widely thought of as the beginning of modern chemistry
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Joseph Black isolated molecules of Carbon Dioxide, which he named "Fixed Air"
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Antione Lavosier does extensive research to create a list of 33 elements, noting non-metals and metals
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John Dalton describes the relationship between the makeup of a gas, and calls it "Dalton's Law"
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Jacob Berzelius creates a table of the elements, formed in order of Atomic Weight, and also devises a system of single letters to be symbols for the elements to be identified by
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Johan Dobereiner discovers that certain elements has very similar properties, and that these triplets of elements also follow a pattern of density
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Dmitri Mendeleev creates a new table of the elements, with the elements being ordered by atomic weights, but with them also being ordered "Periodically" with their properties, creating the forerunner to the modern Periodic Table, with 66 elements
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William Ramsay, a Scottish chemist, discovers the newest elements, the Noble Gases
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Marie and Pierre Curie are able to isolate Polonium and Radium from a useless ore called "pitchblende", these being some of the first radioactive elements found
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Ernest Rutherford, a Chemist and Physicist who postulated the idea that Radiation came from atoms, and that these elements were not as indestructable as first thought
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Henry Moseley uses X-Ray spectrometry to correctly find the atomic numbers for almost all known naturally occuring elements, with a few gaps, these being elements that are incredibly rare or not naturally occurring.
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Edwin McMillan and Phillip Abelson, two Physicists, are the first to synthesize an element after Uranium on the Periodic Table, Neptunium, the lightest of these "Transuranic Elements", after smashing Uranium Atoms together
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Glenn Seaborg is able to produce the first Plutonium atom, a P - 239 isotope, synthesized after the bombardment of Uranium atoms
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Glenn Seaborg puts forth the idea that elements above 89 have a different electron configuration than those below it, and with the Lanthanides, they should be seperated into their own group, forming our current periodic table
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Physicists of the world continue to synthesize more and more of th elements found on the periodic table, taking us from the know 94 up to the current 118 elements that we know of to this very day
