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400 BCE
Democritus
Democritus first comes up with the idea that all matter is made of particles, or atoms. -
Dalton
John Dalton discovers the first useful atomic theory, based on the law of conservation of mass, and the law of constant composition -
Crookes
William Crookes discovers a fourth state of matter, which led to modern electronic theory, by charging electricity through a gas in a sealed tube -
Becquerel
Henri Becquerel discovered radiation through investigating fluorescence in uranium salts -
Thomson
J.J Thomson discovered electrons by using a cathode ray tube -
Rutherford
Ernest Rutherford discovers that atoms have a dense nucleus, which contains protons, with the gold-foil experiment. -
Bohr
Niels Bohr introduces the theory that electrons move around the nucleus of an atom in orbits, and introduced the idea that an electron could move from a higher energy orbit to a lower energy one -
Electron Cloud Model
It was discovered that electrons move in orbitals, in different levels. The Electron Cloud Model is the model currently being used today. -
Chadwick
James Chadwick discovered the neutron by bombarding elements with alpha particles
