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400
Democritus 400 B.C.
Approximate Date. Democritus believed atoms were small hard particles made out of a single material formed into different shapes and sizes. -
John Dalton
He discovered that: All matter is made of atoms. Atoms are indivisible and indestructible. All atoms of a given element are identical in mass and properties, compounds are formed by a combination of two or more different kinds of atoms. A chemical reaction is a rearrangement of atoms. Dalton's model was that the atoms were tiny, indivisible, indestructible particles and that each one had a certain mass, size, and chemical behavior that was determined by the element they were. -
J. J. Thomson
He discovered the electron. His model was the 'plum-pudding model' which showed an atom as having negatively charged electrons clumped into a positively charged rest of the atom, which appeared similar to plum-pudding. -
Robert Millikan
He discovered the charge of an electron is slighly negative using the oil-drop experiment, and that the charge to mass ratio was very high, meaning that although the mass of an electron is much smaller than that of a Hydrogen atom it has about the same size charge as a negative ion of hydrogen. -
Ernest Rutherford
He discovered that atoms were mostly empty space using his Geiger–Marsden (Gold Foil) experiment, which disproved Thomson's plum-pudding model. The Rutherford Model showed an atom as having a central nucleus that contained most of the atom's mass, despite it being extremely tiny, and electrons in the empty space surrounding the nucleus. This was different than Thomson's model because it showed atoms as having empty space instead of being one big mush. -
Niels Bohr
His model, the Bohr Model stated that electrons travel around the atom in discrete orbits and that they can switch between orbits, in the process emitting a photon. This became the basis for quantam theory. His model was different than the Rutherford model because it showed that electrons have definitive orbits that they follow. -
Proton
Rutherford discovered the proton in 1917 when he bombarded nitrogen gas with alpha particles and observed hydrogen nuclei being emitted from the gas. -
Louis de Broglie
He proposed that all moving particles- especially sub atomic particles- exhibit a degree of wavelike behavior. -
James Chadwick
He discovered the prescence of neutrons in an atom. This led to the creation of the atomic bomb. It also changed the atomic model to show the presence of neutrons in the nucleus of an atom. His model was different than its predecessors because it included neutrons in its nucleus. -
Modern Atomic Theory
Modern Atomic Theory is a combination of past discoveries made about atoms. According to it, atoms are mostly empty space, but have a nucleus that contains most of the atom's mass. Weknow this because of Rutherford's Gold Foil experiment. We also know that atoms have electrons, due to Thomson's discovery, neutrons, due to Chadwick's discovery, and protons. We know that electrons follow discrete orbits due to Bohr's discovery.
