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430 BCE
Democritus
Democritus proposed that all matter was made up of tiny, particles called atoms that could not be destroyed. -
John Dalton
Dalton proposed the first atomic theory. This provided that all matter is made up of tiny invisible atoms and that atoms of the same element are the same. He also argued that atoms combine in whole number ratios to form compounds -
J. J. Thompson
Thompson used cathode rays to discover the electron and he measured the bending of the rays to determine the charge to mass ratio of the electron. -
Ernest Rutherford
Rutherford studied substances, especially alpha particles. He conducted a foil experiment in which he bombarded gold foil with alpha particles. Most particles passed through the foil but some were deflected. He concluded that the atom is mostly empty space, with a very small positive nucleus core. -
Niels Bohr
Bohr improved upon the atomic model by trying to answer the question why electrons aren't pulled into the nucleus of atoms.
He realized that electrons in various orbits have certain amounts of energy. -
Bohr Planetary Model
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Erwin Schrödinger
Schrödinger used mathematical equations to describe the likelihood of finding an electron in a certain position. This predicts the odds of finding the location of the electron. The resulting model can be portrayed as a nucleus surrounded by an electron cloud. -
Quantum Mechanical Model
The Electron Cloud -
James Chadwick
Chadwick contended that the neutron was an uncharged particle. His discovery allowed for a model of the atom that was widely available to chemists. The discovery of a working model of the neutron led to nuclear weapons.
