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460 BCE
Democritus
Democritus' model stated that matter consists of invisible particles of atoms and a void. He stated the atoms are indestructible and unchangeable. His atomic model was solid, and stated all atoms differ in size, shape, mass, position and arrangement, with a void that exists between them. -
John Dalton
Dalton's atomic theory proposed that all matter was composed of atoms, invisible and indestructible building blocks. While all atoms of an element were identical, different elements had atoms of different sizing and mass. -
Dmitri Mendeleev
Mendeleev found that, when all the known chemical events were arranged in order of increasing atomic weight, the resulting table displayed a recurring pattern, or periodicity, of the properties within groups of elements. -
Eugene Goldstein
Goldstein concluded that in addition to the electrons, or cathode rays, that travel from the negatively charged cathode toward the positively charged anode, there is another ray that travels in the opposite direction, from the anode toward the cathode. -
J.J Thomson
J.J Thomson, who discovered electron in 1897, proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model. -
Robert Millikan
Millikan oil drop experiment helped to quantify the charge of the electron, which contributed greatly to our understanding of the structure of the atom and the atomic theory. -
Ernest Rutherford
The Rutherford model was devised to describe an atom. Ernest Rutherford envisioned the atom as a miniature solar system, with electrons orbiting around a massive nucleus, and as mostly empty space, with the nucleus occupying only a very small part of the atom. -
Niels Bohr
The Bohr model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons. Bohr was the first to discover that electrons travel in seperate orbits around the nucleus and the number of electrons in the outer orbit determines the properties of an element. -
James Chadwick
This atomic model is known as the quantum mechanical model of the atom. In 1932, James Chadwick bombarded beryllium atoms with alpha particles. An unknown radiation was produced. He interpreted this radiation as being composed of particles with a neutral electrical charge and the approximate mass of a proton.
