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Greek Atomic Models: Around 400 BC, most Greek philosophers believed in one of two theories of matter: the discontinuous theory of matter, which stated that all matter is made of tiny particles, and the continuous theory of matter, which states that matter can be infinitely broken apart into smaller and smaller pieces. Philosopher Democritus supported the discontinuous model, and Aristotle supported the continuous model.
John Dalton's Atomic Model: In 1803, scientist John Dalton proposes that all matter is made of tiny particles called atoms. At this time protons, electrons, and neutrons were unknown and the atom was only a sphere, an almost identical model to the one made in 450 BC by the Greek scientist and philosopher Democritus.
Thomson's "Plum Pudding" Model: In 1897, J. J. Thomson discovered the electron, and it was found out the atoms are made of even smaller particles. The atomic model made by Thomson had electrons floating in an area of positive charge, similar to raisins in a plum pudding. The atom had no nucleus, and protons or neutrons had not been discovered.
Rutherford Model: In 1911, Ernest Rutherford conducted a number of experiments that showed that atoms have a nucleus about which electrons orbit. The model made had a dense nucleus of positive particles (protons), and electrons orbitting the nucleus on unique orbits, similar to planets around a star. Rutherford also later predicted that a second kind of particle was present in the nucleus.
Bohr Model: In 1922, physicist Niels Bohr won the Nobel prize for his theory that electrons orbit the nucleus in set orbits, having a nucleus surrounded by rings of electrons. In 1926, Shrödinger theorized that electron orbitals are less strict, and electrons are found in "clouds" around the nucleus.
Discovery of Neutron: Rutherford predicted in 1920 that a second particle must be in the nucleus of an atom, as if the nucleus was entirely positive the protons would push each other apart. James Chadwick, in 1932, discovered that particle, the neutron, and found its mass.
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