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440
Democritus
Democritus discovered the atom. He believed it could not be divided, He thought they were small hard particles, with different shapes and sizes. Democritus explained the origin of the universe through atoms moving randomly and colliding to form larger bodies and worlds. Not able to be divided means "atomos" in greek. -
Period: 440 to
Atomic Theory
The Progression of the Atomic Theory by Rachael Stowell -
Dalton
Dalton did not mean to intentionally discover anything about the atom at first. He was originally wanting to know how and why hydrogen and oxygen formd into water. While researching, he found the atom as well. He thought the atom could not be created and divided, or destroyed. He also thought that all atoms are exactly alike and atoms join together with other atoms. He believed the atoms were solid spheres. -
Ernest Rutherford
He discovered alpha and beta rays, created the laws of radioactive decay, and identified alpha particles as helium nucluis. Most importantly, he found the nuclear structure of the atom. -
Joseph Thomson
Joseph Thomson discovered the electron by using a high-vacuum cathode-ray tube. Thomson found electrons by the deflection of the rays by electrically charged plates and magnets as proof that they are "bodies much smaller than atoms." His design of the atom was called the "plum pudding model." It was a sphere with dots all over it. -
Joseph Thomson
Thomson suggested a model of the atom as a sphere of positive matter. Thomson also wondered about how many electrons were inside an atom. -
Niels Bohr
Niels Bohr studied under J. J. Thomson, who discouraged his ideas, and under Ernest Rutherford, whose work was expanded by Bohr into a new theory on the structure of the atom in 1913. Bohr suggested that electrons travel in orbits around the atom's nucleus, and that further explained how electrons put out or absorb energy, Bohr's design of the atom was the actual middle of todays atom with surrounding rings, very close to what we have today. -
James Chadwick
James Chadwick proved the existence of neutrons, the elementary particle without any electrical charge and a fundamental building block of the atom's nucleus. He studied under Hans Geiger before he was imprisoned in World War Two in Poland. -
Darleane Hoffman
Hoffman discovered that the atoms of one element, could split spontaneously. She has helped investigate nuclear weapons test sites for dangerous leakage of radioactive materials into the environment and is interested in the safety issues surrounding the solid radioactive waste produced by nuclear power and bombs. -
Helen Vaghn Michel
Scientists believed that 65 million years ago, an asteroid hit the Earth sending particles and dust everywere. Helen helped prove this hypothesis to be right by studing the the radioactive emissions and radioactive decay paths of the atoms which she discovered how to do in 1977.
