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400
Democritus 400 BC
- All matter consists of invisible particles called atoms.
- Atoms are indestructible.
- Atoms are solid but invisible.
- Atoms are homogenous.
- Atoms differ in size, shape, mass, position, and arrangement. ->Solids are made of small, pointy atoms. ->Liquids are made of large, round atoms. ->Oils are made of very fine, small atoms that can easily slip past each other.
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John Dalton
Atomic theory – that is, the belief that all matter is composed of tiny, indivisible elements – has very deep roots. -
Robert Millikan 1868
Found the electric charge of the electron. -
JJ Thomson
Discovered the electron and developed the plum-pudding model of the atom. -
Ernst Rutherford
Used the result of his gold-foil experiment to state that all the mass of an atom were in a small positively- charged ball at the center of the atom. -
Neils Bohr
Stated that the electrons moved around the nucleus in successively large orbits. He also presented the Bohr atomic model which stated that the atoms absorb or emit radiation only when the electrons abruptly jump between allowed, or stationary, states. -
James Chadwick
Established that atomic number is determined by the numbers of protons in an atom.
He also discovered the fourth subatomic particle,the neutron.
