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400 BCE
Democritus
- Democritus was the first person to suggest that all matter was composed of tiny particles called atoms.
- This term comes from the Greek word “atoms” meaning indivisible. (cannot be divided)
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John Dalton
- John Dalton described atoms as being solid indivisible spheres.
- Dalton’s atoms were not made up of smaller particles.
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J.J. Thompson
- J.J. Thompson proposed the “plum pudding” model of the atom.
- Thompson’s atoms were spheres (“pudding”) with a positive charge, studded with negative particles. (“plums”)
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Ernest Rutherford
- Ernest Rutherford proposed the planetary model of the atom.
- In this model, electrons orbit protons in the nucleus like planets orbit the Sun.
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Niels Bohr
- The Niels Bohr model of the atom consisted of successive shells of electrons orbiting the nucleus.
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James Chadwick
- James Chadwick discovered the neutron. Neutrons are located in the nucleus of atoms with protons.