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400 BCE
Democritus
Greek philosopher who lived between 470-380 B.C. He believed everything in the universe was made of atoms. Had no evidence to back it up. Thought atoms were microscopic and indestructible. -
John Dalton
Dalton was an English chemist. John Dalton's atomic theory said that all matter is made up of atoms. All atoms of an element were identical, different elements had different atoms. -
J. J. Thomson
Thompson was an English scientist. Disovered the electron when experimenting with gas discharge tubes. There was movement in a tube. He called the movement cathode rays. The rays moved from the negative end of the tube to the positive end. He realized that the rays were made of electrons. -
Niels Bohr
Bohr was a Danish physicist. He proposed a model of the atom that is similar to the model of the solar system. All electrons have their energy levels- a certain distance from the nucleus. Level 1 can hold 2 electrons, level 2 can hold 8 electrons, level 3 can hold 18 electrons, level 4 can hold 32 electrons. The energy of electrons goes up from level 1 to other levels. When electrons release energy they go down a level. When electrons absorb energy they go to a higher level.
