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442 BCE
Democritus
Ancient Greek philosopher Democritus did an experiment to test a theory he learned from his teacher. He thought that if you take something and you cut it in half again and again, you would come to a point where you can't do that anymore. He tested his theory by taking a seashell and continuously cut it in half until he was left with powder. He took this powder and tried to cut it, but realized he couldn't. He called this unit an atom, or atomos, meaning "invisible." -
John Dalton
English chemist John Dalton forms the atomic theory of matter.
The theory states:
1. All elements are composed of atoms. It is impossible to divide or destroy an atom
2. All atom of the same elements are alike
3. Atoms of different elements are different
4. Atoms of different elements combine to form a compound. These atoms have to be in definite whole number ratios -
J.J. Thompson
English scientist J.J. Thompson discovered the electron when experimenting with gas discharge tubes. He noticed a movement in the tube, and called it cathode rays. The cathode rays moved from the negative end of the tube to the positive end, and he realized that the rays were made of negatively charged particles - electrons -
Niels Bohr
Danish physicist Niels Bohr proposed a model of the atom that is similar to the solar system. With this model, he said that the electrons orbit the nucleus like planets orbit the sun. He also said that all electrons are a certain distance from the nucleus because of their different energy levels. Level 1 can hold 2 electrons, Level 2 can hold 8, Level 3 can hold 18, Level 4 can hold 32. When electrons release energy, they go down a level. But when electrons absorb energy, they go up a level. -
Lord Ernest Rutherford
Rutherford conducted an experiment known as the Gold Foil Experiment, where he used a thin sheet of gold foil and special equipment to shoot alpha particles (positively charged particles) at the foil. Most of the particles went through the foil as if it wasn't there, but some went back or deflected like they hit something. This experiment showed two things.
1. Atoms are made of a small positive nucleus. Positive nucleus repels positive alpha particles
2. Atoms are mostly empty space
