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460 BCE
Democritus
Democritus was a Greek philosopher who proposed the idea that matter is made of tiny, indivisible and indestructible particles that he called atomos, which in English is pronounced as atoms. However, his ideas were rejected because he did not have the equipment to prove them. -
John Dalton
Dalton created a theory which consisted of three ideas: number one, all ideas are composed of atoms. Number two, all atoms of the same element are alike and atoms of different are different. Finally, he said that elements combine to form a compound. These atoms have to be in definite whole number ratios. For ex., 2 atoms of H and 3 atoms of O make water. In other words, 3 atoms of H and 1 atom of O cannot make water! -
J.J. Thompson
J.J Thompson was an English scientist, He discovered the electron when he was experimenting with gas discharge tubes. He noticed a movement in a tube. He called the movement cathode rays. The rays moved from the negative end of the tube to the positive ones. He realized that the rays were made of negatively charged particles called electrons. -
Rutherford
Ernest Rutherford conducted a famous experiment called the gold foil experiment. He also used special equipment to shoot alpha particles (positively charged particles) at the gold foil. Most particles passed right through the foil like the foil wasn't there. Some particles went straight back or were deflected as if they had hit something. His ideas were that that atoms are mostly empty space and atoms are made of a small positive nucleus, positive nucleus repels positive alpha particles. -
Neils Bohr
He was a Danish physicist. He proposed a model that look like the solar system. The electrons go around the nucleus like planets orbit the sun. All electrons have their energy levels, in other words, a distance from the nucleus. When the electrons release energy they go down a level. When electrons absorb energy, they go to a higher level.
