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460 BCE
Democritus
Democritus was a Greek philosopher who proposed the idea that matter is made of tiny, indivisible, indestructible particles called atomos, from English word atom (meaning indivisible) is derived. Since he was such an early scientist, he didn't have the necessary equipment to test his idea by using controlled experiments, and his idea were eventually rejected. https://www.youtube.com/watch?v=TgWn91ap99A -
John Dalton
John Dalton was an English chemist that believed all matter are made of atoms which are indivisible and indestructible. All atoms of a given element are identical in mass and properties. He said that compounds are formed by a combination of two or more different kind atoms. A chemical reaction is a re-arrangement. Even though our modern atomic theory is a little more involved, the essence of Dalton's theory is still valid. https://www.youtube.com/watch?v=kJ-6Qy05u_Q -
JJ Thompson
JJ Thompson was an English scientist that discovered electrons. He proved his theory by using the cathode ray tube. He placed two electric rays on his path, one being positive and other is negative. The glowing light and material bent towards the positive plate. Therefore, the electrons are negative charged. Since the particles were smaller then atoms, he called it electrons. https://www.youtube.com/watch?v=JwdGFZA3WOs -
Lord Ernest Rutherford
https://www.youtube.com/watch?v=kJ-6Qy05u_Q
Lord Ernest Rutherford was the first student of JJ Thompson. HE conducted a famous experiment called the gold foil experiment. He used a thin sheet of gold foil and equipment to shoot alpha particles which has a positive charge. Most particles past straight through the foil, others went straight back or got deflected as if they had hut into something. This experiment shows how atoms are made of a small positive nucleus and atoms are mostly empty space -
Niels Bohr
Niels Bohr was a Danish physicist. His greatest contribution to modern physics was the atomic model. He was first to discover the electrons travel in separate orbits around the nucleus and that the number of electrons in the outer orbit determines the property of the element. Therefore, his model shows the atom as a small, positively charged nucleus surrounded by orbiting electrons. https://www.youtube.com/watch?v=Cn6v5ygyZHQ
