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460 BCE
Democritus
Greek philosopher, Democritus, developed the idea of the atom. He realized that you can't break something in half forever and at some point you would be left with the smallest possible bit of matter, the atom. However, Aristole dismissed his idea and for more than 2000 years no one did anything to explore the atom. -
John Dalton
John Dalton was a British chemist who revived Democritus's ideas about the atom around the 1800s. He developed one of the most important theories in science, the atomic theory. The atomic theory Dalton developed consisted of 3 ideas, all substances are made of atoms and atoms are the smallest particles of matters, all atoms of the same element are alike and have the same mass, and that atoms join together to form compounds. Dalton's theory was soon widely excepted. -
John Dalton pt 2
Dalton did many experiments to prove that atoms were real. He studied the pressure of gases and concluded that gases must consist of tiny particles in motion. Dalton also researched the properties of a compound and showed that a compound always has the same elements in the same ratio.
Dalton designed atomic models to model atoms and how they are tiny solid particles of matter. When later scientists discovered subatomic particles they realized Dalton's models were too simple. -
J. J. Thomson
J.J. Thomson, a British physicist, was the 1st to discover electrons around 1897. Electrons were the first subatomic particles to be identified. Thomson was interested in electricity, he did experiments where an electric current would past through a vacuum tube. When connected to a power source one end would become positive and the other would become negative. Thomson discovered that electric current flowed through the tube from the negative plate to the positive plate in negative charge. -
Ernest Rutherford
Ernest Rutherford was a physicist from New Zealand. He discovered the nucleus. He composed a Gold foil experiment. Rutherford discovered that some elements give off positively charged particles. He named them alpha particles. In 1911 he used the alpha particles to study atoms. He aimed a beam of alpha particles at a very thin sheet of gold foil. Outside the foil, he placed a screen of material that glowed when alpha particles struck it. -
J. J. Thomson pt 2
J. J. Thomson made the plum pudding model, he knew that atoms are neutral in an electric charge, so he wondered how atoms could contain negative particles. Thomson thought that the rest of the atom must be positive to cancel out the negative charge. He said an atom is like plum pudding, which has plums scattered through it. That's why he called it the plum pudding model. -
Ernest Rutherford pt 2
Based on Rutherford's results, he was able to conclude that all of the positive charge of the atom is located in a small center, which he called the nucleus. He named the positive particles protons. He also predicted the neutrons but failed to find them. -
Niels Bohr
Niels Bohr was a Danish physicist. He composed the theory for the hydrogen atom based on quantum theory that energy is transferred only in a certain well defined quantities. Electrons should move around the nucleus but only in certain orbits. Bohr's theory could explain why atoms emitted light in fixed wave lengths.
