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450 BCE
Democritus
Democritus discovered the atom, or atomos. He experimented with an apple. He started by cutting an apple into smaller and small pieces. He did this because he believed that there would be a point where matter could not be cut any smaller. -
John Dalton
Dalton discovered the atomic theory. The atomic theory states that all substances are made of atoms, all atoms of the same element are alike and have the same mass, and that atoms join together to form compounds. He came up with properties of compounds, stating that compounds consist of the same elements in the same ratio. He also studied the pressure of gases and concluded that gases must consist of tiny particles in constant motion. Dalton modeled his discovery with wooden balls. -
J.J. Thomson
Thomson discovered the first subatomic particle, the electron. He experimented with electricity by passing electric currents through a vacuum tube. He concluded many things from that experiment. He concluded that electric currents consist of flowing negatively charged particles. Also that negative particles are all alike and smaller than atoms. And since they are all alike, they couldn't be fundamental units of matter. -
J.J. Thomson
Thomson is most popular for his Plum pudding model of the atom. Plum pudding has plums scattered all throughout it so the plums represented the electrons. Thomson had thought that the rest of the atom had to have been positive to cancel out all of the negative so he had the rest of the plum pudding be postive charge. -
Ernest Rutherford
Rutherford discovered that some elements give off positively charged particles and also the nucleus. He experimented by aiming a beam of alpha particles at a thin sheet of gold foil. He placed a screen of material thepat glowed when alpha particles struck it. Some of the particles passed through the foil while others bounced back, or back scattering. He realized that they would only bounce back if they hit very small areas at the center of the foil. -
Ernest Rutherford
Rutherford concluded that there is a positive charge of an atom in a small central area. He called it a nucleus. He also discovered positive charged particles and named them protons. Finally he predicted the existence of neutrons. -
Niels Bohr
Bohr discovered what happens inside an atom and he developed a picture of the atomic structure. He discovered that electrons travel in separate orbits.
