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200
Democritus 440 B.C.
Democritus thought you would be able to find a particle that could not be cut. He believed that this particle was an atom. He said that all atoms are small, hard particles and thought that they were made of a single material formed into different shapes and sizes. -
200
Aristotle 382 B.C.
Aristotle believed that you could never end up with a particle that could not be cut. This disagreed with Democritus' theory. He had a very strong influence that made people believe him for a long time. But, it turned out that Democritus was correct. -
Dalton
Daltons atomic theory stated: That all substances are made of atoms. Atoms are small particles that cannot be created, devided, or destoyed. Atoms of the same element are exactly alike, and atoms of different elements are different. Atoms join with other atoms to make new substances. -
Thomson
Thomson discovered that Dalton's theory had a mistake. He found out that there are small particles inside of the atom, which means that atoms can be divided into smaller particles. Thomson expiremented with a cathode-ray tube. He then discovered that a positively charged plate attracted the beam. He concluded that the beam was made up of particles with negative charges. Also he figured out the negatively charged particles are in every kind of atoms, these particles are now called electons. -
Rutherford 1909
Rutherford, a former student of Thomson, decided to test Thomson's theory. He created an experiment to study the parts of an atom. He pointed a beam of small, positively charged particles at a thin sheet of gold foil with a special coating behind it. This coating glowed when it was hit by the positively charged particles. He could then see where the particles went after hitting the gold. -
Rutherford 1911
Rutherford revised the atomic theory and made a new model of the atom. He said that in the center of an atom is a tiny, dense, positively charged part called the nucleus. Since like charges repel, he found that positively charged particles that passed close by the nucleus were pushed away by the positive charges in the nucleus. He calculated that the diameter of a nucleus was 100,000 times smaller than the diameter of the gold atom. -
Bohr
Bohr studied the way that atoms react to light. His results made him propose that electrons move around the nucleus in certain paths, or energy levels. In his model there are no paths between levels. But, the electrons can jump from a path in one level to a path in another level. His model was a good tool in predicting some atomic behavior. -
Schrodinger and Heisenberg
Schrodinger and Heisenberg explained the nature of electrons in the atom. They discovered that electrons did not travel in paths like Bohr said. Actually, the found out that the path of an atom cannot be predicted.
