The History of The Atom

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  • Atoms - Lesson 4 - History of the Atom
    • John Dalton published his ideas about atoms in 1803. He thought that all matter was made of tiny particles called atoms, which he imagined as tiny spheres that could not be divided.
      • Nearly 100 years later, J J Thomson carried out experiments and discovered the electron. This led him to suggest the plum pudding model of the atom. In this model, the atom is a ball of positive charge with negative electrons embedded in it - like currants in a Christmas pudding.
    • In 1909 Ernest Rutherford designed an experiment to test the plum pudding model. In the experiment, positively charged alpha particles were fired at thin gold foil. Most alpha particles went straight through the foil. But a few were scattered in different directions.
      • This evidence led Rutherford to suggest a new model for the atom, called the nuclear model. In the nuclear model:the mass of an atom is concentrated at its centre, the nucleusthe nucleus is positively charged
    • Niels Bohr adapted Ernest Rutherford's nuclear model. Bohr did calculations that led him to suggest that electrons orbit the nucleus in shells. The shells are at certain distances from the nucleus. The calculations agreed with observations from experiments.
      • This evidence led Rutherford to suggest a new model for the atom, called the nuclear model. In the nuclear model:the mass of an atom is concentrated at its centre, the nucleusthe nucleus is positively charged
      • Further experiments led to the idea that the nucleus contained small particles, called protons. Each proton has a small amount of positive charge.
        • In 1932 James Chadwick found evidence for the existence of particles in the nucleus with mass but no charge. These particles are called neutrons. This led to another development of the atomic model, which is still used today.

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