Discovery of Atomic Structure

1897: J. J. Thomson discovered the electronApplying large voltages to gases at low pressure in a glass tube caused the gas to glow as it was struck by 'cathode rays' from the negative electrode which were electrons emitted from the cathode.J. J. Thomson thought that the atom was a cloud of positive charge with tiny little negative electrons embedded in the cloud rather like the bits of fruit in a plum pudding.1909-1911: Rutherford's nuclear modelIn 1909 Geiger and Marsden, working with Rutherford, fired positively charged alpha particles at a thin gold foil. Most of the alpha particles passed straight through the foil. About 1 in 10,000 were deflected through large angles or bounced straight back. The results suggested that most of the atom was empty space, but the positive charge must be concentrated in a tiny, dense, positive nucleus which repelled the alpha particles and deflected them through large angles. Rutherford published this nuclear model in 1911.1917: Rutherford discovered the proton, the positive particle in the nucleus.1932: Chadwick discovered the neutronScientists had noticed that the mass of the nucleus could not be accounted for by protons alone. Neutrons were difficult to detect as they had no charge. James Chadwick discovered the neutron in 1932.