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The history of the atom
The first model of the atom was put forward by John Dalton in 1808. He knew that matter was made up of atoms and guessed at the shape of an atom as a solid ball therefore the “billiard ball” model.
In 1897 the physicist J.J. Thompson discovered the electron and brought forward a new model. This model showed the negatively charged electrons stuck in positive matter. This improved on Dalton’s model by showing electrons which were unknown in Dalton’s time and by showing the negative and positive charges. His model was called the “Plum pudding” model.
The next model of the atom was created by Ernest Rutherford in 1911 who worked on the findings of Geiger and Marsden. They found that when alpha particles were beamed through gold foil that some were reflected by the foil even though it was only 0.00004 cm thick. The reflected particles hit a screen they had set up to one side. One in twenty thousand alpha particles had been deflected 45 degrees or more. Rutherford wondered why so many got through but the ones that were reflected were reflected so greatly. He thought about it and came up with the idea that there was a small nucleus, positively charged, which had electrons orbiting it. This improved on J.J. Thomson’s model by showing the electrons moving instead of staying stuck and by having a small nucleus. Rutherford’s model was often called the “Planetary model”
In 1913 Niels Bohr proposed the quantized shell theory which stated that the electrons moved in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. The atom will be completely stable in the case of the lowest orbit since there is less energy for a jump. This model improved on Rutherford’s theory by stabilizing the atom and by creating the atomic shells.