Our picture of the atom has changed as new evidence has appeared. In 1897 J. J. Thomson discovered the electron. He suggested the plum pudding model: an atom is a ball of positive charge with tiny negative electrons embedded in it, like fruit in a pudding.
In about 1909 Geiger and Marsden, working with Rutherford, fired alpha particles at a very thin sheet of gold foil. Most alpha particles passed straight through. A few were deflected through large angles, and a very small number bounced almost straight back. The plum pudding model could not explain this.
Rutherford explained the results with the nuclear model. Most of the atom is empty space, which is why most alpha particles went straight through. All the positive charge and almost all the mass is squeezed into a tiny, dense nucleus, which repelled the few alpha particles that came close. The plum pudding model was replaced.
Niels Bohr later improved the nuclear model. He suggested that electrons orbit the nucleus only at fixed distances, called shells (or energy levels). Scientists tested each model against evidence, and a model is changed when new evidence does not fit it.
