3) The development of the model of the atom

• In 1898, Thomson discovered electrons and the representation of the atom had to be changed.

  Overall, an atom is neutral, i.e. it has no charge.
• Thomson thought atoms contained tiny, negative electrons surrounded by a sea of positive charge. This was the 'plum- pudding' model.
• Later, Geiger and Marsden carried out an experiment in which they bombarded a thin sheet of gold with alpha particles. Although most of the positively charged alpha particles passed straight through the atoms, a tiny number were deflected back towards the source.
• Rutherford looked at these results and concluded that the positive charge in an atom must be concentrated in a very small

  area. This area was named the 'nucleus' and the resulting model became known as the 'nuclear' model of the atom.
• Bohr deduced that electrons must orbit the nucleus at specific distances, otherwise they would spiral inwards.