Scientific Models of the Atom

• In early models, atoms were thought to be tiny spheres that could not be divided into simpler particles
• In 1898, Thompson discovered electrons and the representation of the atom had to be changed
• Overall, an atom is neutral, i.e. it has no charge
• Thompson 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 was deflected back to 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
• Later experiments showed that the nucleus is made of smaller particles:

  • Some of which have a positive charge and are called protons
  • Some of which have no charge and are called neutrons