An atom contains three particles: electrons, and protons and neutrons which are located in the nucleus.
The Geiger-Marsden Experiment
In this experiment a beam of alpha particles is aimed at a thin piece of gold foil. A zinc sulphide screen mounted on a rotatable microscope is used to detect the alpha particles. This experiment must be carried out in a dark room in order to see a small flash of light which can be seen whenever an alpha particle strikes the zinc sulphide screen. A majority of the particles will pass straight through the gold foil. A small fraction will bounce back to the source.
Rutherford’s model of the atom
Based on the G-M experiment, Rutherford made a model of the atom with a very dense core (or nucleus). All the positively charged particles and most of the mass of the atom are concentrated in this atom. As the atom is neutral, this model suggested an equal number of electrons to the protons that orbit around the nucleus. The nucleus and electrons occupy 1 × 10-12 of the volume of an atom which would mean that most of the atom is unoccupied. This explains why most of the alpha particles pass, and only a small number is affected by a strong repulsive force when they come near or head on to the positively charged nucleus. This repulsion causes the particles to deviate.