The experimental set up used by Rutherlord and his, colaborators, Geiger and Marsden is shown in fig.
2) The `alpha`-particles emitted by radio active source contained in a lead cavity are collimated into a narrow beam with the help of a lead slit (collimator).
3) The collimated beam is allowed to fall on a thin gold foil of thickness of the order of `2.1 xx 10^(-7)m`.
4) The `alpha`-particles scattered in different directions are observed through a rotatable detector consisting of zinc sulphide screen and a microscope.
5) The `alpha`-particles produce bright flashes or scintillations on the ZnS screen.
6) These are observed in the microscope and counted at different angles from the direction of incidence of the beam.
7) The angle `theta` of deviation of an `alpha`-particle from its original direction is called its scattering angle `theta`
Observations : We find that
1) Most of the alpha particles pass straight through the gold foil. It means they do not suffer any collision with gold atoms.
2) Only about 0.14% of incident `alpha`-particles scatter by more than `1^(@)`
3) About one `alpha`-particle in every 8000 `alpha`-particles deflect by more than `90^(@)`.
Estimation of size of the nucleus :
1) This led to Rutherford postulate, that the entire positive charge of the atom must be concentration in a tiny central core of the atom. This tiny central core of each atom was called atomic nucleus.
2) The electrons would be moving in orbits about the nucleus just as the planets do around the sun.
3) Rutherford's experiments suggested the size of the nucleus to be about `10^(-15)" m to "10^(-14)m`, From kinetic theory, the size of an atom was known to be `10^(-10) m`, about 10,000 to 1,00,000 times larger than the size of the nucleus.
