Draw a schematic arrangement of Geiger-Marsden experiment showing the scattering of C-particles by a thin foil of gold. Why is it that most of the O-particles go right through the foil and only a small fraction gets scattered at large angles ?

Explanation

A schematic arrangement of the Geiger-Marsden experiment is shown in Fig. 12.11.
In their experiment it was observed that most of the alpha-particles pass through the gold foil without any appreciable deflection. Some a particles are scattered through different angles and a very small number of particles suffers large angle scattering of about `180^(@)`
Large angle scattering can be explained if we consider the entire positive charge and almost whole mass of gold foil atom to be concentrated in a tiny central core of the atom. When an `alpha`-particle with kinetic energy K approaches the gold nucleus directly along its central line it slows down, as it approaches the a-particles gold nucleus, due to coulombian repulsion force between nucleus and `alpha`-particle. The `alpha` -particle can come up to a certain minimum distance is called the distance of closed approach . Obviously value of distance of closest approach provides the upper limit on the size of the nucleus .