Pictorial representation of the a particles through different angles.
A radioactive source S emitting a particles was collimated into a fine beam and made to fall on a thin gold foil. The a particles scattered in all directions. These produced bright flashes on falling on the fluorescent screen. A detector was used to observe this. On plotting a graph between the angles of scattering q and number of alpha particles N, the following observations were noted:
(i) Most alpha particles pass straight through the gold foil or suffer very small angle of scattering.
(ii) Here few a particles scattered through large angles (greater than 90o)
(iii) An a particle is rarely scattered through 180o.
Explanation
These observations explained that there was a coulomb interaction between a particles and electron. As a particles were 7000 times more massive than electrons, a strong force is needed to deflect them through large angles. Therefore the entire +ve charge of the gold foil was concentrated at the center called the atomic nucleus. If a a particle had a head on collision with the nucleus it scattered through 180o. Thus it was seen that a particles, which passed at larger distance from the nucleus, go almost undeviated. The scattering being due to elastic collision between nucleus and a particles, it was observed that due to Coulomb's repulsive force the K.E. of the alpha particles kept on decreasing as it approached the gold nucleus.
At a certain distance ro from the nucleus, the K.E. of a particle was completely converted to electric potential energy and this prevented the a particle to go further closer and so it retraced its path. This distance was called the distance of closest approach.
