a– particles emitted by a radioactive substance were shown to be dipositive Helium ions (He++) having a mass of 4 units and 2 units of positive charge.
Rutherford allowed a narrow beam of a–particles to fall on a very thin gold foil of thickness of the order of 0.0004 cm and determined the subsequent path of these particles with the help of a zinc sulphide fluorescent screen. The zinc sulphide screen gives off a visible flash of light when struck by an alpha particle, as ZnS has the remarkable property of converting kinetic energy of a particle into visible light. [For this experiment, Rutherford specifically used a particle because they are relatively heavy resulting in high momentum].
i) Majority of the alpha–particles pass straight through the gold strip with little or no deflection.
ii) Some alpha–particles are deflected from their path and diverge.
iii) Very few alpha–particles are deflected backwards through angles greater than 90 degrees.
iv) Some were even scattered in the opposite direction at an angle of 180 degrees.