-rays consists of high energy particles carrying two units of positive charge and four unit of atomic mass. He concluded that α - particles are helium nuclei as when α - particles combined with two electrons yielded helium gas. β -rays are negatively charged Fig. .
Thomson model of atom can be visualised as a pudding or watermelon of positive charge with plums or seeds (electrons) embedded into it. An important feature of this model is that the mass of the atom is assumed to be uniformly distributed over the atom . Although this model was able to explain the overall neutrality of the atom, but was not consistent with the results of later experiments. Thomson was awarded Nobel Prize for physics in , for his theoretical and experimental investigations on the conduction of electricity by gases.
Table . Properties of Fundamental Particles Name Symbol Absolute charge/C Relative charge Mass/kg Mass/u Approx. mass/u Electron Proton Neutron e p – .602176× – + .602176× – – + .109382× – .6726216× – .674927× – .00054 .00727 .00867 represented in Fig. .
. A stream of high energy α –particles from a radioactive source was directed at a thin foil (thickness ∼ nm) of gold metal. The thin gold foil had a circular fluorescent zinc sulphide screen around it. Whenever α –particles struck the screen, a tiny flash of light was produced at that point.
The results of scattering experiment were quite unexpected. According to Thomson model of atom, the mass of each gold atom in the foil should have been spread evenly over the entire atom, and α –particles had enough energy to pass directly through such a uniform distribution of mass. It was expected that the particles would slow down and change directions only by a small angles as they passed through the foil. It was observed that: (i) most of the α –particles passed through the gold foil undeflected.
(ii) a small fraction of the α –particles was deflected by small angles. (iii) a very few α –particles ( ∼ in , ) bounced back, that is,