In order to understand the arrangement of electrons and protons in an atom, Rutherford and his coworkers performed a series of experiments known as Rutherford's scattering experiments. They bombarded alpha particles emitted from a radioactive substance on a piece of a thin film of gold.
- In this experiment, a piece of a radioactive substance is placed in a lead block. The block is constructed in such a way with slits that only a narrow beam of alpha-particles could escape. The beam then passes through the thin gold foil. In order to detect the alpha-particles after scattering a movable circular screen coated with zinc sulphide is placed around the gold foil. When alpha-particles strike the zinc sulphide screen, they produce flashes of light or scintillation which can be detected. By examining different protons of the screen, it was possible to determine the proportions of the alpha-particles which got deflected through various angles. The following observations were made from this experiment.
- Most of the alpha particles passed through the gold foil undeflected.
- Some of them got deflected through small angles.
- Very few did not pass through the foil at all but suffered large deflections or even came back suffering a deflection of 180 degrees.
Rutherford explained these observations as follows:
- Since most of the alpha-particles passed through the gold foil undeflected, it means that there must be a large empty space within the atom.
- Since some of the alpha-particles are deflected to some angles, it means that there is a heavy positively charged mass present in the atom. Moreover, this mass must be occupying a very small space within the atom because only a few alpha-particles suffered large deflections.
- The strong deflections or even bouncing back of alpha-particles from the foil were explained to be the direct collision with the heavy positively charged mass.
On the basis of the above experiment and observations, Rutherford proposed a model for the structure of the atom called Rutherford's nuclear model of the atom.
Limitations of Rutherford's Model
The objection to Rutherford's postulate was put forward by Clark Maxwell. According to him a charged particle (electron) continuously revolving around the nucleus under the influence of attractive forces, loses energy in the form of electromagnetic radiations. Due to this loss of energy, the speed of the electron would slow down and its orbit of revolution would become smaller and smaller and finally, the electron would fall into the nucleus.