Rutherford's Scattering Experiment

Rutherford in 1911 performed an experiment which led to the downfall of Thomson's model. The experiment involved the bombardment of a thin sheet of gold (thickness 4 * 10–5 cm) by α-particles. These particles were obtained in the form of a narrow beam by passing through a slit. A circular screen coated with ZnS was placed around the foil to detect the deflection suffered by α – particles (whenever, an α particle struck the screen, tiny flashes of light known as Scintillations are produced at that point).
From the experiment, Rutherford observed that;
(i). Most of the α – particles, nearly 99% passed through the gold foil un – deflected.
(ii). Some of the α – particles were deflected by small angles.
(iii). A few particles, usually one among 20,000 were either deflected by a very large angle or were usually reflected back along their path.

EXPLANATION OF OBSERVATIONS

(i). The gold foil used for scattering experiment is very thin, but it still contains almost 1000 layers of gold atoms, which means that there is hardly any gaps between the atoms through which α – particles can pass. Thus, it means that the α – particles striking the gold foil should bounce back but, most of α – particles pass through the gold foil indicating that the atom is extraordinarily hollow with a lot of empty space inside.
(ii). Some of the α – particles were deflected from their path. The α – particles being positively charged suggests that the centre of atom must be positively charged which repels the α – particles from their paths causing deviations. This positively charged central part is known as Nucleus.
(iii). Among 20,000 α – particles, only one retraced the path or were deflected at very large angles. This indicates that the nucleus must be very small as compared to an atom. The size of nucleus is found to be 10–15 m and that of an atom is 10–10 m.