Ernest Rutherford and his co-workers were working in the area of radioactivity. They were studying the effect of alpha (α) particles on matter. The alpha particles are helium nuclei, which can be obtained by the removal of two electrons from the helium atom.
In 1910, Hans Geiger (Rutherford’s technician) and Ernest Marsden (Rutherford’s student) performed the famous α-ray scattering experiment. This led to the failure of Thomson's model of atom.
In this experiment, a stream of α particles from a radioactive source was directed on a thin (about 0.00004 cm thick) piece of gold foil. On the basis of Thomson's model it was expected that the alpha particles would just pass straight through the gold foil and could be detected by a photographic plate placed behind the foil.
However, the actual results of the experiment were quite surprising. It was observed that:
The results of α-ray scattering experiment were explained by Rutherford in 1911 and another model of the atom was proposed.
According to Rutherford’s model:
On the basis of the proposed model, the experimental observations in the scattering experiment could be explained. The α particles passing through the atom in the region of the electrons would pass straight without any deflection. Only those particles that come in close vicinity of the positively charged nucleus get deviated from their path. Very few α-particles, those that collide with the nucleus, would face a rebound.
On the basis of his model, Rutherford was able to predict the size of the nucleus. He estimated that the radius of the nucleus was at least 1/10000 times smaller than that of the radius of the atom.
According to Rutherford’s model the negatively charged electrons revolve in circular orbits around the positively charged nucleus. However, according to Maxwell’s electromagnetic theory, if a charged particle accelerates around another charged particle then it would continuously lose energy in the form of radiation. The loss of energy would slow down the speed of the electron. Therefore, the electron is expected to move in a spiral fashion around the nucleus and eventually fall into it.
In other words, the atom will not be stable. However, the atom is stable and such a collapse does not occur. Thus, Rutherford's model is unable to explain the stability of the atom.
An atom may contain a number of electrons. The Rutherford’s model also does not say anything about the way the electrons are distributed around the nucleus.
Another drawback of the Rutherford’s model was its inability to explain the relationship between the atomic mass and atomic number (the number of protons). This problem was solved later by Chadwick by discovering neutron, the third particle constituting the atom.
The problem of the stability of the atom and the distribution of electrons in the atom was solved by Neils Bohr by proposing yet another model of the atom.