The series of experiments conducted by Rutherford and his co-worker to understand the arrangement of proton and electron in an atom is called Rutherford atomic model. It is also called as alpha particle scattering experiment.
In this experiment, a radioactive metal is placed in a lead cavity as shown in figure. The cavity is made in such a way with the silt that only a small beam of alpha particle is passed through the slit. The alpha particle is made to pass through a gold foil. To study the alpha particle after scattering, a circular movable foil is used around the gold foil. This movable foil is coated with the zinc sulphate so that alpha particles can produce flashes of light or scintillation in the foil. Some proportion of alpha particle that gets reflected through different angle is possible to determine by analyzing the different parts of zinc sulphide foil.
Following observation were made in this experiment.
- Most of the gold foil passed through the goil foil without deflection.
- Some of the alpha particles passes through the gold foil with deflection of some angle.
- Very few of the alpha particles do not pass through the foil at all but get deflected by large angle or even get rebound through an angle of 180°.
Above observation can be explained as below.
- Since most of the alpha particle passed without deflection, most of the space within the atom is empty.
- Since, some of the alpha particles were deflected though small angles, this means there is a small positively charged mass at the centre of atom. Moreover, the size of atom is extremely small in compared to the size of atom.
- The large deflection and the rebounding of alpha particle can be explained as the result of direct collision of alpha particle to the positively charged mass.
Rutherford gave the following model of atom:
- In an atom, there is a heavy positive mass in which whole mass is concentrated called nucleus. Moreover, the size of nucleus is extremely small in comparison to the size of atom. The radius of nucleus is taken in the order of 10-10m.
- The negatively charged electron revolves around the nucleus in circular path.
- The total negative charge in revolving the electron is exactly equal to the total positive charge present in the nucleus.
- The centrifugal force of revolving electron is balanced by the electrostatic attraction between the nucleus and electron.
Drawbacks of Rutherford’s atomic model:
- According to the classical theory of electrodynamics, an accelerated charged particle continuously emits electromagnetic radiation. So, the electron revolving around the nucleus most emit the electromagnetic radiation and it should gradually drift towards nucleus and finally should collapse into the nucleus. But, atom never collapse in this way.
- It could not explain the line spectra produced by hydrogen on application of strong electric or magnetic field.