2.2.2 RUTHERFORD'S NUCLEAR MODEL OF ATOM
Rutherford’s Nuclear Model of Atom
Rutherford proposed an atom model based on his α–particle scattering experiment. He bombarded a very
thin gold foil with α–particles.
The Experiment: A stream of high energy α–particles from a radioactive source was directed at a thin gold
foil. The thin gold foil was surrounded by a circular fluorescent zinc sulphide screen. Whenever α–particles
struck the screen, a tiny flash of light was produced at that point.
1. Most of the α– particles passed through the gold foil without any deviation.
2. A small fraction of the α–particles was deflected by small angles.
3. A very few α– particles (∼1 in 20,000) bounced back, that is, were deflected by nearly 180°.
Observations: The important observations made by Rutherford are:
1. Most of the α– particles passed through the gold foil without any deviation.
2. A small fraction of the α–particles was deflected by small angles.
3. A very few α– particles (∼1 in 20,000) bounced back, that is, were deflected by nearly 180°.
Conclusions: From the above observations, Rutherford made the following conclusions:
1. Since most of the α–particles passed through the foil without any deviation, most space in the atom
is empty.
2. A few positively charged α– particles were deflected. This is because the positive charge of the atom
is concentrated in a very small volume at the centre called nucleus.
3. The volume occupied by the nucleus is negligibly small as compared to the total volume of the atom.
The radius of the atom is about 10–10 m, while that of nucleus is 10–15 m.
On the basis of above observations and conclusions, Rutherford proposed the nuclear model (Planetary
model) of atom. According to this model:
1. All the positive charge and most of the mass of the atom are concentrated in an extremely small
region called nucleus.
2. Electrons are revolving round the nucleus with a very high speed in circular paths called orbits.
3. Electrons and the nucleus are held together by electrostatic forces of attraction.
Drawbacks or Limitations of Rutherford’s atom model
1. Rutherford’s model cannot explain the stability of the atom.
2. He could not explain the electronic structure of atom.: From the above observations, Rutherford made the following conclusions:
1. Since most of the α–particles passed through the foil without any deviation, most space in the atom
is empty.
2. A few positively charged α– particles were deflected. This is because the positive charge of the atom
is concentrated in a very small volume at the centre called nucleus.
3. The volume occupied by the nucleus is negligibly small as compared to the total volume of the atom.
The radius of the atom is about 10–10 m, while that of nucleus is 10–15 m.
On the basis of above observations and conclusions, Rutherford proposed the nuclear model (Planetary
model) of atom. According to this model:
1. All the positive charge and most of the mass of the atom are concentrated in an extremely small
region called nucleus.
2. Electrons are revolving round the nucleus with a very high speed in circular paths called orbits.
3. Electrons and the nucleus are held together by electrostatic forces of attraction.
Drawbacks or Limitations of Rutherford’s atom model
1. Rutherford’s model cannot explain the stability of the atom.
2. He could not explain the electronic structure of atom.
