The nuclear model of the atom was first suggested by Rutherford in 1910. The first exploration of the structure of atom was made by Rutherford by making use of $\alpha$ particles. On the basis of the results obtained from the scattering experiments, Rutherford suggested an atomic model, according to which the entire positive charge in the atom is concentrated to a very small region called nucleus.
On the basis of his analysis, Rutherford introduced his model of the planetary atom or solar atom according to which the positive charge and the majority of the atomic mass is concentrated in a tiny nucleus at the center of the atom. The electrons on the other hand, are distributed throughout a sphere of atomic dimensions.
Thomson ModelBack to Top
This model satisfactorily explained as to how on heating a substance, it starts radiating light. Though this was the first attempt at understanding what an atom could be, it failed to satisfy the results of the experiments performed later by Rutherford and others. Thomson suggested in 1911 that his model just cannot answer how and why the $\alpha$-ray scattering should take place, on the basis of his atomic model. So this model is discarded.
Ernest Rutherford's DiscoveryBack to Top
Rutherford discovered that most of the mass of the atom is concentrated at its center in a very tiny nucleus. The electrons move around the nucleus but most of the atoms is in empty space.
Rutherford Atomic TheoryBack to Top
The electrons revolve round the nucleus with high velocities to counterbalance the electrostatic forces of attraction between protons and electrons. Rutherford's atomic model resembles the planetary motion in solar system. Therefore, Rutherford's model of an atom called planetary model.
Gold Foil ExperimentBack to Top
In his gold foil experiment, Rutherford bombarded a piece of thin gold foil with alpha particles. These particles would do one of two things: they would either hit the foil and continue moving in the same direction or they would bounce off of the foil and be reflected at acute angles.
Rutherford found that alpha particles travel right through the foil, while only a few were deflected. He realized that these deflections were the result of electrical repulsions between the positive alpha rays and the positively charged components of the atom. Rutherford was able to conclude that the positive charges inside the atoms of the foil had to be concentrated otherwise the alpha particles would would not have been deflected backwards as he had observed. He call this small positively charged region the nucleus.
The graph between the angle of scattering and the number of $\alpha$-particles scattering in the corresponding direction is shown below.
The Rutherford formula used for scattering experiments is given below.
- The attraction of electrons
- The repulsion by positive charge in the atom
Ernest Rutherford Atomic ModelBack to Top
- The atom consists mainly of space
- The mass of the atom is concentrated in the nucleus which is a small core at the center of the atom.
- The nucleus has positive charges.
- Electrons move around the nucleus like planets orbiting the sun.
- The atom is neutral as its has the same number of positive charges and negatively charged electrons.
The Rutherford atomic model features are as follows.
- Mass and positive charge of the atom are concentrated in the nucleus the size of which is of the order of 10-15m = 1 Fm
- Negatively charged electrons revolve about the nucleus in a cloud, the radius of which is of the order of 10-10m = 1Ao
The Rutherford atomic model picture is shown below.
Rutherford Nuclear ModelBack to Top
Rutherford nuclear model assumed the electrons outside the nucleus, either to be stationary or moving in a circle. Thus if electrons were stationary they would have been attracted towards nucleus and would have fallen into the nucleus.
The drawbacks of this model is
- The theory did not explain the distribution of electrons in the orbit.
- The theory did not explain the stability of the atom as a whole.
Rutherford Contribution to the Atomic TheoryBack to Top
After doing many calculations Rutherford concluded that the radiation could only come back if that atom had a hard positively charged core at the center of the atom.