Rutherford's α ray scattering experiment and nuclear model of atom; limitation

After the discovery of the radioactive particles, Rutherford performed an experiment where he bombarded a thin sheet of gold with a particles obtained from a radioactive substance. On striking the gold foil some of the a particles scattered and produced flashes on a zinc sulphide (ZnS) screen placed at the back of the gold foil. These tiny flashes were observed by a movable microscope. The observations made in this scattering experiment were as follows:

• Most of the a particles pass through the metal and are undeflected.

• Some of the a particles are deflected through small angles.

• Only a very few of them are deflected through as much as 90^o or even larger angles.

The results of the scattering experiment could not explain the Thomson's atomic model. Hence, Rutherford

concluded that:

• As most of the a particles passed undeflected, most of the space inside the atom is empty or hollow.

• Some that deflected with large angles show that heavy positively charged body must be present inside the body of the atom, which repelled the like charge of the a particle. This heavy, positively charged body was named as nucleus.

• The number of heavy positively charged particles that undergo deflection is very small. The volume occupied by the nucleus is very small compared to the volume of the atom.

• When heavy particles like the a get deflected, the nucleus of the atom must also have an appreciable mass.

Rutherford's nuclear model

On the basis of the scattering experiment, Rutherford described the structure of the atom as:

• An atom consists of a positively charged nucleus surrounded by electrons that move around it. The positive charge of the nucleus is due to the protons.

• Electrons and neutrons are held together by coulombic force of attraction.

• The effective volume of the nucleus is extremely small as compared to the effective volume of the atom. The volume occupied by the nucleus is about 10^-12 times the volume of the atom.

• The entire mass of the atom is concentrated at the nucleus.

• Since each atom is electrically neutral, the number of positive charges in the nucleus of an atom is equal to the number of electrons in it.

Fig: 3.12 - Rutherford's model of an atom

Limitations

Rutherford's model suffered some drawback, as it could not explain the stability of the atom in-spite of the revolving electrons around the nucleus. The moving electrons should emit radiations and lose energy. They should slow down, gradually move towards the nucleus, follow a spiral path and then fall into the nucleus. This would make the atom collapse and hence unstable, which is not true.