Thomson's and Rutherford's Atomic Model

Thomson’s Model of Atom

According to J.J. Thomson, the electron is a constituent of all matter. It has a negative charge of 1.602 × 10^-19 C and a mass of 9.1085 × 10^-31 kg at rest. Electrons are distributed along with positive charges in the atom in a sphere of radius 10^-10 m like a plum in the pudding. Atom is electrically neutral as there is an equal number of positive and negative charges.

Failure of Thomson’s Atom Model

1. It could not explain the origin of spectral series as in a case of a hydrogen atom.
2. It could not explain the large angle scattering of α-particles from thin metal foils, as observed by Rutherford.

Rutherford’s Experiment on scattering of α-particles

In this experiment, α-particles are emitted by some radioactive material (polonium) R, kept inside a thick lead box. This well-collimated beam is then allowed to fall on the gold foil F. while passing through the gold foil, the α-particles are scattered through different angles. The scattered α-particles are in a particular direction are allowed to strike on a screen coated with zinc sulphide. When a α-particle is incident on zinc sulphide, it produces fluorescence and is detected with the help of the microscope M.

Observation

1. Most of the α-particles pass straight through the gold foil.
2. Some of the α-particles were scattered by small angles.
3. There were a few α-particles that were scattered through large angles.
4. A few of the α-particles were even scattered directly in the backward direction.

   

Rutherford’s Atomic Model

On the basis of α-particle scattering experiment, Rutherford suggested his atomic model

1. An atom consists of a positive nucleus in which entire positive charge and almost whole mass of the atom are concentrated.
2. The size of the nucleus is of the order of 10^-15 m which is small compared to the size of the atom (10^-10 therefore the nucleus occupies only a small portion of the available space.
3. As atom, on the whole, is electrically neutral, so the total negative charge of electrons surrounding the nucleus is equal to the total positive charge in the nucleus.
4. Electrons moves around the nucleus in different orbits like the planets moves around the sun. the necessary centripetal force for the electrons to revolve around the nucleus is provided by the electrostatic force of attraction between them and the nucleus.

Conclusion

1. The large angle scattering of α-particles could happen, only if the positively charged α-particles were being repelled by a massive positive charge concentrated in a very small region of space.
2. Since very few particles were scattered through large angles, the probability of a head on approach is small and indicates that the nucleus occupies only a small portion of the available space.
3. A large number of α-particles passing undeviated through the atom indicates that the atom indicates that the remaining part of the atom around the nucleus should be empty or contain electrons which are too light to affect the motion of the α-particles which are quite heavy as compared to the electrons.

Drawbacks of Rutherford’s Atom Model

1. According to Maxwell’s theory of electromagnetism, a charge that is accelerating radiates energy as electromagnetic waves. The electron moving around the nucleus under constant accelerating radiates energy as electromagnetic waves, the electrons in Rutherford’s model were radiated. Hence, Rutherford’s model cannot be stable. However in actual practice, an atom is stable. Therefore, Rutherford’s atom model could not explain the stability of the atom.
2. Electrons can revolve in orbits of all possible radii and it should emit radiation of all wavelengths, however, in a case of an atom like hydrogen, experimentally it was found that line spectra have been observed. Therefore, Rutherford failed to explain line spectra of hydrogen.

   

Reference

Manu Kumar Khatry, Manoj Kumar Thapa, Bhesha Raj Adhikari, Arjun Kumar Gautam, Parashu Ram Poudel.Principle of Physics. Kathmandu: Ayam publication PVT LTD, 2010.

S.K. Gautam, J.M. Pradhan. A text Book of Physics. Kathmandu: Surya Publication, 2003.