The Experiment

In 1911, Rutherford and his students Geiger and Madsen performed an experiment known as the alpha particle scattering experiment. They aimed to understand the arrangement of electrons and protons in an atom. In this experiment, they bombarded a thin gold foil with a stream of high-energy alpha particles, using a circular fluorescent ZnS screen around it. A small flash of brightness was observed where the alpha particles struck the screen.

At that time, the neutron had not yet been discovered, so the experiment did not reveal the position of neutrons in the atom. In 1920, Rutherford only speculated about the presence of a neutral particle in the nucleus.

Observations

• 99 percent of alpha particles passed through the foil undeflected.
• A small number of particles were deflected at small angles.
• Very few particles were deflected at large angles, and only 0.001 percent of the particles bounced back.

Conclusions

From these observations, Rutherford drew the following conclusions:

1. As most alpha particles passed through the foil, it indicated that most of the atom is empty.
2. Alpha particles are positively charged. Deflection occurs only when they come close to a positively charged mass due to repulsion. The deflection of some particles indicated the presence of a small, dense, positively charged mass in the atom.
3. Alpha particles are heavy, approximately four times heavier than a hydrogen atom. They can only be deflected back by a denser positive body. The large-angle deflections suggested that the positive mass in the atom has a high density.
4. The nucleus occupies a very small volume compared to the atom. The radius of an atom is around 0.0000000001 meters, while the radius of the nucleus is about 0.000000000000001 meters.

Rutherford’s Atomic Model

Based on these observations, Rutherford proposed a new atomic model known as the nuclear model of the atom. The key postulates of this model are:

• An atom is spherical, with most of it being empty space.
• At the center of the atom is a very small, dense, positively charged region called the nucleus. The entire mass of the atom is concentrated in the nucleus.
• The nucleus is surrounded by electrons that revolve around it at high speeds in circular orbits.
• Electrons and the nucleus are held together by electrostatic forces of attraction.
• The size of an atom is approximately 10^5 times larger than the size of the nucleus.

Limitations

Rutherford’s atomic theory had several limitations:

1. Stability of an Atom: Rutherford’s model does not explain the stability of the atom. According to electrodynamics, a moving electron should lose energy in the form of electromagnetic radiation and spiral into the nucleus, leading to atomic collapse. However, atoms remain stable, which contradicts this theory.
2. Formation of Line Spectra of Hydrogen: If electrons continuously lose energy, the atomic spectra would be continuous. However, actual atomic spectra are discontinuous, as observed in hydrogen spectra.
3. Electronic Structure and Energy of Electrons: The model does not provide information about the electronic structure of atoms, the distribution of electrons around the nucleus, or the energy levels of electrons.

References

• Rutherford, E., Geiger, H., & Marsden, E. (1911). “On the Scattering of α and β Particles by Matter and on the Structure of the Atom”. Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences.
• Bohr, N. (1913). “On the Constitution of Atoms and Molecules”. Philosophical Magazine.
• Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics. Addison-Wesley.
• Haken, H., & Wolf, H. C. (2004). The Physics of Atoms and Quanta. Springer.
• Serway, R. A., & Jewett, J. W. (2013). Physics for Scientists and Engineers with Modern Physics. Cengage Learning.
• Tilley, D. R., Tilley, J. C., & Ramamurthi, A. (2014). Nuclear Physics: Principles and Applications. Wiley.
• Krane, K. S. (1988). Introductory Nuclear Physics. Wiley.
• Schiff, L. I. (1968). Quantum Mechanics. McGraw-Hill.
• Walker, J., & Davies, R. (2017). Physics: Principles with Applications. Pearson.
• Perkins, D. H. (2003). Introduction to High Energy Physics. Cambridge University Press.