Understanding Rutherford's Gold Foil Experiment

Rutherford's famous gold foil experiment provided crucial insights into the structure of the atom. In this experiment, alpha particles (which are positively charged helium nuclei) were directed at a very thin sheet of gold foil.

Observations of the Gold Foil Experiment

The experiment yielded several key observations:

• Most alpha particles passed straight through the foil without any deflection. This indicated that most of the atom is empty space.
• Some alpha particles were deflected by small angles. This suggested that there is a positive charge concentrated in a small region within the atom, repelling the positively charged alpha particles.
• A very small number of alpha particles (approximately 1 out of every 12000) were deflected back by nearly 180 degrees. This was a surprising result.

Conclusion on Nuclear Size

The observation that only a tiny fraction of alpha particles deflected back significantly was the most important finding for determining the size of the nucleus. Here's why:

• Alpha particles are relatively heavy and have high energy. For one to be deflected back, it must have encountered a head-on collision with something equally massive and positively charged concentrated in a very small region.
• If the positive charge and mass were spread throughout the atom, as proposed by earlier models like the plum pudding model, most alpha particles would only experience small deflections, or none at all.
• The fact that almost all alpha particles went straight through means that the obstacle causing the large deflection or bouncing back must occupy only a minuscule volume compared to the total volume of the atom.

This observation led Rutherford to conclude that the positive charge and most of the mass of the atom are concentrated in an extremely small, dense region at the center, which he called the nucleus. The calculation based on the number of particles deflected back allowed Rutherford to estimate that the radius of the nucleus is about $$10^{-15} \text{ m}$$, while the radius of the atom is about $$10^{-10} \text{ m}$$. This means the nucleus is about $$10^{5}$$ times smaller in radius than the atom.

Therefore, the specific observation that only about 1 in 12000 alpha particles deflected back strongly supported the conclusion that the size of the nucleus is very small compared to the size of the atom.