Rutherford’s Gold Foil Experiment

Abstract

This report discusses the gold foil experiment, which was conducted in 1909 to further existing research about the atomic model. At the time, the “Plum Pudding” model suggested that positive charge was spread evenly throughout the atom. In the experiment, alpha particles from a radioactive radium source were directed at a thin sheet of gold foil, and their deflections were observed using a zinc sulfide screen and a microscope. Nearly all of the particles passed through the foil without deflection, but some were deflected at small angles, and a few were reflected back towards the radium at large angles. These results showed that the atom was mostly empty space with a small, dense, positively charged nucleus at its center. This discovery changed the existing view of atomic structure and led to the development of the Rutherford model of the atom.

Introduction
The experiment is known as the gold foil experiment due to the use of gold foil to serve as a screen to deflect alpha particles. In the early 20th century, J.J. Thomson proposed the “Plum Pudding” model of the atom, suggesting an even distribution of positive charge throughout the atom, with negatively charged electrons spread throughout, like raisins in a pudding, hence the name. Since the positive charge was evenly distributed, it was expected that the alpha particles would pass through the foil without deflection. The experiment tested this by using a radioactive radium source, emitting particles towards a thin sheet of gold foil, and their deflections were observed using a zinc sulfide screen and a microscope. However, the experiment produced surprising results, and paved the way for a new, more accurate atomic model to be developed, including the discovery of the nucleus.

Procedures and Materials

Materials:

• Gold foil
• Radium sample
• Lead block
• Zinc sulfide screen
• Microscope

Gold was chosen as the metal due to its malleability and its ability to be hammered into very thin sheets, which was necessary for the experiment. This experiment also used a radioactive sample of the element radium, which emitted alpha particles. Alpha particles are positively charged particles which are identical to the nucleus of a helium atom and approximately 7000 times larger than an electron. The configuration of all the materials is shown in Figure 1. 

Using the lead block, the alpha particles from the radium were directed towards the thin sheet of gold foil. The zinc sulfide screen was used to enclose the gold foil to detect the particles and if they were deflected. The microscope was used to detect flashes of light produced when alpha particles struck the zinc sulfide screen. 

Results

A large majority, approximately 99% of the alpha particles passed straight through the gold foil. However, a small percentage of the particles were slightly deflected, and an even smaller percentage, approximately 1 out of 20,000, of the particles were deflected at angles greater than 90 degrees, deviating significantly from their initial trajectory (Fig.2). 

Discussion

Since nearly all of the particles passed through the foil without deflection, it indicated that most of the atom consists of empty space. The few particles that the gold deflected, however, did so at large, unexpected angles. This contradicted the Plum Pudding model, and suggested a small concentration of positive charge rather than an equal distribution of it throughout the atom. The concentration of charge was discovered to be the nucleus, which gained its charge from protons. The new model theorized that the atom behaved almost like a solar system, with the nucleus in the middle, and electrons orbiting in the periphery. 

Conclusion

Overall, this experiment was pivotal in the development of the atomic model, and succeeded in proving the existence of a positively charged nucleus, leading to the development of the Rutherford model of the atom (Fig.3). These discoveries laid the groundwork for future atomic models, and Rutherford’s experiment was hailed as one of the most significant in the field of physics.

Appendix

Figure 1. Setup of the Experiment

Figure 2. Results of the Experiment                                    
   

Figure 3. Rutherford Model

References

“Rutherford Model.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 5 Feb. 2025, www.britannica.com/science/Rutherford-model.

Brunning, Andy, et al. “The History of the Atom – Theories and Models.” Compound Interest, 13 Sept. 2023, www.compoundchem.com/2016/10/13/atomicmodels/.