Gold Foil Experiment Overview

Experimental Setup

• The experiment involved bombarding a thin gold foil with a beam of alpha particles.
• Alpha particles are positively charged particles emitted by radioactive elements.
• The gold foil was extremely thin, typically a few thousand atoms thick.
• A zinc sulfide screen was positioned around the foil to detect the scattered alpha particles. The screen emitted a flash of light (scintillation) when hit by an alpha particle.
• A detecting device (e.g., a microscope) was used to observe the scintillations.
• The experiment was meticulously designed to determine the paths of the scattered alpha particles.

Expected Outcome (Classical Model)

• According to the then-prevailing plum pudding model of the atom, alpha particles should pass straight through the foil with minimal deflection.
• This is because the positive charge in the atom was thought to be spread uniformly throughout, and the negatively charged electrons were small and numerous to screen the positive charge.
• The model predicted a mostly undeflected path for the majority of alpha particles.

Unexpected Observations

• The experiment revealed that a small fraction of the alpha particles were deflected at large angles, even back towards the source.
• This was unexpected and inconsistent with the plum pudding model.

Interpretation and Conclusion

• The large-angle scattering indicated a concentrated positive charge concentrated in a very small volume within the atom, called the nucleus.
• Rutherford proposed the nuclear model of the atom.
• This model suggested that most of the atom's mass and positive charge are concentrated in a tiny, dense nucleus at the atom's center.
• Electrons orbit the nucleus much like planets orbit the sun.
• The "empty space" characteristic of the atom was a vital observation since the majority of the alpha particles passed through the foil with little to no deviation.
• The nucleus's small size and positive charge were sufficient to deflect some particles at large angles, but a majority passed through unaffected.

Significance

• Rutherford's experiment was a landmark experiment in modern physics.
• It fundamentally changed our understanding of atomic structure.
• The results refuted the prevailing plum pudding model.
• The nuclear model opened up the field of nuclear physics and laid the groundwork for subsequent discoveries and studies.