Rutherford's Alpha Scattering Experiment

Rutherford’s alpha scattering experiment (1909) provided the first evidence for the nuclear model of the atom, replacing the earlier plum pudding model.

The experiment

A beam of alpha particles was fired at a very thin gold foil. Detectors (zinc sulfide screens) around the foil recorded where the alpha particles arrived.

Observations

Observation	Percentage
Most alpha particles passed straight through	> 99%
Some were deflected through small angles	Small %
Very few were deflected through large angles (> 90$^\circ$)	About 1 in 8000

Conclusions

• Most pass through: The atom is mostly empty space.
• Small deflections: The alpha particles pass close to a positive charge and are repelled slightly by electrostatic repulsion.
• Large deflections (backscattering): A few alpha particles hit something very small, very dense and very positively charged. This is the nucleus.

The nuclear model

• The atom has a very small, dense, positively charged nucleus at its centre.
• The nucleus contains most of the atom’s mass.
• Electrons orbit the nucleus at relatively large distances.
• Most of the atom is empty space.

Why the plum pudding model was replaced

The plum pudding model (Thomson) had positive charge spread evenly throughout the atom. If this were true, alpha particles would only be deflected by small angles. The large-angle deflections could only be explained by a concentrated central charge (the nucleus).

Common errors and how to correct them

• Saying the experiment used beta particles. It used alpha particles (positively charged, so they are repelled by the positive nucleus).
• Saying most alpha particles bounced back. Most passed straight through; only about 1 in 8000 bounced back.

How to apply this in an exam

Describe the setup. State the three key observations. Link each observation to a conclusion about atomic structure. Explain why this disproved the plum pudding model.