How did the alpha scattering experiment reveal that the atom has a tiny, dense, positive nucleus?
The Rutherford alpha particle scattering experiment, the observations and conclusions, and how they led to the nuclear model of the atom.
A focused answer to AQA A-Level Physics 3.8.1.1, covering the Rutherford and Geiger and Marsden alpha scattering experiment, the key observations and the conclusions they support about the nuclear model of the atom.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
AQA specification point 3.8.1.1 wants you to describe the alpha particle scattering experiment, state the observations, and explain the conclusions that led to the nuclear model of the atom, replacing the earlier plum pudding model.
The experiment
A narrow beam of alpha particles from a radioactive source was directed at a very thin gold foil in an evacuated chamber. A movable detector (a zinc sulphide scintillation screen viewed through a microscope) recorded how many alpha particles arrived at different scattering angles. Gold was chosen because it can be beaten into an extremely thin foil, only a few hundred atoms thick, so that most alpha particles encounter at most one nucleus. The vacuum prevented the alpha particles being absorbed by air.
The observations
The conclusions
The backward scattering is the most striking result: Rutherford famously compared it to firing a shell at tissue paper and having it bounce back. A diffuse charge could never reverse the motion of a fast alpha particle.
Why it replaced the plum pudding model
In the plum pudding model the positive charge was spread throughout the atom, so the field anywhere was weak and could never have produced the large-angle deflections that were observed. The nuclear model, with a concentrated positive nucleus, explains all the results.
Try this
Q1. State what the deflection of a small number of alpha particles through large angles tells us. [1 mark]
- Cue. The positive charge of the atom is concentrated in a small region (the nucleus).
Q2. Explain why most alpha particles passed straight through the foil. [1 mark]
- Cue. The atom is mostly empty space, so most alpha particles do not pass close to a nucleus.
Q3. State why gold foil was used in the experiment. [1 mark]
- Cue. It can be made extremely thin (a few hundred atoms thick), so most alpha particles meet only one nucleus.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20185 marksDescribe the alpha particle scattering experiment and explain how each of the main observations led to a conclusion about the structure of the atom.Show worked answer →
A narrow beam of alpha particles from a radioactive source was directed at a very thin gold foil in a vacuum, and a movable detector recorded the number of alpha particles scattered at different angles.
Most alpha particles passed straight through with little or no deflection, which shows the atom is mostly empty space. A small number were deflected through large angles (greater than 90 degrees), which shows the positive charge is concentrated in a small region. A very few bounced almost straight back, which shows that almost all the mass and the positive charge are concentrated in a tiny, dense, central nucleus, because only a concentrated charge could repel a fast alpha particle backwards.
Markers reward the experimental setup, each of the three observations, and pairing each observation with the correct conclusion.
AQA 20213 marksExplain why the results of the alpha scattering experiment could not be explained by the plum pudding model of the atom.Show worked answer →
In the plum pudding model the positive charge was spread thinly throughout the whole volume of the atom, so the electric field anywhere would be weak. Such a diffuse charge could only produce small deflections of fast, massive alpha particles.
It could not produce the observed large-angle deflections and backward scattering, which require a strong, concentrated electric field, that is, a small dense positive nucleus. The nuclear model explains all the results, so the plum pudding model was rejected.
Markers reward the diffuse charge giving only small deflections, the inability to explain large-angle and backward scattering, and the need for a concentrated nucleus.
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Sources & how we know this
- AQA A-level Physics (7408) specification — AQA (2017)