In 1909, Hans Geiger and Ernest Marsden performed experiments where they directed a collimated beam of alpha particles (24He2+^{4}_{2}\text{He}^{2+}24He2+) from a radioactive bismuth source towards a thin gold foil sheet. Under J.J. Thomson’s atomic model, the atom was envisioned as a uniform, low-density sphere of positive charge containing embedded electrons.
Calculations based on this uniform positive cloud model suggested that the electric field inside the atom would be far too weak to significantly deflect fast-moving, massive alpha particles. Therefore, scientists predicted that the particles would experience negligible scattering. However, Geiger and Marsden surprisingly observed that roughly 1 in every 8,000 8,000\,8,000 incident alpha particles bounced backward, experiencing a deflection angle greater than 90∘90^\circ90∘.
State the prediction regarding the path of the alpha particles based on J.J. Thomson's uniform sphere model of positive charge.
Define what is meant by an anomalous result in a scientific investigation.
Suggest one appropriate step a scientist should take upon first detecting an apparent anomalous data point during an experiment.
Explain how these extremely rare, large-angle deflections led Rutherford to conclude that the positive charge and mass of an atom must be concentrated in a tiny central nucleus.
Suggest two reasons why conducting experimental trials is essential for the advancement of theoretical physics.
65 exam-style questions on Edexcel IGCSE Physics Radioactivity. Each one has a worked solution and a mark scheme showing where the marks go.