3.8.1.1
Rutherford fired alpha particles at gold foil to probe atomic structure
Nuclear Structure & Radiation — AQA A-Level Physics
Key Definition
Rutherford scattering — The experiment in which alpha particles are directed at thin gold foil. The deflection pattern reveals the structure of the atom.
- A beam of alpha particles from a radioactive source in a lead container was aimed at a thin sheet of gold foil (~10⁻⁶ m thick).
- A movable detector recorded the number of alpha particles deflected at each angle.
- The apparatus was placed in an evacuated chamber so alpha particles would not collide with air molecules.
- Gold was chosen because it is malleable enough to be hammered into extremely thin sheets.
Key observations and conclusions
- Most alpha particles passed straight through undeflected. Conclusion: the atom is mostly empty space.
- Some alpha particles were deflected through small angles (<10°). Conclusion: there is a concentration of positive chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). at the centre (electrostatic repulsion).
- Very few alpha particles were deflected straight back (>90°). Conclusion: the nucleus is extremely small and dense, containing most of the atom's mass and chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C)..
- The atom is around 100,000 times larger than its nucleus.
Examiner Tips and Tricks
- The graph of number of particles detected (n) against angle (θ) drops very steeply.
- At θ = 0° n is highest; it falls rapidly and approaches zero at large angles.
- Only graph A (exponential-type decay) fits this pattern.