Geiger and Marsden fired alpha particles at thin gold foil and observed the scattering pattern
Nuclear & Particle Physics - OCR A-Level Physics
- Geiger and Marsden fired alpha particlesHelium-4 nuclei (2 protons + 2 neutrons) emitted during alpha decay. Charge +2e, strongly ionising, short range. at thin gold foil and observed the scattering pattern using a zinc sulfide screen.
- Most alpha particles passed straight through, indicating the atom is mostly empty spaceRutherford scattering showed that atoms are mostly empty space, with almost all mass concentrated in a tiny nucleus..
- A small fraction were deflected through large angles, indicating a concentrated positive chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). at the centre.
- About 1 in 8000 were deflected back (θ > 90°), indicating the nucleus is very small and very dense.
- This disproved the plum pudding modelThomson's early atomic model: a sphere of positive charge with electrons embedded in it, like plums in a pudding. (Thomson) and established the nuclear modelRutherford's model: a tiny, dense, positively charged nucleus surrounded by orbiting electrons in mostly empty space..
Examiner Tips and Tricks
When explaining the experiment, always link each observation to a specific conclusion: 'most pass through → mostly empty space', 'small fraction deflected → small concentrated positive chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).', 'some bounce back → nucleus is massive and dense'.