Fire alphas at gold foil.

Alpha particles fired at a foil of Thomson spread-charge atoms or Rutherford nuclei; hyperbolic deflection paths and a histogram of scattering angles build up.

With the nuclear atom, set an aim offset that turns one alpha back through more than 150°; then show that the Thomson atom never exceeds 5° in 200 shots.

Thinking most alphas bounce back, or that the Thomson model could produce back-scattering.

• Thomson's model (1898): a sphere of radius about 10⁻¹⁰ m with positive charge spread evenly and electrons set in it (plum pudding, raisin pudding or watermelon); the mass is spread evenly too. It explains overall neutrality and little else.
• In Rutherford's scattering experiment, α-particles were fired at gold foil about 100 nm thick, with a zinc sulphide screen around it to catch each one as a flash.
• Most α-particles went straight through, a small fraction turned through small angles, and about 1 in 20,000 bounced back through nearly 180°.
• Conclusions: the atom is mostly empty space, and the positive charge with nearly all the mass sits in a tiny nucleus, radius about 10⁻¹⁵ m against about 10⁻¹⁰ m for the atom. If the nucleus were a cricket ball, the atom would be about 5 km in radius.
• Rutherford's nuclear model places electrons in circular orbits around the nucleus at high speed, held by electrostatic attraction, like planets round the sun.
• Drawback 1: an orbiting electron is accelerating, and by Maxwell's theory an accelerating charge radiates; it should lose energy and spiral into the nucleus in about 10⁻⁸ s. Atoms are stable, so the model fails.
• Drawback 2: the model says nothing about how electrons are arranged around the nucleus or what energies they have. A stationary electron is no fix, since it would simply be pulled into the nucleus.

Teaching this idea? Put the simulation on your own page or LMS with this code.