Firing alpha particles at gold foil: the experiment that found the nucleus

1. Before this experiment, an atom was pictured as a ball of positive charge with electrons dotted through it, with the positive charge and the mass spread evenly through the whole atom.
2. Geiger and Marsden, working under Rutherford, tested that picture by firing a narrow beam of alpha particles at a sheet of very thin gold foil and recording where the particles went afterwards.
3. Three features of the set-up matter.
   1. The foil is only a few atoms thick, so any large deflection is caused by a single close encounter rather than by many small ones adding up.
   2. The whole apparatus is in a vacuum, because alpha particles are absorbed by a few centimetres of air and would never reach the foil.
   3. A movable detector is swung round the foil so that particles can be counted at every angle, including angles behind the foil.
4. The alpha particle is the ideal probe here because it is positively charged, so anything positive inside the atom will repel it and change its path.

Three observations, each one ruling something out

1. The vast majority of the alpha particles passed straight through the foil with no measurable change of direction.
2. A small number were deflected through noticeable angles as they passed through.
3. A very few, roughly one in every eight thousand, were turned through more than $90^{\circ }$ and came back towards the source.
4. That last result was the shock: an alpha particle is fast and heavy, so bouncing one backwards needs a concentrated force, which the old evenly spread model could never supply.

Reading each observation as evidence for the nuclear model

1. Most pass straight through, so the atom must be mostly empty space with a very small nucleus that is a tiny target most particles simply miss.
2. A few bounce back, so the nucleus must hold most of the mass of the atom; something lighter than the alpha particle would be knocked aside instead of reversing it.
3. Deflections are repulsions, and the alpha particle is positive, so the nucleus must be positively charged to push it away rather than pull it in.
4. The angle also depends on the distance of closest approach: a particle heading almost straight at a nucleus is turned through the largest angle, while one passing further away is nudged only slightly.
5. Put together, the results replaced the evenly spread positive ball with the nuclear model: a tiny, dense, positive nucleus surrounded by orbiting electrons and a great deal of empty space.