The nuclear atom and alpha scattering

Last time

Before we start: last time, in Alpha decay, energy and beam current, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

A tiny, heavy centre

At the start of the twentieth century, people thought an atom was a soft ball of positive charge with electrons dotted through it. Then Rutherford's team fired alpha particles at a thin gold foil, and a few bounced almost straight back. It was, said Rutherford, like firing a shell at tissue paper and having it come back. The atom had to have a tiny, dense, charged centre.

The scattering experiment

A narrow beam of alpha particles was directed at a very thin gold foil, in a vacuum. A detector moved round to count the particles scattered at different angles. A vacuum was used so that the alpha particles were not absorbed or scattered by air. A thin foil was used so that each alpha particle was scattered at most once.

Observations and inferences

Observation one: most alpha particles pass straight through, or with very small deflections. Inference: most of the atom is empty space. Observation two: a small proportion are deflected through large angles, more than ninety degrees. Inference: the nucleus is charged, positive, to repel the alpha particles. Also, those that bounce back mean the nucleus is dense and massive: most of the atom's mass is concentrated in it. The nucleus is very small compared with the atom: about ten to the minus fifteen metres, against ten to the minus ten.