OCR GCSE Physics

## How the model of the atom changed Scientists build models of the atom to explain the evidence they have, and change the model when new evidence does not fit. The order matters because each model was a response to an experiment the one before could not explain. After the electron was discovered, J. J. Thomson proposed that an atom is a ball of positive charge with negatively charged electrons embedded in it, which is often called the plum pudding model. It explained why atoms are neutral overall, since the positive charge and the electrons balance. Hans Geiger and Ernest Marsden, working with Ernest Rutherford, fired alpha particles (which are positively charged) at a very thin sheet of gold foil and detected where they went. Most passed straight through, a small number were deflected through large angles and a very few bounced almost straight back. The plum pudding model predicted only small deflections, so it could not explain this. Rutherford concluded that the positive charge and nearly all the mass are concentrated in a tiny nucleus at the centre, and that the rest of the atom is mostly empty space with electrons around it. A particle only bounces back if it meets something small, dense and positive, which repels it. Niels Bohr then changed the model again to say that electrons orbit the nucleus at set distances, in fixed energy levels, rather than anywhere. This explained why atoms emit and absorb radiation only at particular frequencies, and it is the model most…