Edexcel GCSE Chemistry

## How the model of the atom changed At the start of the nineteenth century John Dalton described atoms as tiny solid spheres that could not be divided, with a different kind of sphere for each element. That model explained why elements combine in fixed proportions, but it had no room for anything smaller than an atom, so it had to change when smaller particles were found. In 1897 J. J. Thomson showed that atoms contain negatively charged particles, later called electrons, which are far lighter than any atom. Atoms are neutral overall, so he pictured a ball of positive charge with electrons embedded in it, like fruit in a pudding. In 1909 Geiger and Marsden, working with Rutherford, fired positively charged alpha particles at thin gold foil. Most went straight through, a few were deflected through large angles and a very few bounced almost straight back. A ball of spread-out positive charge could not do that, so Rutherford proposed that the positive charge and nearly all the mass are packed into a tiny nucleus at the centre, with the electrons around it and mostly empty space in between. Niels Bohr then suggested that the electrons move in fixed orbits, or shells, at set distances from the nucleus, which fitted observations of the light that atoms give out. Later work showed that the nucleus itself contains positively charged protons, and in 1932 James Chadwick discovered the neutron, a particle with no charge and about the same mass as a proton. That completed the model…