Early ideas about atoms

Ideas about atoms have changed over time. Scientists developed new atomic models as they gathered new experimental evidence.

John Dalton published his ideas about atoms in 1803. He thought that all matter was made of tiny particles called atoms, which he imagined as tiny spheres that could not be divided.

Nearly 100 years later, J J Thomson carried out experiments and discovered the electron. This led him to suggest the plum pudding model of the atom. In this model, the atom is a ball of positive charge with negative electrons embedded in it - like currants in a Christmas pudding.

Image of a plum pudding model, with a large blue circle with a positive symbol behind six red smaller circles with negative symbols.The plum pudding model

In 1909 Ernest Rutherford designed an experiment to test the plum pudding model. In the experiment, positively charged alpha particles were fired at thin gold foil. Most alpha particles went straight through the foil. But a few were scattered in different directions.

Alpha particles travel from the alpha source and bounce off gold atoms.The alpha particle scattering experiment

This evidence led Rutherford to suggest a new model for the atom, called the nuclear model. In the nuclear model:

  • the mass of an atom is concentrated at its centre, the nucleus
  • the nucleus is positively charged