Developing models of atoms

Dalton’s model (1803)

John Dalton thought that all matter was made of tiny particles called atoms, which he imagined as tiny solid balls. Dalton’s model included these ideas:

  • atoms cannot be broken down into anything simpler
  • the atoms of a given element are identical to each other
  • the atoms of different elements are different from one another
  • during chemical reactions atoms rearrange to make different substances

Thomson’s model (1897)

J.J. Thomson discovered the electron. Atoms are neutral overall, so in Thomson’s ‘plum pudding model’:

  • atoms are spheres of positive charge
  • electrons are dotted around inside
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

The Geiger-Marsden experiment (1909 - 1911)

Hans Geiger and Ernest Marsden tested the plum pudding model. They aimed beams of positively-charged particles at very thin gold foil. These particles should have passed straight through, according to the plum pudding model. However, many of them changed direction. Ernest Rutherford explained these results in his ‘planetary model’:

  • atoms have a central, positively charged nucleus with most of the mass
  • electrons orbit the nucleus, like planets around a star

Bohr’s model (1913)

Niels Bohr improved Rutherford’s model. Using mathematical ideas, he showed that electrons occupy shells or energy levels around the nucleus.

curriculum-key-fact
The Dalton model has changed over time because of the discovery of subatomic particles.