Developing models of atoms

Dalton's model

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 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

The Dalton model has changed over time because of the discovery of subatomic particles.

A timeline of discovery

DateDiscovery
1897Electrons
1909 - 1911Atoms have a nucleus with electrons around it
1913Electrons occupy shells (energy levels)
1918Protons
1932Neutrons

As a result of these discoveries, we now know that:

  • atoms can be broken down into three smaller particles: protons, neutrons and electrons
  • atoms of an element have identical numbers of protons and electrons, but can have different numbers of neutrons
  • atoms of different elements have different numbers of protons and electrons

Remember that in Dalton's time, the technology did not exist to discover the detailed structure of atoms.