Developing models of atoms

Ancient Greek thinkers believed that all matter was made of a combination of earth, air, fire and water. These substances were called 'elements' but they were not the same as modern elements. Later Greek thinkers suggested that matter could be made up of invisible particles. They called these particles atoms but they had no experimental evidence for their model.

The first atomic model

The scientist John Dalton carried out a series of experiments. He concluded that all matter was made of tiny particles called atoms. He suggested that an atom was a tiny solid ball. He published his ideas in 1803.

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
  • the atoms of different elements join to make compounds
  • during chemical reactions atoms rearrange to make different substances

This atomic model has changed over time. Scientists used the model to make predictions. Sometimes the results of their experiments were a surprise and they did not fit with the existing model. Scientists changed the model so that it could explain the new evidence.

A timeline to show how the atomic model has changed

YearScientist(s)New evidenceChanges to the atomic model
1897ThomsonThe discovery of electrons. Atoms can be broken down into smaller parts. An atom is made of tiny negatively charged electrons dotted about a positively charged sphere like a plum pudding.
1909-1911Rutherford (and Geiger and Marsden) Some positively charged particles fired at gold foil bounced back when they were expected to pass straight through.Atoms have a central positive nucleus. Most of the mass of an atom is found in the nucleus.
1913BohrIn-depth work on Rutherford's model showed it had limitations. The electrons should just spiral in towards the positive nucleus.Electrons move in fixed orbits, called electron shells, around the nucleus.

Scientists then discovered that the nucleus is made up of two types of subatomic particles called protons and neutrons.

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