New experimental evidence may lead to a scientific model being changed or…

The idea

Scientific models are simplified explanations of observations. When new experimental evidence conflicts with an existing model, scientists may modify the model or replace it. For the atom, early models included Dalton's solid sphere and Thomson's plum pudding model. Rutherford's alpha-particle scattering experiment provided evidence that most of the atom is empty space with a small, dense, positively charged nucleus. This led to the nuclear model, which replaced Thomson's model. Later evidence from the discovery of the proton and neutron refined the model further. Bohr's model introduced electron shells to explain atomic spectra. The process is iterative: evidence tests predictions, and models change to fit the evidence. A model is not proved true forever; it is the best current explanation.

What earns the marks

• A scientific model is a representation or explanation of observations that can be used to make predictions.
• New experimental evidence can support, modify or contradict an existing model.
• If evidence cannot be explained by the current model, the model may be changed or replaced.
• The development of the atomic model includes the plum pudding model, the nuclear model and later refinements such as electron shells.
• Rutherford's alpha-particle scattering experiment provided evidence for a small, dense, positively charged nucleus.
• Scientific models are provisional and may be replaced when further evidence is obtained.

Common mistakes and how to put them right

• Thinking that a model is a proven fact.

  Put it right: A model is a provisional explanation that is accepted while it fits the available evidence.

• Believing that one experiment immediately replaces a model without review.

  Put it right: New evidence is evaluated by the scientific community and may lead to modification or replacement over time.

• Confusing the plum pudding model with the nuclear model.

  Put it right: The plum pudding model has positive charge spread throughout the atom with electrons embedded, while the nuclear model has a small, dense, positive nucleus with electrons outside it.

Exam tips

• Link each model to the evidence that supported it, for example alpha-particle scattering and the nuclear model.
• Use the phrase 'new experimental evidence' when explaining why a model changed, and state what the evidence showed.
• When describing a sequence of models, use words such as 'before', 'then' and 'after' to make the development clear.