This nuclear model replaced the plum pudding model.

The idea

The plum pudding model predicted that alpha particles would sail through a foil with only slight deflections, because its positive charge is spread thinly everywhere. The observed large deflections and rebounds could not be produced by such a diffuse charge, so the model failed on its own prediction and was replaced. Rutherford's nuclear model puts a tiny, positively charged, massive nucleus at the centre with the electrons well outside it and empty space in between, which accounts for all three observations at once. Comparison questions are level marked: a top answer links each observation to the feature of the model that explains it, rather than simply listing the two models side by side.

What earns the marks

• Stating that the plum pudding model predicted only small deflections of alpha particles.
• Stating that the large deflections observed could not be explained by charge spread through the whole atom.
• Describing the nuclear model as a tiny positively charged nucleus with electrons outside it.
• Linking each experimental observation to the feature of the nuclear model that explains it.

Common mistakes and how to put them right

• Listing the features of the two models without ever comparing them, which caps the answer at the lowest level.

• Describing the experiment but never saying what the results showed.

• Claiming the nuclear model was accepted immediately by everyone.

• Saying the plum pudding model failed because it did not include neutrons.

Exam tips

• In a six-mark comparison, alternate: what the old model said, what was seen, what the new model says.
• Use the word because in every comparison sentence to force the logical link the level descriptors demand.
• Mention empty space, concentrated charge and concentrated mass; these three ideas carry the answer.