Students should be able to describe:

The idea

This statement introduces the descriptive skills students need for the development of the atomic model. A good description names the model or scientist, outlines the evidence, and states how the model changed. For example, alpha-particle scattering showed that most of the atom is empty space with a tiny positive nucleus, replacing the plum pudding idea. Descriptions should distinguish observation from conclusion and should use correct terms such as nucleus, electron, proton and neutron. The skill is assessed by asking students to outline a sequence, explain a change, or match evidence to a model.

What earns the marks

• Describe the plum pudding model as a ball of positive charge with electrons embedded in it.
• Describe Rutherford's nuclear model as a tiny, dense, positive nucleus surrounded by mostly empty space.
• Describe Bohr's contribution as electrons occupying fixed shells or energy levels around the nucleus.
• Describe Chadwick's contribution as evidence for neutral neutrons in the nucleus.
• Explain how new experimental evidence led to each change in the model.

Common mistakes and how to put them right

• Describing the plum pudding model as having a nucleus.

  Put it right by stating that the nucleus was introduced later by Rutherford.

• Saying that Bohr discovered the electron.

  Put it right by stating that Bohr proposed electron shells, while the electron was discovered earlier.

• Listing scientists without linking each to the evidence or the change in the model.

  Put it right by pairing each name with the observation and the resulting model.

Exam tips

• Use a clear sequence: plum pudding, nuclear model, electron shells, neutron.
• For each change, state the evidence and the conclusion in one sentence.
• Use command words carefully: 'describe' needs a clear account, while 'explain' needs a reason or mechanism.