Why the new evidence from the scattering experiment led to a change in the…

The idea

This statement asks you to explain the reasoning that connects the results of the alpha particle scattering experiment to the replacement of the plum pudding model. In the experiment, positively charged alpha particles were fired at thin gold foil. Most passed straight through, some were deflected through small angles, and a very small number bounced back. The plum pudding model predicted only tiny deflections because its positive charge was spread thinly throughout the atom. The observed large deflections and rebounds could only happen if positive charge were concentrated in a tiny, dense region, so Rutherford proposed the nuclear model with a small positive nucleus and mostly empty space. Explaining means linking each observation to a conclusion about atomic structure.

What earns the marks

• States that most alpha particles passed straight through the foil, showing the atom is mostly empty space.
• States that some alpha particles were deflected, showing they passed near a concentrated positive charge.
• States that a very small number were deflected back, showing a tiny, dense, positively charged centre.
• Explains that the plum pudding model could not account for large deflections because its positive charge was spread out.
• Concludes that the evidence led to the nuclear model with a small positive nucleus and electrons outside it.
• Uses the idea that like charges repel to explain why alpha particles were deflected by the nucleus.

Common mistakes and how to put them right

• Saying most particles were deflected.

  Put it right by stating most passed straight through and only a few were deflected.

• Claiming the nucleus is negative.

  Put it right by stating the nucleus is positive, which is why positive alpha particles are repelled.

• Saying the experiment proved electrons orbit in shells.

  Put it right by stating the scattering evidence supported a small positive nucleus, while electron shells came from later work.

Exam tips

• Link each observation to its conclusion in a because sentence, for example most passed through because the atom is mostly empty space.
• Use the word repel when explaining why alpha particles changed direction near the nucleus.
• Compare the prediction of the plum pudding model with the actual result to show why the model changed.