AQA A-Level Physics: Rutherford Scattering and the Evolution of Nuclear Structure Understanding — mark scheme explained

The question

Explain why Rutherford's experiment provided evidence for a small, dense nucleus. [Paraphrased for study — not reproduced from any exam paper.]

Mark scheme, decoded

What each mark is really for — in plain English — and the wording trap that loses it.

• S1

  Rutherford observed that most alpha particles passed through the gold foil with minimal deflection, which is consistent with them passing through regions of the atom where the positive charge was not concentrated.

• S2

  However, a small number of alpha particles were deflected at large angles or even bounced back towards the source. This suggested that these particles must have come very close to a region of strong positive charge.

• S3

  The only way this could happen is if there was a tiny, dense nucleus containing most of the mass and all of the positive charge of the atom.

Model answer

Worked through, with each step tagged to the mark it earns.

1. S1

   Rutherford observed that most alpha particles passed through the gold foil with minimal deflection, which is consistent with them passing through regions of the atom where the positive charge was not concentrated.

2. S2

   However, a small number of alpha particles were deflected at large angles or even bounced back towards the source. This suggested that these particles must have come very close to a region of strong positive charge.

3. S3

   The only way this could happen is if there was a tiny, dense nucleus containing most of the mass and all of the positive charge of the atom.

4. Final answer: Rutherford's experiment provided evidence for a small, dense nucleus because it showed that while most alpha particles passed through with minimal deflection, a few were strongly deflected or bounced back, indicating they encountered a region of strong positive charge.

Common mistakes

• Thinking that Rutherford's experiment disproved the existence of electrons. — Clarify that Rutherford's experiment primarily provided evidence for a small, dense nucleus and did not directly address the existence or behavior of electrons.
• Believing that all alpha particles were deflected at large angles in Rutherford's experiment. — Emphasize that only a small number of alpha particles were deflected at large angles, while the majority passed through with little or no deflection.
• Confusing the plum pudding model with the nuclear model. — Clearly distinguish between the plum pudding model (uniform positive charge with embedded electrons) and the nuclear model (small, dense nucleus with orbiting electrons).
• Thinking that Rutherford's experiment directly measured the size of the nucleus. — Explain that the large deflections indicated the presence of a small, dense region of positive charge but did not directly measure the size of the nucleus.
• Believing that isotopes have different chemical properties. — Clarify that isotopes of the same element have the same number of protons and electrons, so their chemical properties are identical, but they can differ in nuclear stability and radioactive decay.
• Thinking that quarks were discovered before Rutherford's experiment. — Emphasize that quarks were discovered much later, after the development of advanced experimental techniques like particle accelerators.

Where the marks go

• Full worked solution (all marking points)4 marks

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