AQA A-Level Physics Paper 2, June 2023: Question 26

Alpha Particle Scattering: Rutherford Demonstration

📋 What this question tests

This question assesses your understanding of the experimental setup, observations, and theoretical deductions of the Rutherford alpha scattering experiment. Specifically, it tests your knowledge of particle collision dynamics, the frequency of large-angle deflections, and the role of foil thickness.

Question 26 Analysis

Multiple Choice: Identifying the incorrect statement [1 mark]

✅ Correct Option: C

"The number of alpha particles deflected backwards is greater than the number that pass straight through the foil."

The question asks which statement is not true. Statement C is completely false because the vast majority of α particles (approximately 99.9% or about 1 in 8,000) pass straight through with little to no deflection, demonstrating that the atom is mostly empty space.

Mark scheme: Award 1 mark for C.

🧠 Exam Technique: "NOT True" Questions

Underline the negative word: Notice not true immediately. Your goal is to spot the factually incorrect statement, not an accurate fact.
Evaluate each option methodically: Mark each statement with a quick "T" (True) or "F" (False) in the margin beside the options.
Select the single "F": Do not get distracted by true, scientifically accurate statements (A, B, and D).

Detailed Evaluation of Each Statement

Option	Statement	Truth Value	Physics Explanation
A	The foil is thin enough to assume that alpha particles are deflected only once.	TRUE	The gold foil is only a few hundred atoms thick (~10⁻⁷ m) to avoid multiple scatterings, which would disguise the true single-nucleus deflection pattern.
B	Nuclei are more massive than alpha particles which allows the alpha particles to be deflected by more than 90°.	TRUE	A gold nucleus (mass number 197) has ~50 times the mass of an α particle (mass number 4). If the target were lighter, conservation of momentum prevents backscattering (>90°).
C	The number of alpha particles deflected backwards is greater than the number that pass straight through the foil.	NOT TRUE	This is false: Only ~1 in 8000 particles is deflected by >90°. Backscattering is exceptionally rare because the nucleus occupies a tiny fraction of atomic volume.
D	Deflections of alpha particles by electrons in the foil are much smaller than deflections due to nuclei.	TRUE	An electron's mass is ~1/7300 of an α particle's mass. Any collision with an electron imparts virtually zero change in momentum to the incoming α particle.

Key Revision Points

💡 Rutherford's Key Deductions

Most α pass straight through: Atom is mostly empty space.
A small fraction deflected through large angles (>90°): The atom's positive charge and most of its mass are concentrated in a tiny central region (the nucleus).
Radius comparison: Nuclear radius is ~10⁻¹⁵ m, whereas atomic radius is ~10⁻¹⁰ m (a factor of 10⁵ difference).

❌ Common Student Pitfalls

Misreading the prompt: Students often choose option A or D simply because they recognise them as valid, correct physics statements from class.
Confusing foil purpose: Forgetting why thin foil is required (preventing absorption and ensuring single collisions only).
Momentum mechanics: Overlooking that backscattering (>90°) physically requires target mass (m_target) > projectile mass (m_projectile).