AQA GCSE Physics Physics Paper 1 (Higher), November 2021: Question 6

📐 Step-by-Step Calculation

✅ Correct Answer

148

🧠 Exam Technique

Always double-check axis labels on Figure 6. The y-axis shows Mass number (total nucleons), not neutrons. Never stop at reading just one number off the graph!

❌ Common Errors

A frequent error is writing down 234 (the mass number) or 86 (the proton number) directly without subtracting.

💡 Key Knowledge

• An element is defined purely by its atomic number (number of protons).
• Atoms with the same atomic number but different mass numbers are isotopes of the same element.
• On Figure 6, atoms of the same element lie on the same vertical line.

✅ Correct Answer

Tick the box for: D and E

• D: Atomic number = 92, Mass number = 238
• E: Atomic number = 92, Mass number = 235

❌ Common Errors

Confusing mass number with atomic number and choosing A and C because they share the same mass number (234). Remember: identical mass number does not mean the same element!

💡 Key Knowledge: Alpha Particle Properties

An alpha particle is a helium nucleus: ⁴₂He (or ⁴₂α).

• Proton / Atomic number decreases by 2 ( ΔZ = -2 )
• Mass number decreases by 4 ( ΔA = -4 )

✅ Correct Drawing / Answer

An arrow drawn starting at point B (88, 226) and pointing directly to the coordinates:

Atomic number = 86, Mass number = 222

🧠 Exam Technique & Diagram Details

Use a ruler and pencil. Draw an arrow with a clear arrowhead:

• Start point: Exactly on dot B (88, 226) .
• End point: Exactly on the intersection of vertical gridline 86 and horizontal gridline 222 .

❌ Common Errors

• Decreasing mass number by 2 instead of 4 (confusing protons and neutrons).
• Drawing an arrow that shifts right instead of left (increasing atomic number).
• Omitting the arrowhead, which leaves ambiguity about the direction of decay.

✅ Correct Answer (Either statement scores)

• Cannot predict which nucleus will decay next
• — OR —
• Cannot predict when a (particular) nucleus will decay

❌ Common Errors & Examiner Traps

• Vague wording: Writing "it happens randomly" or "it happens by chance" gains 0 marks as it simply repeats the question.
• Confusing with half-life: Stating "we don't know how long half-life takes" is scientifically incorrect; half-life is predictable for large samples.

✅ Model Answer (Full 3 Marks)

• Mark 1: One alpha decay decreases the proton number (atomic number) by 2.
• Mark 2: Two beta decays each increase the proton number by 1, so together they increase the proton number by 2.
• Mark 3: Therefore, the final proton / atomic number is the same as the original nucleus, making it the original element (polonium).

🧠 How to Structure a 3-Mark Explanation

Break the decay sequence down mathematically:

❌ Common Errors

• Failing to mention protons or atomic number and only discussing mass numbers (the final polonium nucleus is an isotope of the original, with a different mass number: 4 units lighter!).
• Thinking beta decay decreases protons (in beta decay, a neutron turns into a proton, so proton number increases by 1).