AQA GCSE Combined Science: Trilogy Physics Paper 1 (Higher), 2024: Question 2

✅ Correct answer

2 protons and 2 neutrons

💡 Key knowledge

• An alpha particle is a small, heavy, positively charged particle.
• It is made of 2 protons and 2 neutrons.
• Because it has 2 protons, it has a charge of +2.

🧠 Exam technique

• Give the particle’s composition, not just its name.
• One precise phrase is enough for 1 mark.

❌ Common errors

• Saying only “helium atom” or “helium nucleus” without stating the particles may lose the mark if the examiner wants the composition.
• Writing “2 protons and 2 electrons” is incorrect.
• Confusing an alpha particle with a beta particle or gamma ray.

📐 Calculations

1. Use: number of neutrons = mass number − atomic number
2. Substitute the values: 197 − 79
3. Work it out: 118

Answer: 118

💡 Key knowledge

• Mass number = protons + neutrons
• Atomic number = protons
• So neutrons = mass number − atomic number

🧠 Exam technique

• Always identify the top number as the mass number.
• Always identify the bottom number as the atomic number.
• For 1 mark, the final number is enough, but showing the subtraction helps avoid mistakes.

❌ Common errors

• Using 79 as the number of neutrons — that is the number of protons.
• Doing 197 + 79 instead of subtracting.
• Forgetting that a neutral atom still has the same number of protons and electrons.

✅ What a top answer needs

Use the actual results to explain why the old model was wrong, then state what the new model said instead.

• The plum pudding model said the atom was a ball of positive charge with electrons embedded in it.
• The results showed that most alpha particles passed straight through, so atoms are mostly empty space.
• Some alpha particles were deflected, which showed the positive charge is not spread out evenly.
• A tiny number were deflected through large angles, which suggested a very small, dense centre.
• This led to the nuclear model: the atom has a small, positively charged nucleus.
• Most of the mass is concentrated in the nucleus, and electrons orbit around it.

💡 Key knowledge

• Plum pudding model: positive charge spread throughout the atom; electrons embedded in it.
• Nuclear model: almost all the mass is in a tiny nucleus; the nucleus is positively charged; most of the atom is empty space.
• Large deflections happen because alpha particles are repelled by the concentrated positive charge in the nucleus.

🧠 Exam technique

• To reach the top level, you need to link evidence to conclusion.
• Don’t just list observations — explain what they mean.
• Good structure:
  • State the old model.
  • Describe the result.
  • Explain why that result does not fit the old model.
  • State the new nuclear model idea.

❌ Common errors

• Saying only “the model changed” without explaining why.
• Missing the key idea that most particles went straight through, showing empty space.
• Forgetting the small, dense, positively charged nucleus.
• Writing that electrons are in the nucleus — this is wrong.
• Not mentioning the large-angle deflections, which are crucial for full marks.

✅ Correct answer

James Chadwick

💡 Key knowledge

• James Chadwick discovered the neutron.
• The neutron helped explain why atoms have extra mass without extra charge.

❌ Common errors

• Isaac Newton is not correct here.
• Niels Bohr is linked to electron shells, not the neutron.
• Be careful not to confuse scientists from different atomic models.

🧠 Exam technique

• In multiple-choice style questions, read the options carefully.
• Only one name is needed for the mark.

Atomic structure

• Protons and neutrons are in the nucleus.
• Electrons move around the nucleus.
• Most of the atom is empty space.

Alpha particle

• 2 protons + 2 neutrons
• Charge = +2
• Quite massive and strongly ionising

Isotope calculations

• Neutrons = mass number − atomic number
• Use the top number minus the bottom number
• For ¹⁹⁷₇₉Au, neutrons = 118