OCR GCSE Physics A: Nuclear structure, isotopes and decay equations (J249) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover OCR GCSE Physics A (Gateway Science, J249), specification Version 5.0 (July 2026), learning outcomes P6.1a to P6.1g. All of them test content assessed in both Foundation and Higher Tier papers. Read the study guide first, or use the revision notes. Course links: OCR GCSE Physics hub, printable checklist and free 10-minute diagnostics. Use a periodic table where you need an element’s name or atomic number.

Questions

1. An atom of lead has the nuclear symbol ²⁰⁷₈₂Pb. It is electrically neutral. State the number of protons, the number of neutrons and the number of electrons. [3]

2. Four nuclei are written below.

²⁸₁₄Si ³⁰₁₄Si ³⁰₁₅P ³²₁₆S

(a) Name the two nuclei that are isotopes of the same element. [1] (b) Explain why ³⁰₁₄Si and ³⁰₁₅P are not isotopes of each other, even though their top numbers match. [2]

3. The nucleus of a sodium atom has a relative charge of +11. One isotope has 12 neutrons.

(a) State which particles are in the nucleus and which of them gives it its charge. [2] (b) Write the nuclear symbol for this isotope of sodium. [2]

4. Describe each of the four emissions from unstable nuclei: alpha particle, beta particle, neutron and gamma ray. [4]

5. Copy and complete the table to show how the nucleus changes when each emission leaves it. [4]

Emission	Change in mass number	Change in atomic number
Alpha
Beta
Neutron
Gamma

6. A nucleus of thorium, ²³⁰₉₀Th, decays by emitting an alpha particle. Write a balanced equation and name the new element. [3]

7. Potassium-42 has the symbol ⁴²₁₉K. It decays by beta emission. Write the balanced equation and name the new element. [3]

8. A nucleus of calcium has 20 protons and 25 neutrons. It emits a single neutron.

(a) State the number of protons, the number of neutrons and the mass number of the nucleus that is left. [2] (b) Explain whether the nucleus that is left is the same element as the original, and say how the two nuclei are related. [2]

9. Caesium-137, ¹³⁷₅₅Cs, decays by beta emission to an excited nucleus of barium. The barium nucleus then emits a gamma ray.

(a) Write the equation for the beta emission. [2] (b) Write the equation for the gamma emission. [2] (c) State how many neutrons the caesium nucleus and the barium nucleus each contain. [1]

10. A nucleus of thorium-232, ²³²₉₀Th, goes through a series of decays and ends as lead-208, ²⁰⁸₈₂Pb. Only alpha and beta emissions occur.

(a) Work out how many alpha particles are emitted. [3] (b) Work out how many beta particles are emitted. [3] (c) State why the number of beta particles cannot be found from the mass numbers alone. [1]

11. A student writes this equation for the decay of lead-214.

²¹⁴₈₂Pb → ²¹⁴₈₂Bi + ⁰₋₁e

(a) Explain what is wrong with it. [2] (b) Write the correct equation. [2]

12. Two isotopes of neon are written ²⁰₁₀Ne and ²²₁₀Ne. Compare the two nuclei in terms of their identities, their charges and their masses. [4]

Answers

1. Protons = 82 (the bottom number) [1]. Neutrons = 207 - 82 = 125 [1]. An uncharged atom has as many electrons as protons, so electrons = 82 [1]. [3] Examiner insight: Show the subtraction for the neutrons. A wrong answer with no working leaves nothing to credit.

2. (a) ²⁸₁₄Si and ³⁰₁₄Si, which are both silicon with 14 protons [1]. (b) They have different numbers of protons, 14 and 15 [1], so they are different elements, and isotopes must be the same element [1]. [3] Examiner insight: The explanation has to refer to protons. Saying only “different symbols” restates the question and gives no reason.

3. (a) The nucleus contains protons and neutrons [1]. The protons give it its positive charge, because neutrons have no charge [1]. (b) A = 11 + 12 = 23 [1], so the symbol is ²³₁₁Na [1]. [4] Examiner insight: A symbol needs both numbers in the right places. Writing 11 on top and 23 below gives a different nucleus.

4. Alpha: two protons and two neutrons, the same as a helium nucleus [1]. Beta: a fast electron emitted from the nucleus [1]. Neutron: a single neutron with no charge [1]. Gamma: electromagnetic radiation with no mass and no charge [1]. [4] Examiner insight: Be specific about the beta particle. “An electron” alone may not show that it comes from the nucleus, so add where it is emitted from.

5. Alpha: A down by 4, Z down by 2 [1]. Beta: A no change, Z up by 1 [1]. Neutron: A down by 1, Z no change [1]. Gamma: A no change, Z no change [1]. [4] Examiner insight: Give direction and size, such as “down by 4”. A bare 4 does not say whether the number rises or falls.

6. Mass number 230 - 4 = 226 [1]. Atomic number 90 - 2 = 88, which is radium [1]. ²³⁰₉₀Th → ²²⁶₈₈Ra + ⁴₂He [1]. Check: 226 + 4 = 230 and 88 + 2 = 90. [3] Examiner insight: Write the alpha particle with both of its numbers, ⁴₂He. Then the balance can be checked on both lines.

7. The mass number stays at 42 [1]. Atomic number 19 + 1 = 20, which is calcium [1]. ⁴²₁₉K → ⁴²₂₀Ca + ⁰₋₁e [1]. Check: 20 + (-1) = 19. [3] Examiner insight: The charge line is where beta equations usually go wrong. Show 20 + (-1) = 19 so the working is visible.

8. (a) Protons = 20 (unchanged) [1]. Neutrons = 24 and mass number = 44 [1]. (b) It is still calcium, because it has 20 protons [1]. It is a different isotope of calcium, with one fewer neutron [1]. [4] Examiner insight: Start from the original nucleus: 20 + 25 = 45. Then lose 1 from the mass number and none from the protons.

9. (a) Mass number stays at 137 and Z = 55 + 1 = 56, which is barium [1]. ¹³⁷₅₅Cs → ¹³⁷₅₆Ba (excited) + ⁰₋₁e [1]. (b) ¹³⁷₅₆Ba (excited) → ¹³⁷₅₆Ba + γ [1]. Both numbers are unchanged and gamma has none [1]. (c) Caesium-137: 137 - 55 = 82 neutrons. Barium: 137 - 56 = 81 neutrons [1]. [5] Examiner insight: In a gamma equation keep the nuclear symbol the same on both sides. Changing the element there suggests a particle was emitted.

10. (a) The mass number falls from 232 to 208, a drop of 24 [1]. Only alpha emission changes the mass number, by 4 each time [1]. 24 ÷ 4 = 6 alpha particles [1]. (b) Six alpha particles lower Z by 12, so Z would be 90 - 12 = 78 [1]. Lead has Z = 82, which is 4 higher [1]. Each beta raises Z by 1, so 4 beta particles [1]. (c) Beta emission does not change the mass number [1]. [7] Examiner insight: Set the working out as two separate stories, one for mass number and one for charge. Mixing them in one line is where slips happen.

11. (a) A beta particle has a charge of -1, so the product must have a charge one higher [1]. The student left the atomic number at 82, so the charge does not balance (82 on the left, 81 on the right) [1]. (b) The atomic number rises to 83, which is bismuth [1]. ²¹⁴₈₂Pb → ²¹⁴₈₃Bi + ⁰₋₁e [1]. [4] Examiner insight: State the numbers that fail to match. Saying “it does not balance” gives less than saying 82 on the left and 81 on the right.

12. Identities: both are neon, with 10 protons [1]. Charge: both nuclei are +10 [1]. Mass: neon-22 has a larger mass number, 22 against 20 [1]. It has 12 neutrons against 10, so two more [1]. [4] Examiner insight: Use the three words the question gives you as headings. Each one needs its own statement for its own mark.

Where marks are usually lost

• Reading the symbol the wrong way round, giving the atomic number as the top number.
• Giving the neutron number as the mass number rather than A - Z.
• Saying isotopes differ in protons or electrons when they differ only in neutrons.
• Treating the same mass number as proof of the same element.
• Using +1 or a mass of 1 for the beta particle in a balance.
• Failing to raise Z by 1 in beta emission, or lowering it.
• Not naming the new element after finding its atomic number.
• Changing the mass number in a gamma equation.
• Counting alpha and beta emissions together instead of taking mass number first, then charge.

Next steps

• Condense the topic with the revision notes.
• Re-read the full explanations in the study guide.
• Course page: OCR GCSE Physics hub, and the printable checklist.
• Take the free 10-minute diagnostics to find your weakest topics.
• Book a free trial class if you want help with any of these.

Official syllabus

OCR GCSE (9-1) Gateway Science Physics A (J249), Version 5.0 (July 2026), for first assessment in 2018, published by OCR. These questions cover learning outcomes P6.1a to P6.1g.