IGCSE Physics Nucleus Questions and Worked Solutions (5.1.2) | 0625

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IGCSE Physics 0625 · Topic 5.1.2

IGCSE Physics Nucleus Questions and Worked Solutions

50 topic-wise practice questions on The Nucleus (Unit 5: Nuclear Physics), written to the Cambridge IGCSE Physics 0625 syllabus for 2026–2028, each with a step-by-step worked solution.

50 questions21 Core19 Extended10 Challenge118 marksTextbook §5.1 · pp. 252–259
Questions tagged Core are for all candidates (Papers 1 & 3). Questions tagged Extended are Supplement content (Papers 2 & 4). Questions 41–50 are harder Challenge questions. Try each question first, then tap Show solution to check your working.

Topic guide: The Nucleus

The nucleus contains protons and neutrons. The proton number identifies the element; the nucleon number gives the total number of particles in the nucleus. Isotopes have the same proton number but different neutron numbers. (Extended) Fission splits heavy nuclei; fusion joins light nuclei.

Key formulas

number of neutrons = A − Znucleon number A, proton number ZCore
ᴬ_Z Xnuclide notationCore
balance A and Z on both sidesnuclide equationsExtended

Prerequisite knowledge

  • Atomic structure (5.1.1)

Exam tips & common mistakes

  • A is the top number, Z the bottom
  • Isotopes have the same chemical properties
  • Check both totals when balancing equations

Key facts from the syllabus (5.1.2)

  • Relative charges: proton +1, neutron 0, electron −1
  • Proton number (atomic number) Z: number of protons; nucleon number (mass number) A: protons + neutrons
  • Isotopes: same element (same Z), different numbers of neutrons; an element can have several isotopes
  • Extended fission — a heavy nucleus (e.g. U-235) absorbs a neutron and splits into two smaller nuclei plus neutrons, releasing energy
  • Extended fusion — light nuclei join to form a heavier nucleus, releasing energy
  • Extended in both, the products have slightly less mass; the mass difference is released as energy
  • Extended relative charge of a nucleus = Z; relative mass ≈ A

Textbook: §5.1 The nuclear model of the atom, pp. 252–259

1
Core[1 mark]

Name the two particles in the nucleus.

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Method: Protons and neutrons.

Protons and neutrons
2
Core[2 marks]

State the relative charges of a proton, a neutron and an electron.

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Method: +1, 0, −1.

+1, 0, −1
3
Core[1 mark]

Define proton number Z.

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Method: Number of protons.

The number of protons in the nucleus
4
Core[1 mark]

Define nucleon number A.

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Method: Protons + neutrons.

The total number of protons and neutrons in the nucleus
5
Core[2 marks]

A nucleus of uranium is written ²³⁸₉₂U. Calculate the number of neutrons.

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Method: N = A − Z.

N = 238 − 92 = 146
6
Core[2 marks]

A carbon nucleus is ¹⁴₆C. State the number of protons, neutrons and nucleons.

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Method: Z = 6, A = 14.

6 protons, 8 neutrons, 14 nucleons
7
Core[2 marks]

Write the nuclide notation for an atom with 19 protons and 20 neutrons (symbol K).

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Method: A = 39.

³⁹₁₉K
8
Core[2 marks]

Define an isotope.

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Method: Same Z, different N.

Atoms of the same element with the same number of protons but different numbers of neutrons
9
Core[2 marks]

State which of these are isotopes of the same element: ¹²₆C, ¹⁴₆C, ¹⁴₇N.

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Method: Same Z.

¹²₆C and ¹⁴₆C
10
Core[2 marks]

State the number of electrons in a neutral atom of ²³₁₁Na.

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Method: Equals Z.

11
11
Core[2 marks]

Chlorine has isotopes ³⁵Cl and ³⁷Cl. Proton number 17. Calculate the neutrons in each.

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Method: N = A − Z.

18 and 20
12
Core[2 marks]

State the relative masses of a proton, neutron and electron.

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Method: 1, 1, ~1/1840.

1, 1 and about 1/1840 (negligible)
13
Core[2 marks]

A nucleus has 26 protons and 30 neutrons. State its nucleon number.

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Method: Add.

56
14
Core[2 marks]

Write nuclide notation for an iron nucleus with 26 protons and 30 neutrons.

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Method: A = 56.

⁵⁶₂₆Fe
15
Core[2 marks]

State the number of neutrons in ²⁷₁₃Al.

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Method: 27 − 13.

14
16
Core[2 marks]

State the number of protons and neutrons in ¹⁶₈O.

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Method: Read notation.

8 protons, 8 neutrons
17
Core[2 marks]

State the number of neutrons in ²³⁵₉₂U and in ²³⁸₉₂U.

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Method: A − Z.

143 and 146
18
Core[2 marks]

Explain why ²³⁵U and ²³⁸U are isotopes.

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Method: Same Z.

They have the same number of protons (92) but different numbers of neutrons
19
Core[2 marks]

Hydrogen has isotopes ¹H, ²H and ³H. State the number of neutrons in each.

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Method: A − 1.

0, 1 and 2
20
Core[2 marks]

Explain why isotopes of an element have the same chemical properties.

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Method: Same electrons.

They have the same number of protons and electrons
21
Core[2 marks]

State the particle with no charge in the nucleus.

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Method: Neutron.

Neutron
22
Extended[1 mark]

State the relationship between the proton number and the relative charge on a nucleus.

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Method: Equal.

The relative charge equals the proton number Z
23
Extended[1 mark]

State the relationship between the nucleon number and the relative mass of a nucleus.

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Method: Approximately equal.

The relative mass is approximately equal to the nucleon number A
24
Extended[2 marks]

Describe nuclear fission.

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Method: Splitting.

A large nucleus (e.g. uranium-235) absorbs a neutron and splits into two smaller nuclei, releasing neutrons and energy
25
Extended[2 marks]

Describe nuclear fusion.

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Method: Joining.

Two light nuclei (e.g. hydrogen) join to form a heavier nucleus, releasing energy
26
Extended[2 marks]

State what happens to the total mass during fission and fusion.

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Method: Mass decreases.

The total mass decreases slightly; the lost mass is released as energy
27
Extended[3 marks]

Complete: ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + x ¹₀n. Find x.

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Method: Balance A.

236 = 141 + 92 + x, so x = 3
28
Extended[3 marks]

Complete the fusion equation ²₁H + ³₁H → ⁴₂He + ? .

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Method: Balance A and Z.

¹₀n (a neutron)
29
Extended[2 marks]

Explain why fusion needs very high temperatures.

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Method: Overcome repulsion.

Nuclei are positively charged and repel; very high speeds (temperatures) are needed to bring them close enough to fuse
30
Extended[2 marks]

A nucleus has relative charge +26 and relative mass 56. State its proton number and nucleon number.

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Method: Z = charge, A ≈ mass.

Z = 26, A = 56
31
Extended[2 marks]

A nucleus has relative charge +8 and relative mass 17. State Z and A.

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Method: Read.

Z = 8, A = 17
32
Extended[2 marks]

Write the nuclide notation for a neutron.

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Method: A = 1, Z = 0.

¹₀n
33
Extended[3 marks]

Balance: ²³⁵₉₂U + ¹₀n → ¹⁴⁴₅₆Ba + ⁸⁹₃₆Kr + x¹₀n.

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Method: Balance A.

x = 236 − 144 − 89 = 3
34
Extended[3 marks]

Balance: ²³⁵₉₂U + ¹₀n → ¹⁴⁰₅₄Xe + ᴬ₃₈Sr + 2¹₀n. Find A.

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Method: Balance A.

A = 236 − 140 − 2 = 94
35
Extended[2 marks]

State what happens to the neutrons released in fission in a reactor.

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Method: Chain reaction.

They can cause further fissions (a chain reaction)
36
Extended[3 marks]

Write the equation for two ²₁H nuclei fusing to form ³₂He and one other particle.

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Method: Balance.

²₁H + ²₁H → ³₂He + ¹₀n
37
Extended[2 marks]

Explain why the total mass of products in fusion is less than that of reactants.

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Method: Energy released.

Some mass is converted into the energy released
38
Extended[2 marks]

State one difference between fission and fusion other than splitting/joining.

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Method: Elements, conditions.

Fission uses heavy nuclei (e.g. uranium); fusion uses light nuclei (e.g. hydrogen) and needs very high temperature
39
Extended[2 marks]

Explain why fusion is not yet used in commercial power stations.

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Method: Conditions hard.

It needs extremely high temperatures and pressures that are hard to achieve and contain
40
Extended[2 marks]

State the relationship between the nucleon number and the approximate relative mass of a nucleus.

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Method: Equal.

The relative mass is approximately equal to the nucleon number

Challenge questions

Harder, multi-step questions at the top end of the exam. Attempt these once you are confident with the questions above.

41
ChallengeCore[4 marks]

Chlorine has isotopes ³⁵₁₇Cl and ³⁷₁₇Cl. State the number of protons, neutrons and electrons in a neutral atom of each and explain why they are isotopes.

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Method: A − Z.

Cl-35: 17 p, 18 n, 17 e; Cl-37: 17 p, 20 n, 17 e; same proton number, different neutron numbers
42
ChallengeCore[4 marks]

A nucleus has 82 protons and 126 neutrons. Write its nuclide notation (symbol Pb) and state its relative charge and approximate relative mass.

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Method: Z and A.

²⁰⁸₈₂Pb; relative charge +82; relative mass about 208
43
ChallengeCore[3 marks]

Explain why isotopes of the same element cannot be separated by chemical reactions.

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Method: Same electrons.

They have the same number of electrons and therefore identical chemical properties
44
ChallengeExtended[4 marks]

Complete and balance: ²³⁵₉₂U + ¹₀n → ¹⁴¹₅₆Ba + ᴬ_Z Kr + 3¹₀n, finding A and Z.

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Method: Balance both.

A = 236 − 141 − 3 = 92; Z = 92 − 56 = 36
45
ChallengeExtended[4 marks]

Complete the fusion equation ³₂He + ³₂He → ⁴₂He + 2X, identifying X.

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Method: Balance.

X = ¹₁H (a proton): mass 6 = 4 + 2, charge 4 = 2 + 2
46
ChallengeExtended[4 marks]

Describe nuclear fission in a reactor, including what starts it and how a chain reaction occurs.

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Method: Neutron absorption.

A uranium-235 nucleus absorbs a slow neutron, becomes unstable and splits into two smaller nuclei, releasing 2–3 neutrons and energy; these neutrons cause further fissions
47
ChallengeExtended[4 marks]

Explain where the energy released in fission and fusion comes from, without using numbers.

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Method: Mass decrease.

In both, the total mass of the products is less than that of the reactants; this mass difference is released as energy
48
ChallengeExtended[4 marks]

Compare fission and fusion in terms of nuclei involved, conditions needed and products.

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Method: Comparison.

Fission: heavy nuclei split, triggered by neutrons, products radioactive. Fusion: light nuclei join, need very high temperature and pressure, products mostly not radioactive
49
ChallengeExtended[3 marks]

Explain why the relative charge of a nucleus equals its proton number.

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Method: Neutrons neutral.

Each proton has charge +1 and neutrons are uncharged
50
ChallengeExtended[4 marks]

Explain why fusion requires extremely high temperatures, using ideas about charge and kinetic energy.

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Method: Overcome repulsion.

Nuclei are positively charged and repel; only at very high temperature do they move fast enough to get close enough to fuse
All IGCSE Physics topics← 5.1.1 The Atom5.2.1 Detection of Radioactivity →

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