Physics 0625 · syllabus 5.1
Cambridge IGCSE Physics 0625 — The nuclear model of the atom topical past papers
The structure of the atom, protons, neutrons and electrons, proton and nucleon numbers, isotopes and nuclide notation, alpha-particle scattering, and fission and fusion.
- Questions
- 781
- Years
- 2009–2026
- With mark scheme
- 99%
- Questions 2022–2026
- 307
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What The nuclear model of the atom covers
An atom has a tiny, positively charged nucleus, with negatively charged electrons orbiting around it. The nucleus is made of protons and neutrons, whose relative charges are +1 and 0, while an electron's relative charge is −1. An atom that loses electrons becomes a positive ion, and one that gains electrons becomes a negative ion; the nucleus is unchanged in both cases.
The proton number Z, also called the atomic number, is the number of protons, and it decides which element the atom belongs to. The nucleon number A, or mass number, counts the protons and neutrons together, so the number of neutrons is A − Z. Nuclide notation writes the element's symbol with A at the top left and Z at the bottom left, as in ¹⁴₆C. Isotopes of an element have the same number of protons but different numbers of neutrons, and many elements have more than one. Extended candidates link the charge of a nucleus to its proton number and its mass to its nucleon number.
Extended candidates also describe the evidence for this model. When α-particles are fired at a very thin metal foil, most pass straight through, showing that the atom is mostly empty space; a few are deflected through large angles and a very small number bounce back, showing that the atom's positive charge and almost all of its mass are packed into a tiny nucleus. They describe nuclear fission, in which a heavy nucleus such as uranium-235 absorbs a neutron and splits into two smaller nuclei and a few neutrons, and nuclear fusion, in which light nuclei join, as they do in the Sun. In both, the products have slightly less mass than the nuclei they came from, and energy is released; fission is what drives nuclear power stations.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 5.1 The nuclear model of the atom
What you should be able to do
- Describe the structure of an atom and how atoms become positive or negative ions
- State the relative charges of the proton, the neutron and the electron
- Work out the numbers of protons, neutrons and electrons from nuclide notation
- Explain what isotopes are, with examples
- Explain what firing α-particles at a thin metal foil reveals about the atom (Extended)
- Describe fission and fusion, and complete nuclide equations for them (Extended)
Key ideas
- Relative charges: proton +1, neutron 0, electron −1
- Proton number Z = number of protons; nucleon number A = protons + neutrons; neutrons = A − Z
- Isotopes: the same proton number, different nucleon numbers
- α-scattering: most pass straight through (mostly empty space); a few are deflected strongly (a tiny, positive, massive nucleus) (Extended)
- Fission splits a heavy nucleus and fusion joins light ones; both release energy as the total mass falls slightly (Extended)
How The nuclear model of the atom has been examined
From 2022 to 2026, The nuclear model of the atom was on Paper 4 · Theory (Extended) in 13 of 14 exam series, worth about 10% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 122 | 10% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 125 | 10% | 13 of 14 |
| Paper 3 · Theory (Core) | 28 | 7% | 13 of 14 |
| Paper 4 · Theory (Extended) | 32 | 10% | 13 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1. The bank files this topic together with neighbouring ones, so these figures count every question in its chapters, including some on those topics.
Filed in Momentum under: Atoms and Radioactivity.
A worked example
A uranium-235 nucleus, ²³⁵₉₂U, absorbs a neutron and splits into rubidium-93, ⁹³₃₇Rb, a caesium nucleus with nucleon number 141, and some neutrons. Find the proton number of the caesium, the number of neutrons released, and the number of neutrons in the uranium-235 nucleus.
- Proton numbers balance: 92 + 0 = 37 + Z, so caesium has proton number Z = 55
- Nucleon numbers balance: 235 + 1 = 93 + 141 + n, so n = 236 − 234 = 2 neutrons released
- Neutrons in uranium-235 = A − Z = 235 − 92 = 143
Answer: Proton number 55; 2 neutrons released; 143 neutrons in uranium-235
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Swapping the proton and nucleon numbers — the nucleon number is the larger, top number, so ¹⁴₆C has 6 protons and 8 neutrons
- Saying isotopes have different numbers of protons — they would then be different elements; isotopes differ only in their neutrons
- Saying an ion forms when an atom gains or loses protons — ions form when electrons are gained or lost, and the nucleus does not change
- Pairing the scattering observations with the wrong conclusions — the many undeflected α-particles show empty space; the rare large deflections show a small, positive, massive nucleus
- Mixing up fission and fusion — fission splits heavy nuclei, as in a power station; fusion joins light nuclei, as in the Sun
Exam technique
- In nuclide equations, check that the top numbers balance and the bottom numbers balance before moving on
- When describing α-particle scattering, put each observation and its conclusion in the same sentence
- Say 'proton number' and 'nucleon number' rather than 'atomic mass', which is a different quantity
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.