IGCSE Physics: The Nuclear Model of the Atom Practice Questions
An atom has a tiny, dense, positively charged nucleus containing protons and neutrons, surrounded by electrons. The alpha particle scattering experiment provided the evidence for this model.
Sub-topic 5.1 of Cambridge IGCSE Physics 0625 is examined mainly through the alpha scattering experiment and nuclide notation. The scattering question is marked observation by observation, so each result must be paired with the conclusion it supports. The questions below train that pairing.
What you need to know for The Nuclear Model of the Atom
- Structure of the atomA central nucleus containing protons and neutrons, with electrons orbiting around it. The nucleus is extremely small compared with the atom, and almost all the mass is concentrated in it.
- Alpha particle scatteringAlpha particles fired at thin gold foil. Most passed straight through, some were deflected through small angles, and a very few bounced back.
- Nuclide notationThe nucleon number is written above and the proton number below the chemical symbol. Nucleon number is the total of protons and neutrons, and proton number identifies the element.
- IsotopesAtoms of the same element with the same number of protons but different numbers of neutrons. They have identical chemical properties but different masses and different nuclear stability.
- Nuclear fissionA large unstable nucleus splits into two smaller nuclei plus two or three neutrons, releasing energy. Those neutrons can cause further fissions, producing a chain reaction.
- Nuclear fusionTwo light nuclei join to form a heavier nucleus, releasing energy. This is the process powering the Sun and other stars.
IGCSE Physics The Nuclear Model of the Atom questions and answers
4 exam-style questions written to the 0625 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
In the alpha particle scattering experiment, most alpha particles passed straight through the gold foil, a small number were deflected, and a very few bounced back. State the conclusion drawn from each observation.
Show the worked answer
- Most alpha particles passed straight through with no deflection. This shows the atom is mostly empty space, since nearly all the particles met nothing to deflect them.
- A small number were deflected through small angles. This shows there is a concentration of positive charge in the atom, which repelled the positively charged alpha particles that passed close to it.
- A very few bounced almost straight back. This shows the concentration of positive charge is extremely small in volume, because so few particles came close enough to be turned around.
- It also shows the nucleus is very massive compared with an alpha particle, since a light target could not reverse the particle's direction.
An atom is written with a nucleon number of 40 and a proton number of 19. State the number of protons, neutrons and electrons in a neutral atom of this element, and explain what would change in an isotope of it.
Show the worked answer
- Protons equal the proton number, so there are 19 protons.
- Neutrons equal nucleon number minus proton number: 40 minus 19 = 21 neutrons.
- The atom is neutral, so the number of electrons equals the number of protons: 19 electrons.
- An isotope of the same element has the same 19 protons and therefore the same 19 electrons, but a different number of neutrons, so its nucleon number differs.
Describe what happens during nuclear fission and explain how a chain reaction is produced.
Show the worked answer
- A slow moving neutron is absorbed by a large unstable nucleus such as uranium-235, making it more unstable.
- The nucleus splits into two smaller nuclei of roughly similar size, releasing a large amount of energy.
- Two or three additional neutrons are also released in the process.
- Each of these neutrons can be absorbed by another large nucleus and cause it to undergo fission in turn. The number of fissions therefore multiplies rapidly, producing a chain reaction. In a reactor, control rods absorb surplus neutrons to keep the rate steady.
Explain why the nuclear model replaced the earlier model in which positive charge was spread evenly throughout the atom.
Show the worked answer
- In the earlier model the positive charge and mass were spread thinly and evenly across the whole atom.
- Such a diffuse charge would exert only a weak force on a fast, massive alpha particle, so every particle would have passed through with at most a very small deflection.
- The observation that a very small number of alpha particles were deflected through more than 90 degrees could not be explained by that model.
- Concentrating all the positive charge and nearly all the mass into a tiny central nucleus does explain it, so the model was revised. This is how experimental evidence changes scientific models.
Common mistakes in this topic
- Listing the scattering conclusions without pairing them to the observations.
- Confusing nucleon number with proton number.
- Saying isotopes have different numbers of protons.
- Mixing up fission and fusion.
- Describing electrons as part of the nucleus.
Exam tips
- Answer the scattering question in three pairs: observation, then conclusion, three times.
- Check nuclide arithmetic by making sure the neutron count is not negative and is usually similar to or greater than the proton count.
- Fission splits, fusion joins. Fusion powers the Sun, fission powers reactors.
- For any question about a model being replaced, name the observation the old model could not explain.
- State that the nucleus contains almost all the mass of the atom but occupies a tiny fraction of its volume.
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The Nuclear Model of the Atom FAQs
What did the alpha particle scattering experiment show?
Most alpha particles passed straight through the foil, showing the atom is mostly empty space. Some were deflected, showing a concentration of positive charge. A very few bounced back, showing that concentration is extremely small in volume and very massive. Together these established the nuclear model.
How do I work out the number of neutrons in an atom?
Subtract the proton number from the nucleon number. An atom with a nucleon number of 40 and a proton number of 19 has 21 neutrons. In a neutral atom the number of electrons equals the number of protons, in this case 19.
What is the difference between fission and fusion?
Fission is the splitting of a large unstable nucleus into two smaller nuclei plus two or three neutrons, releasing energy. Fusion is the joining of two light nuclei to form a heavier one, also releasing energy. Fission powers nuclear reactors and fusion powers the Sun.
What is a chain reaction?
When a nucleus undergoes fission it releases two or three neutrons. Each of those can be absorbed by another large nucleus and cause it to split in turn, releasing more neutrons. The number of fissions multiplies rapidly. In a reactor, control rods absorb surplus neutrons to hold the rate steady.
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Written to the published Cambridge IGCSE Physics (0625) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.