4 high-yield, syllabus-aligned questions on Nuclear chemistry, covering 12 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
State3 marks
State three differences between a nuclear reaction and an ordinary chemical reaction.
TopMarks model answer
A nuclear reaction involves changes in the nucleus of the atom, whereas a chemical reaction involves only the electrons in the outermost shell. A nuclear reaction produces a new element, whereas a chemical reaction rearranges atoms but the elements themselves remain unchanged. A nuclear reaction releases enormously more energy than a chemical reaction, and its rate is unaffected by temperature, pressure or catalysts, which do affect chemical reactions.
Suggested marking guide
1Nucleus versus outer electrons
2A new element is formed
3Far more energy, and unaffected by conditions
Why it scores
Each mark is a paired contrast, and the third is the one that shows real understanding: chemical reaction rates depend on temperature, pressure and catalysts, while nuclear decay rates depend on none of them. That is why a half-life is a fixed constant for an isotope.
A common incomplete answer
“A nuclear reaction takes place in the nucleus while a chemical reaction involves the electrons.”
What it costs: Two marks of three. One properly paired difference is credited; the formation of a new element and the energy comparison are missing.
Key ideas to include: nucleus, outermost electrons, new element, transmutation, half-life.
State3 marks
Uranium-238 decays by emitting an alpha particle. Write the equation for the decay and state the mass number and atomic number of the product.
TopMarks model answer
An alpha particle is a helium nucleus with mass number 4 and atomic number 2. In alpha decay the mass number therefore falls by 4 and the atomic number by 2. Uranium-238 has atomic number 92, so the product has mass number 234 and atomic number 90: U-238 → Th-234 + He-4. The product is thorium-234.
Suggested marking guide
1An alpha particle is a helium nucleus of mass number 4
2Mass number of the product is 234
3Atomic number of the product is 90, giving thorium
Why it scores
Every alpha-decay question is the same two subtractions: mass number minus 4, atomic number minus 2. Once you know the alpha particle is a helium nucleus, you never need to remember any individual decay — you can derive it. Beta decay follows the same logic with mass number unchanged and atomic number plus one.
A common incomplete answer
“Uranium-238 emits an alpha particle and becomes thorium.”
What it costs: Two marks of three. The product element is credited, but the alpha particle is never identified as a helium nucleus and neither the mass number nor the atomic number of the product is given.
Key ideas to include: alpha particle, helium nucleus, mass number, atomic number, transmutation.
State3 marks
State two uses of radioactivity in medicine and one hazard of exposure to radiation.
TopMarks model answer
Radiotherapy uses gamma rays from cobalt-60 to destroy cancerous cells. Radioactive tracers such as iodine-131 are given to a patient and followed with a detector to study the functioning of an organ such as the thyroid gland. Exposure to radiation is hazardous because it ionises the molecules of living cells, damaging DNA and causing mutations, leukaemia and other cancers.
Suggested marking guide
1Radiotherapy using gamma rays
2Radioactive tracers
3A named hazard to cells or DNA
Why it scores
Two uses plus one hazard equals three marks — the structure of the answer is dictated by the question. For the hazard, name the mechanism as well as the outcome: radiation ionises molecules in cells and damages DNA. “It is dangerous” is not a hazard, it is an opinion.
A common incomplete answer
“Gamma rays are used to treat cancer, and radiation can damage cells.”
What it costs: One mark of three. Radiotherapy and the hazard are both credited, but only one use is given where the question asked for two.
Define half-life and calculate the mass of a 40 g sample of a radioactive isotope of half-life 5 days that remains after 20 days.
TopMarks model answer
The half-life of a radioactive isotope is the time taken for half of the atoms in a given sample to decay. In 20 days the number of half-lives is 20 ÷ 5 = 4. The mass remaining is 40 ÷ 2⁴ = 40 ÷ 16 = 2.5 g.
Suggested marking guide
1Correct definition of half-life
2Correct number of half-lives
3Correct final mass with the unit
The working, line by line
1Number of half-lives = total time ÷ half-life = 20 ÷ 5 = 4 — Always work out the NUMBER of half-lives first. It is a mark on its own and it prevents almost every error in this topic.
2Halve four times: 40 → 20 → 10 → 5 → 2.5 — Halving step by step is safer under pressure than using 40 ÷ 2⁴, and it shows the examiner your reasoning.
3Mass remaining = 2.5 g — Give the unit.
Why it scores
The trap is treating decay as though it were linear. After four half-lives the sample is not down by four halves — it is halved four times over, leaving one sixteenth. Writing the halving sequence out makes the error impossible to make.
A common incomplete answer
“The half-life is the time taken for half the sample to decay. After 20 days, 10 g will be left.”
What it costs: Two marks of three. The definition is credited, but the number of half-lives is never worked out and the arithmetic treats the decay as linear rather than as repeated halving.
Key ideas to include: half-life, decay, number of half-lives, repeated halving.
Practise Nuclear chemistry on real questions
Reading a full-mark answer is the first half. Writing one under time is the other. A free account opens exam-standard practice in Chemistry with the full solution on every question.