Isotopes Exam Questions
Practice IGCSE Chemistry exam questions on isotopes. Covers definition, calculating relative atomic mass from isotopic abundances, uses of radioactive isotopes, and identifying isotopes from data with mark schemes and examiner notes.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
These questions cover all aspects of isotopes tested on the 0620 papers. Write each answer in full before revealing the mark scheme.
Question 1 (2 marks, Core)
Define the term isotopes.
Mark scheme
- Atoms of the same element [1]
- with the same number of protons but different numbers of neutrons / same atomic number but different mass numbers [1]
Examiner note: Both parts are needed for full marks. “Same element, different mass” only earns 1 mark — you must specify that the difference is in neutron number (or mass number). “Same protons, different neutrons” is the safest wording.
Question 2 (2 marks, Core)
Chlorine has two isotopes: chlorine-35 and chlorine-37. Both have 17 protons.
(a) State the number of neutrons in each isotope. (1)
(b) Explain why both isotopes have identical chemical properties. (1)
Mark scheme
(a) Chlorine-35 has 18 neutrons; chlorine-37 has 20 neutrons [1] (both needed)
(b) They have the same number of electrons / same electron configuration, and chemical properties depend on electron arrangement [1]
Examiner note: Chemical properties depend on electrons, not neutrons. This is why isotopes react identically despite having different masses. Physical properties (such as density and rate of diffusion) differ because mass differs.
Question 3 (3 marks, Supplement)
Chlorine has two isotopes: ^35Cl (75.0% abundance) and ^37Cl (25.0% abundance).
Calculate the relative atomic mass of chlorine. Show your working.
Mark scheme
- Ar = (35 x 75.0 + 37 x 25.0) / 100 [1]
- = (2625 + 925) / 100 [1]
- = 3550 / 100 = 35.5 [1]
Examiner note: The formula is: Ar = sum of (isotopic mass x percentage abundance) / 100. Even if your final answer is wrong, correct substitution into the formula earns method marks. Do not round early — give the answer to one decimal place.
Question 4 (4 marks, Supplement)
Boron has two naturally occurring isotopes.
| Isotope | Mass number | Abundance (%) |
|---|---|---|
| Boron-10 | 10 | 19.9 |
| Boron-11 | 11 | 80.1 |
(a) State the number of neutrons in boron-10 and boron-11. (Boron has atomic number 5.) (1)
(b) Calculate the relative atomic mass of boron. Show your working. (3)
Mark scheme
(a) Boron-10: 5 neutrons; Boron-11: 6 neutrons [1]
(b) Ar = (10 x 19.9 + 11 x 80.1) / 100 [1] = (199 + 881.1) / 100 [1] = 1080.1 / 100 = 10.8 [1]
Examiner note: The relative atomic mass (10.8) is closer to 11 than to 10 because boron-11 is much more abundant (80.1%). This is a useful check — the Ar should be closer to the more abundant isotope’s mass number.
Question 5 (3 marks, Supplement)
The relative atomic mass of copper is 63.5. Copper has two isotopes: ^63Cu and ^65Cu.
Calculate the percentage abundance of each isotope. Show your working.
Mark scheme
Let the percentage of ^63Cu = x, then percentage of ^65Cu = (100 - x)
63x + 65(100 - x) = 63.5 x 100 [1] 63x + 6500 - 65x = 6350 -2x = -150 [1] x = 75
^63Cu = 75%, ^65Cu = 25% [1]
Examiner note: This is a reverse calculation — working backwards from the Ar to find abundances. Set up the algebra carefully. A common error is forgetting to multiply the Ar by 100. Check: the Ar (63.5) is closer to 63, so ^63Cu should be more abundant.
Question 6 (3 marks, Core)
Carbon has three isotopes: carbon-12, carbon-13, and carbon-14.
(a) Complete the table for each isotope. (Carbon has atomic number 6.) (2)
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| Carbon-12 | ? | ? | ? |
| Carbon-13 | ? | ? | ? |
| Carbon-14 | ? | ? | ? |
(b) State one use of carbon-14. (1)
Mark scheme
(a)
| Isotope | Protons | Neutrons | Electrons |
|---|---|---|---|
| Carbon-12 | 6 | 6 | 6 |
| Carbon-13 | 6 | 7 | 6 |
| Carbon-14 | 6 | 8 | 6 |
[1] for protons and electrons correct for all three (all 6) [1] for neutrons correct for all three (6, 7, 8)
(b) Carbon dating / determining the age of archaeological specimens/fossils [1]
Examiner note: All three isotopes have 6 protons and 6 electrons — only the neutron count changes. This is the defining feature of isotopes.
Question 7 (4 marks, Supplement)
Magnesium has three isotopes with the following data:
| Isotope | Mass number | Abundance (%) |
|---|---|---|
| Mg-24 | 24 | 78.99 |
| Mg-25 | 25 | 10.00 |
| Mg-26 | 26 | 11.01 |
(a) Calculate the relative atomic mass of magnesium. Give your answer to one decimal place. (3)
(b) Explain why the relative atomic mass is not a whole number. (1)
Mark scheme
(a) Ar = (24 x 78.99 + 25 x 10.00 + 26 x 11.01) / 100 [1] = (1895.76 + 250.00 + 286.26) / 100 [1] = 2432.02 / 100 = 24.3 [1]
(b) The relative atomic mass is a weighted average of the masses of all the isotopes, taking into account their relative abundances, so it is not a whole number [1]
Examiner note: “It’s an average” alone is not sufficient — you must say it is a weighted average that accounts for the different abundances. This explains why most Ar values in the periodic table are not whole numbers.
What to revise if you scored below 5
If the definition caused problems (Question 1), memorise: “same protons, different neutrons.” If Ar calculations were difficult (Questions 3-5, 7), practise the formula until it is automatic: sum of (mass x %) / 100. Revisit the isotopes notes for worked examples and remember to check your answer — the Ar must fall between the lightest and heaviest isotope masses.
Studying this yourself? Tutoring arrangements are normally made by a parent or guardian. Message us for the details to share with them, or send them this page.