What are isotopes and how do they differ from each other?

Isotopes are variants of a particular chemical element that have the same number of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers. For example, carbon-12 and carbon-14 are isotopes of carbon, with mass numbers 12 and 14 respectively.

Why are isotopes important in the Cambridge IGCSE Chemistry curriculum?

Isotopes are important in the Cambridge IGCSE Chemistry curriculum because they help students understand the concept of atomic structure and the diversity of elements. They also play a crucial role in various applications such as radiocarbon dating, medical diagnostics, and nuclear energy, which are relevant to both theoretical and practical chemistry.

How do isotopes affect the average atomic mass of an element?

The average atomic mass of an element is calculated based on the relative abundance of its isotopes. Each isotope contributes to the average atomic mass in proportion to its abundance. For instance, if an element has two isotopes, one more abundant than the other, the average atomic mass will be closer to the mass of the more abundant isotope.

Can you explain the concept of stable and radioactive isotopes?

Stable isotopes are those that do not undergo radioactive decay and remain unchanged over time. Radioactive isotopes, on the other hand, are unstable and decay over time, emitting radiation. This property makes radioactive isotopes useful in various applications such as medical imaging and treatment, as well as in determining the age of archaeological finds through radiometric dating.

How are isotopes used in real-world applications?

Isotopes have a wide range of applications in the real world. In medicine, radioactive isotopes are used in diagnostic imaging and cancer treatment. In archaeology, isotopes like carbon-14 are used for dating ancient artifacts. In environmental science, isotopes help trace the origins of pollutants. These applications highlight the significance of isotopes beyond the classroom, making them an essential topic in the Cambridge IGCSE Chemistry curriculum.

Why is "Saying isotopes are different elements" a common mistake in Isotopes (New)?

Why it happens: Different mass numbers. How to avoid it: Same proton number → SAME element. Just different mass. Source: 0620/42 — recurring

Why is "Saying isotopes have different chemical properties" a common mistake in Isotopes (New)?

Why it happens: Different masses suggests different behaviour. How to avoid it: Chemistry depends on electrons, which are the same. So chemistry is identical.

Why is "Forgetting to divide by 100 (or the total abundance)" a common mistake in Isotopes (New)?

Why it happens: Speed. How to avoid it: Always divide by total percentage (usually 100). The answer should be close to the mass numbers, never bigger.

Why is "Adding isotope masses without weighting" a common mistake in Isotopes (New)?

Why it happens: Simplifying. How to avoid it: Use a WEIGHTED mean — multiply each mass by its abundance fraction.

Atoms, Elements and CompoundsCambridge IGCSE Chemistry (0620)

Isotopes (New)

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Isotopes — Cambridge IGCSE 0620 Chemistry Extended (2026)

Different forms of the same element with the same proton number but different nucleon numbers. Same chemistry, different mass — the basis for relative atomic masses.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

1.5 — Define isotope.
1.5 — Explain why isotopes of the same element have the same chemistry.
1.5 — Calculate relative atomic mass from isotopic abundance (Extended).

What is an isotope?

Same proton number, different number of neutrons → same element, different mass.

Isotope. A different form of the same element with the same number of PROTONS (so same $Z$ ) but a different number of NEUTRONS (so different $A$ ).

Examples.

Hydrogen: $^{1}_{1}\text{H}$ (protium, no neutrons), $^{2}_{1}\text{H}$ (deuterium, 1 neutron), $^{3}_{1}\text{H}$ (tritium, 2 neutrons).
Carbon: $^{12}_{6}\text{C}$ , $^{13}_{6}\text{C}$ , $^{14}_{6}\text{C}$ (radioactive).
Chlorine: $^{35}_{17}\text{Cl}$ and $^{37}_{17}\text{Cl}$ .

Worked. $^{35}\text{Cl}$ has 17 protons (chlorine) and $35 - 17 = 18$ neutrons. $^{37}\text{Cl}$ has 17 protons and 20 neutrons.

Same proton number → same number of electrons → same chemistry.

Different mass. Heavier isotopes are slightly less mobile (e.g. $^{2}\text{H}_{2}\text{O}$ — heavy water — is denser and less volatile than ordinary water), but their chemical reactions are identical in type.

Same $Z$ , different $A$ .
Same protons + electrons.
Different neutrons.
Same chemistry, different mass.

Relative atomic mass from isotopes

$A_r$ = weighted average of the natural isotope masses, weighted by abundance.

Why is chlorine's $A_r$ shown as 35.5 on the Periodic Table? Because in nature, chlorine occurs as a mixture of two stable isotopes — neither has mass 35.5. The "35.5" is the WEIGHTED AVERAGE.

Calculation. $A_r = \dfrac{\sum (\text{isotope mass} \times \text{abundance \%})}{100}.$

Worked. Chlorine: $^{35}\text{Cl}$ at 75% abundance, $^{37}\text{Cl}$ at 25%.

$A_r = (35 \times 75 + 37 \times 25)/100 = (2625 + 925)/100 = 35.5$ .

Worked. Boron: $^{10}\text{B}$ at 20%, $^{11}\text{B}$ at 80%.

$A_r = (10 \times 20 + 11 \times 80)/100 = (200 + 880)/100 = 10.8$ .

Tip. $A_r$ is rarely a whole number — that's a sign of multiple isotopes. The Periodic Table values (e.g. Cl 35.5, Cu 63.5) are the standardised weighted averages.

$A_r = \sum(m \times \%) / 100$ .
Weighted by ABUNDANCE.
Cl: 35.5 from 75% $^{35}$ Cl + 25% $^{37}$ Cl.
Non-whole $A_r$ → multiple isotopes.

Radioactive isotopes (briefly)

Some isotopes are unstable — used as tracers, dating, medical treatment.

Radioactive isotopes. Have unstable nuclei that emit radiation (alpha, beta, or gamma) over time.

Common uses.

$^{60}\text{Co}$ (cobalt-60): gamma source; cancer treatment, sterilising medical equipment.
$^{14}\text{C}$ (carbon-14): beta emitter; radiocarbon dating of organic remains (half-life $5730$ years).
$^{131}\text{I}$ (iodine-131): medical tracer; thyroid imaging.
$^{235}\text{U}$ (uranium-235): nuclear power.

Worked qualitative. Why use carbon-14 for dating but not for cancer treatment? Half-life 5730 years means activity is too low for a significant medical effect; useful for archaeology because the long timescale matches sample ages.

Cambridge tip. Cambridge IGCSE Chemistry doesn't go deep into radioactive decay — that's more Physics 0625. Just know: some isotopes are radioactive, used in dating, medicine and energy.

Some isotopes unstable → radioactive.
$^{14}\text{C}$ for archaeological dating.
$^{60}\text{Co}$ for cancer treatment / sterilisation.
$^{131}\text{I}$ for thyroid scans.

How it’s examined

Isotopes appear most years on Paper 2 (3-4 marks: definition, identify isotope pairs) and Paper 4 (4-5 marks: $A_r$ calculation from abundances). Examiner reports flag students saying 'isotopes have different chemistries' — they don't, they have the same chemistry but different mass.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (1.5); 0620/42 Oct/Nov 2024 — Q4 ($A_r$ from isotopic abundances); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

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Step-by-step worked examples — Isotopes (New)

Step-by-step solutions to past-paper-style questions on isotopes (new), written exactly the way a tutor would explain them at the board.

1Identify isotopes
Core• identify
Question
Are $^{35}_{17}\text{Cl}$ and $^{37}_{17}\text{Cl}$ isotopes? Explain.
Step-by-step solution
1. Step 1
  Same proton number ( $Z = 17$ ); different nucleon number → different neutrons.
Answer
Yes — same element (chlorine), different number of neutrons.
2Calculate relative atomic mass
Extended• Adapted from 0620/42 May/Jun 2024 Q4• A_r
Question
Chlorine is $75\%$ ${}^{35}\text{Cl}$ and $25\%$ ${}^{37}\text{Cl}$ . Find $A_{r}$ .
Step-by-step solution
1. Step 1
  Weighted mean of isotope masses.
  $A_{r} = \dfrac{75 \times 35 + 25 \times 37}{100}$
2. Step 2
  Compute.
  $= \dfrac{2625 + 925}{100} = 35.5$
Answer
$A_{r}(\text{Cl}) = 35.5$
3Chemical vs physical properties
Core• properties
Question
Compare the chemical and physical properties of two isotopes of an element.
Step-by-step solution
1. Step 1
  Chemical properties: IDENTICAL (same number of outer-shell electrons).
2. Step 2
  Physical properties: SLIGHTLY DIFFERENT (different mass → different density, slight difference in melting/boiling points, different rates of diffusion).
Answer
Chemical: same. Physical: small differences due to mass.
4Use of a radioactive isotope
Extended• uses
Question
State one use of a radioactive isotope.
Step-by-step solution
1. Step 1
  ${}^{60}\text{Co}$ in radiotherapy / sterilising medical equipment.
2. Step 2
  ${}^{14}\text{C}$ for radiocarbon dating (archaeology).
Answer
Examples: Co-60 in cancer therapy; C-14 in radiocarbon dating.

Key Formulae — Isotopes (New)

The formulae you need to memorise for isotopes (new) on the Cambridge IGCSE 0620 paper, with every variable defined in plain English and a note on when to use it.

Relative atomic mass from isotopes
$A_{r} = \dfrac{\Sigma (\text{abundance}\,\% \times \text{isotope mass})}{100}$
When to use
Several isotopes of the same element with known relative abundances.

Key Definitions and Keywords — Isotopes (New)

Definitions to memorise and the exact keywords mark schemes credit for isotopes (new) answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Isotope
Examiner keyword
Atoms of the same element (same $Z$ ) with different numbers of neutrons (different $A$ ).
Relative atomic mass ( $A_{r}$ )
Examiner keyword
Average mass of an atom, taking abundances of all isotopes into account, on a scale where ${}^{12}\text{C} = 12$ .
Radioactive isotope
Examiner keyword
An isotope with an unstable nucleus that emits radiation. Used in medicine, dating, and industry.

Common Mistakes and Misconceptions — Isotopes (New)

The traps other students keep falling into on isotopes (new) questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying isotopes are different elements
0620/42 — recurring
Why it happens
Different mass numbers.
How to avoid it
Same proton number → SAME element. Just different mass.
✕Saying isotopes have different chemical properties
Why it happens
Different masses suggests different behaviour.
How to avoid it
Chemistry depends on electrons, which are the same. So chemistry is identical.
✕Forgetting to divide by 100 (or the total abundance)
Why it happens
Speed.
How to avoid it
Always divide by total percentage (usually 100). The answer should be close to the mass numbers, never bigger.
✕Adding isotope masses without weighting
Why it happens
Simplifying.
How to avoid it
Use a WEIGHTED mean — multiply each mass by its abundance fraction.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

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Video lesson

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Isotopes (New) — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Isotopes (New)

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Identify isotopes

2Calculate relative atomic mass

3Chemical vs physical properties

4Use of a radioactive isotope

Relative atomic mass from isotopes

Isotope

Relative atomic mass (ArA_{r}Ar​)

Radioactive isotope

✕Saying isotopes are different elements

✕Saying isotopes have different chemical properties

✕Forgetting to divide by 100 (or the total abundance)

✕Adding isotope masses without weighting

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Isotopes (New) — frequently asked questions

Relative atomic mass ( $A_{r}$ )