Who this is for: Cambridge IGCSE Chemistry (0620) students who confuse isotopes with ions — or lose marks calculating relative atomic mass from isotope abundances.
What query it owns: how to understand and revise isotopes in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the isotopes revision-guide angle, while Tutopiya’s Isotopes subtopic page owns the learning resource and the free Isotopes quiz owns the practice.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons — so they have the same atomic number but different mass numbers. Isotopes explain why relative atomic masses on the periodic table are not always whole numbers (e.g. chlorine ≈ 35.5). This guide covers definitions, nuclear symbols, and the relative atomic mass calculation examiners expect.

Key takeaways

• Isotopes — same proton number, different neutron number; same element, different mass number.
• Chemical properties are identical (same electrons); physical properties (e.g. mass) can differ.
• Relative atomic mass = weighted average of isotope masses by natural abundance.
• Do not confuse isotopes (different neutrons) with ions (different electrons).
• Confirm with the Isotopes quiz.

What are isotopes in Cambridge IGCSE Chemistry?

Isotopes are atoms of the same element that contain the same number of protons in the nucleus but different numbers of neutrons. Because proton number defines the element, isotopes are the same element with different mass numbers. Cambridge IGCSE Chemistry (0620) tests definitions, particle counts, and relative atomic mass calculations.

Full notes are on Tutopiya’s Isotopes subtopic page.

Isotopes vs ions — comparison table

Feature	Isotopes	Ions
What differs	Neutron number	Electron number
Proton number	Same	Same
Element identity	Same element	Same element
Charge	Neutral atoms	Charged
Example	³⁵Cl and ³⁷Cl	Na⁺ and Na

How to calculate relative atomic mass — step by step

1. List each isotope with its mass number and percentage abundance.
2. Multiply mass × abundance for each isotope.
3. Add the products.
4. Divide by 100 (if abundances are percentages).
5. Check with the free Isotopes quiz.

Past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical stem
Define	Precise definition	”Define isotopes.”
State	Short fact	”State what is the same in all isotopes of an element.”
Calculate	Numerical answer with working	”Calculate the relative atomic mass of chlorine.”
Explain	Why Ar is not a whole number	”Explain why the relative atomic mass of chlorine is 35.5.”

Worked exam-style stems (how to answer the wording)

1. “Define isotopes.” Target: atoms of the same element with the same number of protons but different numbers of neutrons. Define = both proton sameness and neutron difference.
2. “Chlorine has two isotopes: ³⁵Cl (75%) and ³⁷Cl (25%). Calculate the relative atomic mass.” (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 35.5. Calculate = show full working.
3. “Explain why isotopes of the same element have identical chemical properties.” Target: same number of electrons (same electron arrangement) in neutral atoms; chemical behaviour depends on electrons, not neutrons.

Practise more on the Atoms topical past paper questions and review Atomic Structure if particle counts are weak.

Common mistakes students make

• Confusing isotopes with different elements.
• Using mass number directly as relative atomic mass without abundance weighting.
• Saying isotopes have different numbers of protons — they do not.
• Forgetting to divide by 100 after summing percentage products.
• Skipping the Isotopes quiz.

When you need more support

If relative atomic mass calculations keep failing, drill the topical past paper questions and book a Cambridge IGCSE Chemistry tutor. The Cambridge IGCSE Chemistry hub links all Atoms subtopics.

Frequently asked questions

Do isotopes have different chemical properties? No — chemical properties depend on electron arrangement; isotopes of an element have the same proton and electron count (in neutral atoms).

Why is chlorine’s relative atomic mass 35.5? It is the weighted average of ³⁵Cl and ³⁷Cl based on natural abundances, not a single isotope mass.

What is the difference between mass number and relative atomic mass? Mass number applies to one isotope (protons + neutrons); relative atomic mass is the weighted average of all isotopes.

How do I revise isotopes effectively? Read Isotopes notes, practise define/calculate stems, take the Isotopes quiz, then link to ion formation.

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