GCSE Chemistry Isotopes & Relative Atomic Mass Notes
Isotopes are atoms of the same element with different numbers of neutrons. Their average mass, weighted by abundance, is the relative atomic mass on the periodic table.
Key concepts
- Isotope
- Atoms of the same element with the same protons but different neutrons, so different mass numbers.
- Relative atomic mass (Ar)
- The weighted mean mass of an element's isotopes compared to 1/12 of carbon-12.
- Abundance
- The percentage of each isotope naturally present in a sample of the element.
- Isotopes have identical chemistry because they have the same electron configuration.
- Ar values on the periodic table are rarely whole numbers because they are weighted averages.
- Chlorine's Ar of 35.5 reflects roughly 75% ³⁵Cl and 25% ³⁷Cl.
Key formulas
Relative atomic mass
Ar = Σ(isotope mass × %abundance) ÷ 100
Worked example
Chlorine has two isotopes: ³⁵Cl (75%) and ³⁷Cl (25%). Calculate its relative atomic mass.
- 1Multiply each isotope mass by its abundance: (35 × 75) + (37 × 25).
- 2= 2625 + 925 = 3550.
- 3Divide by 100: 3550 ÷ 100 = 35.5.
Answer: Ar(Cl) = 35.5
Common mistakes
✗ Adding masses and dividing by 2 instead of weighting by abundance.
✓ Multiply each mass by its % abundance first, then divide by 100.
✗ Saying isotopes react differently.
✓ They have identical electron configurations so their chemistry is the same.
✗ Confusing atomic number with mass number when identifying isotopes.
✓ Isotopes share the atomic number; only the mass number changes.
Quick check
Try these, then reveal the answers.
Isotopes of an element differ in what?Reveal answer
The number of neutrons (and therefore mass number).
Boron has ¹⁰B (20%) and ¹¹B (80%). Find Ar.Reveal answer
(10×20 + 11×80)/100 = 10.8
Do isotopes react differently?Reveal answer
No — same electrons, same chemistry.
Exam tips
- Always show the '× abundance ÷ 100' step for full marks.
- Give Ar to the same precision as the data (usually 1 dp).
- State 'weighted mean mass' when defining Ar.
Related practice
Practise this topic
Exam-style questions with instant Gradion feedback.
Flashcards
Quick-fire recall to lock in the key facts.
Worksheets
Printable practice you can work through offline.
Past papers
Full papers with examiner-grade marking.
Gradion
Ask for a step-by-step explanation any time.
All Chemistry
Every Chemistry topic across every exam board.
Related topics
Frequently asked questions
Why is chlorine's Ar 35.5?+
It is a weighted average of its two isotopes, ³⁵Cl and ³⁷Cl, in the ratio present in nature.
Do isotopes have the same chemical properties?+
Yes — chemistry depends on electrons, which are identical in isotopes.
Create a free account to practise isotopes & relative atomic mass with Gradion feedback
Try 5 questions now, get instant examiner-grade marking and ask the Gradion to explain anything. Free during our early launch period — no credit card required.