Relative Atomic Mass – IGCSE Chemistry Definition
IGCSE Chemistry definition of relative atomic mass (Ar): the weighted average mass of an atom compared to 1/12 of carbon-12. Covers isotope calculations.
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.
Relative atomic mass (Ar) is one of the most used values in IGCSE Chemistry. It appears on the periodic table for every element and is the starting point for almost every mole calculation. The 0620 syllabus requires you to define Ar, explain why it can be a decimal, and calculate it from isotope abundance data (Supplement).
The 0620 definition
The relative atomic mass (Ar) of an element is the weighted average mass of the atoms of the element, compared to 1/12 of the mass of an atom of carbon-12.
Key features:
- It is a weighted average, taking into account the relative abundance of each isotope
- It is a ratio, so it has no units
- It is found on the periodic table (usually not a whole number)
Why Ar is not always a whole number
Elements exist as mixtures of isotopes. Since isotopes have different masses but occur in different proportions, the average is not necessarily a whole number. The periodic table shows this average.
Calculating Ar from isotope data
Ar = sum of (isotope mass x percentage abundance) / 100
Example: chlorine
| Isotope | Mass | Abundance |
|---|---|---|
| Cl-35 | 35 | 75% |
| Cl-37 | 37 | 25% |
Ar = (35 x 75 + 37 x 25) / 100 = (2625 + 925) / 100 = 35.5
Example: copper
| Isotope | Mass | Abundance |
|---|---|---|
| Cu-63 | 63 | 69% |
| Cu-65 | 65 | 31% |
Ar = (63 x 69 + 65 x 31) / 100 = (4347 + 2015) / 100 = 63.6
Worked exam question
Silicon has three isotopes: Si-28 (92%), Si-29 (5%), and Si-30 (3%). (a) Define relative atomic mass. (2) (b) Calculate the relative atomic mass of silicon. (2)
Mark scheme
(a) The weighted average mass of the atoms of an element [1]; compared to / relative to 1/12 of the mass of an atom of carbon-12 [1]
(b) Ar = (28 x 92 + 29 x 5 + 30 x 3) / 100 [1]; = (2576 + 145 + 90) / 100 = 28.1 [1]
Common exam mistakes
- Defining Ar as “the mass of an atom” without the comparison to carbon-12. The definition must include “compared to 1/12 of carbon-12.”
- Forgetting the word “weighted” in the definition. A simple average of 35 and 37 gives 36, not 35.5. The weighting by abundance is essential.
- Giving Ar with units. Relative atomic mass has no units — it is a dimensionless ratio.
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