This worked example covers Calculating Relative Atomic Mass (Ar) within Atomic Structure for GCSE Chemistry. Revise Atomic Structure in Atomic Structure for GCSE Chemistry with 28 exam-style questions and 22 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 8 of 14 in this topic. Treat this as a marking guide for what examiners are looking for, not just a fact list.
🧮 Calculating Relative Atomic Mass (Ar)
Formula: Ar = (mass of isotope × % abundance + mass of isotope × % abundance + …) ÷ 100
Worked example — Chlorine:
Chlorine has two isotopes: ³⁵Cl (abundance 75%) and ³⁷Cl (abundance 25%)
This is why the periodic table shows chlorine's relative atomic mass as 35.5 — it is a weighted average of both isotopes.
Practice questions for Atomic Structure
What does the atomic number of an element tell you?
Explain what is meant by the relative atomic mass of an element and how it is calculated from isotopic data. [3 marks]