Calculating Relative Atomic Mass (Ar)
Part of Atomic Structure — GCSE Chemistry
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 25 exam-style questions and 22 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. 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.
Topic position
Section 8 of 14
Practice
25 questions
Recall
22 flashcards
🧮 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.