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Quiz yourself on Atomic StructureGet your flashcardsBuild my revision planTrack my progress🧮 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%)
Ar = (35 × 75 + 37 × 25) ÷ 100 = (2625 + 925) ÷ 100 = 3550 ÷ 100 = 35.5
This is why the periodic table shows chlorine's relative atomic mass as 35.5 — it is a weighted average of both isotopes.
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