Science · Grades 9–12
Model isotope averages
The given masses and abundances describe a hypothetical element; fractions sum to one.
The key idea
The given masses and abundances describe a hypothetical element; fractions sum to one.
average mass = Σ(fraction × isotope mass)
Worked examples
A hypothetical element has isotopes of mass 60 u (90%) and 63 u (10%). Calculate its mean atomic mass in u.
60 × 0.9 + 63 × 0.1 = 60.3
60.3 u
A hypothetical element has isotopes of mass 26 u (60%) and 28 u (40%). Calculate its mean atomic mass in u.
26 × 0.6 + 28 × 0.4 = 26.8
26.8 u
Try it yourself
A hypothetical element has isotopes of mass 80 u (15%) and 83 u (85%). Calculate its mean atomic mass in u.
A hypothetical element has isotopes of mass 50 u (5%) and 53 u (95%). Calculate its mean atomic mass in u.
Check your understanding
Answers and marking notes
- 82.55 u
80 × 0.15 + 83 × 0.85 = 82.55
- 52.85 u
50 × 0.05 + 53 × 0.95 = 52.85
Further reading and review references
These are reference links, not copied exercises, institutional endorsement or proof of independent review.
Learning preview · teacher review required. Grade bands are approximate, not official curriculum alignment.