The idea
Isotopes have the same atomic number but different mass numbers. Their chemical behaviour is broadly similar because neutral atoms have the same electron arrangement. Calculate a weighted mean by multiplying each isotopic mass by its fractional abundance and adding the results. Relative atomic mass is a ratio and has no unit; mass number is a count for one isotope.
Worked example
A sample contains 75% atoms with isotopic mass 35 and 25% with isotopic mass 37. Find its relative atomic mass using these values.
- Convert percentages to fractions: 0.75 and 0.25.
- Weighted mean = (35 × 0.75) + (37 × 0.25).
- Relative atomic mass = 35.5.
A mistake to avoid
Taking the simple average gives the correct result only when the two isotopes are equally abundant. Real isotope masses may also differ slightly from integer mass numbers.
Check your understanding
What differs between carbon-12 and carbon-14?
Show the explanation
Both have 6 protons. Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons.
Explore the lesson
This guide connects to Same Element, Different Atom — Isotopes in the IB Chemistry course. The first two published lessons per subject are free previews; further interactive lessons require an account.