Cambridge IGCSE Chemistry · Syllabus 0620 · Atoms, Elements and Compounds
Relative atomic mass
What is Relative atomic mass?
The relative atomic mass of an element is the average mass of the isotopes of that element, weighted by the abundance of each isotope, compared with one twelfth of the mass of an atom of carbon-12. It is calculated by multiplying each isotopic mass by its percentage abundance, adding the results and dividing by the total abundance, normally 100. Because it is an average across isotopes it is usually not a whole number, and because it is a ratio of masses it has no unit.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge IGCSE Chemistry.
Common mistakes with Relative atomic mass
- 2. “Mass number and relative atomic mass are the same thing.” Supplement 2.3.4 Repair Mass number belongs to one atom and is always a whole number. Relative atomic mass belongs to the element and is an abundance-weighted mean, so it is usually not a whole number. Taught fully in Lesson 2.3B, which is a Supplement — Extended candidates section. Core candidates still need to know that the two quantities are different (mass number is Core 2.2.4); only the calculation is Supplement.