Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level Chemistry.
Common mistakes with Relative atomic mass
- 2. “Mass number and relative atomic mass are the same thing.” 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.
Questions students ask about Relative atomic mass
What is the difference between mass number and relative atomic mass?
Mass number (nucleon number) belongs to one atom: it is the total number of protons and neutrons in that nucleus, so it is always a whole number. Relative atomic mass belongs to the element as a whole: it is the mean mass of its atoms, weighted by the abundance of each isotope, so it is usually not a whole number. Chlorine has atoms of mass number 35 and 37, but a relative atomic mass of 35.5 because the two isotopes occur in different proportions.