Cambridge IGCSE Chemistry · Syllabus 0620 · Atoms, Elements and Compounds
Mass number
What is Mass number?
The total number of protons and neutrons in the nucleus of an atom, also called the nucleon number and given the symbol A. It is always a whole number and belongs to one specific atom, which distinguishes it from relative atomic mass, an abundance-weighted average that belongs to the element as a whole and is usually not a whole number.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge IGCSE Chemistry.
Common mistakes with Mass number
- 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.