Cambridge O Level Chemistry · Syllabus 5070 · 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 O Level Chemistry.
Common mistakes with Mass number
- 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 Mass number
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.