22.2.2—The atomic mass of an element

Relative atomic mass is the weighted mean of an element’s isotopic masses: Ar = Σ(isotopic mass × percentage abundance)/100.

Use fractional or percentage abundances consistently and check that they sum to 100%. The result should lie between the lightest and heaviest isotope masses.

For 75% isotope 35 and 25% isotope 37, Ar = (35×75 + 37×25)/100 = 35.5.

Do not take a simple arithmetic mean unless abundances are equal, and do not confuse Ar with one isotope’s mass number.