GCSE Periodic Tables have integer (whole number) Mass Numbers for most elements except Copper, Chlorine and Bromine. These elements have Mass Numbers that are not integer values because they have two or more common isotopes and a weighted mean is taken (to take into account how abundant each isotope is).
At A level, many more elements will have non-integer Mass Numbers to better refelect reality. For this reason text books will distinguish between Relative Atomic Mass and Relative Isotopic Mass and it is important to distinguish between them. Relative Atomic Mass is shown on Periodic tables and includes all the isotopes. Relative Isotopic Mass is the Mass Number of one particular element.
eg. There are two stable isotopes of Chlorine ( 35Cl (76%) and 37Cl (24%) ) *
- The Relative Isotopic Mass of 35Cl is 35
- The Relative Isotopic Mass of 37Cl is 37
- The Relative Atomic Mass of Cl = (35 x 76/100) + (37 x 24/100) = 35.5
You may be required to read the abundance from a Mass Spectrometry graph.
If the abundance is not given as a percentage the Maths remains much the same.
eg. Boron has two isotopes 10B and 11B in a ratio of 1:4
Relative Atomic Mass = (10 x 1/5) + (11 x 4/5) = 10.8
(The 5 coming from the sum of the 1 and 4 from the ratio)