Calculating Relative Atomic Mass from Isotopes

  • The size of atoms is so tiny that we can’t really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass (Ar) is used.
  • The relative atomic mass unit is equal to ½ the mass of a carbon-12 atom.
  • All other elements are measured by comparison to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units.
  • Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon.

Sub-atomic particle comparison - Mass, IGCSE & GCSE Chemistry revision notes

  • The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element.
  • The equation below is used where the top line of the equation can be extended to include the number of different isotopes of a particular element present.
  • So if there were 3 isotopes present then the top line of the equation would read:
    (% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B) + (% of isotope C x mass of isotope C)


Ar Formula, IGCSE & GCSE Chemistry revision notes


The table shows information about the Isotopes in a sample of rubidium with 72% 85Rb and 28% 87Rb

Isotopes in sample of rubidium table, IGCSE & GCSE Biology revision notes

Use information from the table to calculate the relative atomic mass of this sample of Rubidium. Give your answer to one decimal place:

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.