Specification Point 1.17:
Be able to calculate the relative atomic mass of an element (Ar) from isotopic abundances

Calculating Relative Atomic Mass


Atoms of the same element that contains the same number of protons and electrons but different number of neutrons.


        (% of isotope 1 x mass of isotope 1) + (% of isotope 2 x mass of isotope 2)

Ar =   _______________________________________________________________



The table shows information about the isotopes in a sample of Rubidium

Isotope Number of protons Number of neutrons Percentage of isotope in sample
1 37 48 72%
2 37 50 28%

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

         ( 72   x   85 )   +   ( 28   x   87 )

    ____________________________     =   85.6


Relative atomic mass = 85.6

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Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.