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

Calculating Relative Atomic Mass

Isotopes:

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

Equation:

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

Ar =   _______________________________________________________________

                                                                     100

Example:

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

                         100

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.