IGCSEChemistryEdexcelRevision Notes1. Principles of Chemistry1.3 Atomic Structure1.3.2 Calculate Relative Atomic Mass

Calculate Relative Atomic Mass (Edexcel IGCSE Chemistry)

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Calculate Relative Atomic Mass

How to Calculate Relative Atomic Mass

  • The relative atomic mass of each element is calculated using the mass number and the relative abundances of all the isotopes of a particular element
  • To calculate the relative atomic mass: 
    • Multiply the % abundance of each isotope by its mass 
    • Add these numbers together 
    • Divide by the total abundance (which when using % abundance, will be 100%) 
  • Here it is as an equation: 

A subscript straight r equals fraction numerator left parenthesis percent sign space of space isotope space straight A space straight x space mass space of space isotope space straight A right parenthesis space plus space left parenthesis percent sign space of space isotope space straight B space straight x space mass space of space isotope space straight B right parenthesis over denominator 100 end fraction

  • 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 equation would read:

fraction numerator left parenthesis percent sign space of space isotope space straight A space straight x space mass space of space isotope space straight A right parenthesis space plus space left parenthesis percent sign space of space isotope space straight B space straight x space mass space of space isotope space straight B right parenthesis space plus space left parenthesis percent sign space of space isotope space straight C space straight x space mass space of space isotope space straight C right parenthesis over denominator 100 end fraction

Worked example

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

Isotopes of rubidium, downloadable IGCSE & GCSE Chemistry 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.

Answer:

Relative Atomic Mass = 85.6

Exam Tip

Isotopes are easy to recognize from their notation as they have the same symbol but different mass numbers. For example, the two stable isotopes of copper are 63Cu and 65Cu

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    Stewart

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    Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.