IGCSEChemistryEdexcelRevision Notes1. Principles of ChemistryAtomic StructureCalculate Relative Atomic Mass

Calculate Relative Atomic Mass (Edexcel IGCSE Chemistry)

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Edexcel ChemistryEdexcel Chemistry (Modular): Unit 1

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Calculating relative atomic mass

  • The relative atomic mass of each element is calculated from the mass number and relative abundances of all the isotopes of a particular element

How to work out relative atomic mass

  • 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

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

  • 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

Isotope Mass Number Percentage abundance
1 85 72
2 87 28

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 = fraction numerator open parentheses 72 cross times 85 close parentheses plus open parentheses 28 cross times 87 close parentheses over denominator 100 end fraction
  • Relative Atomic Mass = 85.6

Examiner Tip

Isotopes are easy to recognise 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

    Author: Stewart

    Expertise: Chemistry Lead

    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 Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.