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IGCSEChemistryEdexcelChemistry (Modular)Unit 1Revision Notes1. Principles of Chemistry: Part 1Atomic structureHow to Calculate Relative Atomic Mass

How to Calculate Relative Atomic Mass (Edexcel IGCSE Chemistry (Modular))

Revision Note

Stewart Hird

Written by: Stewart Hird

Reviewed by: Lucy Kirkham

Edexcel Chemistry (Modular): Unit 1Edexcel Science (Double Award): ChemistryEdexcel Chemistry

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

  • 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 Tips and Tricks

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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    Previous:Atoms: Definitions & StructureNext:Periodic Table: Basics
    Author
    Reviewer
    Stewart Hird

    Author: Stewart Hird

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

    Lucy Kirkham

    Author: Lucy Kirkham

    Expertise: Head of STEM

    Lucy has been a passionate Maths teacher for over 12 years, teaching maths across the UK and abroad helping to engage, interest and develop confidence in the subject at all levels.Working as a Head of Department and then Director of Maths, Lucy has advised schools and academy trusts in both Scotland and the East Midlands, where her role was to support and coach teachers to improve Maths teaching for all.