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Relative Atomic Mass (AQA GCSE Chemistry)

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Stewart Hird

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What is relative atomic mass?

  • Atoms are so tiny that we cannot really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass (Ar) is used

  • The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element

  • The relative atomic mass unit is equal to 1/12th the mass of a carbon-12 atom

  • All other elements are measured by comparison to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units

  • For example, hydrogen has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon

How to calculate relative atomic mass (Ar)

  • 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

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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    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.