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GCSEChemistryWJECRevision Notes1. Chemical Substances, Reactions & Essential Resources1.2 Atomic Structure & The Periodic TableRelative Atomic Mass

Relative Atomic Mass (WJEC GCSE Chemistry)

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Alexandra Brennan

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Relative Atomic Mass

Higher Tier

  • 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 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 do I calculate 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

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

You will be provided with the equation to calculate relative atomic mass in an exam so do not need to be able to recall it. 

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    Alexandra Brennan

    Author: Alexandra Brennan

    Expertise: Chemistry

    Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.