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

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

$A_{r} = \frac{(% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B)}{100}$

So, if there were 3 isotopes present then the equation would read:

$\frac{(% of isotope A x mass of isotope A) + (% of isotope B x mass of isotope B) + (% of isotope C x mass of isotope C)}{100}$

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 = $\frac{(72 \times 85) + (28 \times 87)}{100}$
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

Last updated: 27 August 2024

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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.

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.