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

Author

Stewart Hird

Expertise

Chemistry Lead

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

Exam 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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Author: Stewart Hird

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.