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

Revision Note

Written by: Stewart Hird, Chemistry Lead

Reviewed by: Lucy Kirkham, Head of STEM

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

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

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

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:

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

the (exam) results speak for themselves:

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