How to Calculate Relative Atomic Mass (Edexcel IGCSE Chemistry (Modular))
Revision Note
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
So, if there were 3 isotopes present then the equation would read:
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 =
Relative Atomic Mass = 85.6
Examiner 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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