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Relative Atomic Mass (AQA GCSE Chemistry: Combined Science)
Revision Note
What is relative atomic mass?
- 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 of an element is an average value that takes account of the abundance of the isotopes of the element
- 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 to calculate relative atomic mass (Ar)
- 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
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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