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Syllabus
Syllabus
- The relative atomic mass of an element is an average value that takes account of the abundance of the isotopes of the element.
- Students should be able to calculate the relative atomic mass of an element given the percentage abundance of its isotopes.
What does this mean?
What does this mean?
There are three isotopes of Hydrogen, with relative masses of 1, 2 and 3.
On the Periodic Table, the Relative Mass of Hydrogen is given as 1.01
But the average of 1,2 and 3 is 2.
The reason for this is that almost all Hydrogen atoms are 1H.
So the relative mass of the element takes into account the abundance of the isotopes -how common they are.
We calculate the relative mass by:
- Multiplying the % abundance of each isotope by its mass.
- Adding them all together and dividing by 100.
(99.98 x 1) + (0.02 x 2) + ( almost 0 x 3 ) /100 = 1.01
Given the abundances of isotopes for an element we should be able to work out its relative atomic mass.
Example Questions
Example Questions
Example 1
Example 1
Note!
It's a good idea to estimate a sensible answer before starting.
The answer couldn't possibly be below 20, because that's the lightest isotope.
The answer couldn't possibly be above 22, because that's the heaviest isotope.
And it had to be close to 20, because that was the most common isotope.
Example 2
Example 2
Estimated Answer
Estimated Answer
The answer couldn't possibly be below 35, because that's the lightest isotope.
The answer couldn't possibly be above 37, because that's the heaviest isotope.
And it had to be a little closer to 35, because that was the most common isotope.
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