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As you look through the periodic table, you may have noticed that not all atomic masses are written as whole numbers.
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Elements can exist with slightly different numbers of neutrons.
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We call these isotopes of an element.
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It is particularly common for heavier elements, but a familiar lighter example is chlorine.
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The two most common types of chlorine atoms in the world around us are chlorine 35 and chlorine 37.
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If we took a sample of chlorine gas, we would find that it was made up of approximately 75% chlorine 35,
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and 25% of chlorine 37.
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There are other isotopes of chlorine, but these two are the most common.
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The relative abundance of the different isotopes,
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in other words, how common they are is always taken into consideration when calculating the relative atomic mass of an element.
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One way of visualizing this is to imagine that you have a sample of 100 naturally occurring chlorine atoms.
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Based on the abundance in the example already given, 75 of these would be chlorine 35 and 25 would be chlorine 37.
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Knowing this, we can work out the relative atomic mass.
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In other words, the weighted mean atomic mass.
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And we can do this using a formula as follows
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75 times 35
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plus 25 times 37 all divided by 100
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equals 35.5.
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And this explains why chlorine is listed as having a relative atomic mass of 35.5 in the periodic table.
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The relative atomic mass of an element is the weighted mean mass of an element and it is written in atomic mass units.
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Here's another example for you.
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A sample of bromine contains 50% bromine 79, and 50% bromine 81.
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What is its relative atomic mass?
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Pause the video. You may be able to work this out straight away.
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If not, write it down, as in the chlorine example.
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The answer is 80.
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Detailed periodic tables of elements showed that most relative atomic masses aren't actually whole numbers.
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This is all due to the naturally occurring isotopes of each element.
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So to recap, the relative atomic mass of an element is the weighted mean mass of the isotopes of an element.
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And by weighted we mean that we are taking the relative abundance of each isotope into consideration.
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And this is why relative atomic masses aren't always whole numbers