Multiple choiceQuestion 1 · Recall
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By the end you can define relative atomic mass, , explain why it's a weighted mean, and calculate it from isotope data (or work backwards to an abundance).
Look up chlorine in the Data Booklet: . Now find a chlorine atom with a mass of 35.5 u. You can't. There isn't one.
Natural chlorine is a mixture of two isotopes: about three-quarters and one quarter . The 35.5 is the average atom, weighted by how common each isotope is.
Relative atomic mass, Ar
The weighted mean mass of an atom of an element compared with one twelfth of the mass of one atom of carbon-12.
In terms of the unified atomic mass unit:
"Weighted" means each isotope counts in proportion to its abundance.
Imagine a jar of 100 coins: 76 weigh 35 g and 24 weigh 37 g. The mean mass of a coin isn't 36 g, because most coins are the light ones. It's a bit over 35 g.
Isotopes work the same way. Where the analogy breaks: real relative isotopic masses aren't neat whole numbers ( is 34.97), and there are rather more than 100 atoms in the jar.