MediumChemistryHigh School (9–10)GeneralStructure of the Atom › Isotopes and IsobarsNumericalBoard Exam A-Level IB

Copper has two isotopes, with mass numbers 63 and 65…

Try it on paper first. A hint, the key idea and the worked solution are here when you need them.

Copper has two isotopes, with mass numbers $63$ and $65$ . A careful measurement gives the average atomic mass of copper as $63.55\ \text{u}$ . Find the percentage of each isotope.

Need a reminder? The key ideaShow

The atomic mass of an element is the average of its isotopes' masses, weighted by how common each one is: $\text{average} = \dfrac{\sum (\text{mass number} \times \text{percentage})}{100}$ . When the average is known, let $x$ be one percentage and solve for it.

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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