IB Chemistry SL S1.2 The nuclear atom questions

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Question 1

Multiple choiceHard1 mark

An enriched copper sample contains only $^{63}_{29}\mathrm{Cu}$ and $^{65}_{29}\mathrm{Cu}$ Treating the mass numbers as isotope masses, its relative atomic mass is 63.60. Which description gives both their abundances in the sample and the subatomic-particle counts in the corresponding $\mathrm{Cu}^{2+}$ ions?

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Answer: B. The abundance-weighted mean gives $63x+65(1-x)=63.60$ so the fraction of $^{63}\mathrm{Cu}$ is $x=0.70$ Both isotopes have 29 protons; their neutron numbers are $63-29=34$ and $65-29=36$ Each $2+$ ion has lost two electrons, leaving 27.

A: reverses the abundances, which would give a mean mass of 64.40.
C: treats the $2+$ charge as two extra electrons rather than two fewer.
D: reverses the subtraction for neutron number, assigning more neutrons to the lower-mass isotope.
Remember: Isotopes differ in neutron number; ion charge reflects electron gain or loss.

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