# Chapter 4 - The Structure of Atoms - Exercises - Atomic Composition, Isotopes, and Atomic Weights - Page 165: 32

% abundance of $^{63}Cu$ in naturally occurring copper is $69.16\%$

#### Work Step by Step

Atomic mass of Copper $=63.546amu$ Mass of $^{63}Cu = 62.9298amu$ Mass of $^{65}Cu = 64.9278amu$ Let the fraction of $^{63}Cu = x$ The fraction of $^{65}Cu = (1-x)$ The atomic mass of an element is the weighted average of the masses of the constituent isotopes. Therefore, Atomic mass of Copper $= x(62.9298amu)+(1-x)(64.9278amu)$ $63.546=62.9298x+64.9278-64.9278x$ $-1.3818=-1.998x$ $x=0.6916$ The fraction of $^{63}Cu = 0.6916$ % abundance of $^{63}Cu = 69.16\%$

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