Answer
The second naturally occurring isotope of $Cu$ is $^{63} Cu$.
Work Step by Step
Looking on the periodic table, we can see that the weight for copper is equal to "63.5".
If the isotopes are $^{63} Cu$. and $^{65} Cu$, the average weight for copper is between 63 and 65. If the isotopes are $^{65} Cu$ and $^{66} Cu$, that will be between 65 and 66.
Since the periodic table weight is "63.5", which is between 63 and 65, the isotopes are $^{63} Cu$. and $^{65} Cu$.
