Naturally occurring copper exists as two isotopes, $^{63}Cu$ and $^{65}Cu$ - VCE - SSCE Chemistry - Question 16 - 2004 - Paper 1

To determine the ratio of the copper isotopes, we can use the concept of the weighted average based on their relative atomic masses.

Given that the average atomic mass of copper is 63.6, we can set the molecular weights for the isotopes as follows:

• Let the abundance of $^{63}Cu$ be $x$.
• Hence, the abundance of $^{65}Cu$ will be $1 - x$.

The average atomic mass can be expressed by the equation:

$ext{Average mass} = 63.6 = 63x + 65(1 - x)$

Solving for $x$:

$63.6 = 63x + 65 - 65x$ $63.6 = 65 - 2x$ $2x = 65 - 63.6$ $2x = 1.4$ $x = 0.7$

Therefore, the abundance of $^{63}Cu$ is 0.7 (or 70%) and the abundance of $^{65}Cu$ is $1 - 0.7 = 0.3$ (or 30%).

To express this as a ratio, we write:

$\text{Ratio} = \frac{0.7}{0.3} = \frac{7}{3}$

This simplifies to approximately 3:1. Hence, the correct answer is D. 3:1.