# The element copper has naturally occurring isotopes with mass numbers of 63 and 65. The relative abundance and atomic masses are 69.2% for a mass of 62.93 amu and 30.8% for a mass of 64.93 amu. What is the average atomic mass of copper?

Dec 30, 2015

Take the weighted average of the individual isotopes.

#### Explanation:

""^63Cu has 69.2% abundance.

""^65Cu has 30.8% abundance.

So, the weighted average is 62.93xx69.2% $+$ 64.93xx30.8% $=$ $63.55$ $\text{amu}$.

If we look at the Periodic Table, copper metal (a mixture of isotopes but ""^63Cu and ""^65Cu predominate) has an approximate atomic mass of $63.55$ $g \cdot m o {l}^{-} 1$, so we know we are right.