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?

Take the weighted average of the individual .

##""^63Cu## has ##69.2%## abundance.

##""^65Cu## has ##30.8%## abundance.

So, the weighted average is ##62.93xx69.2%## ##+## ##64.93xx30.8%## ##=## ##63.55## ##"amu"##.

If we look at , copper metal (a mixture of isotopes but ##""^63Cu## and ##""^65Cu## predominate) has an approximate of ##63.55## ##g*mol^-1##, so we know we are right.