Finding atomic m of Copper?

The atomic mes given in the periodic table are weighted averages of the isotopes that exist in nature. There is no single atom that weighs 63.5, but if you weigh a mole of a macroscopic quantity of copper it will weigh 63.5 because it will be a combination of the two isotopes. The average is based on the percentage of natural isotope times its atomic m. So you could say there is a greater percentage of Cu-63 than Cu-65 in nature.