Copper has two naturally occurring isotopes: copper-63 and copper-65. Explain why the relative atomic mass of copper is not a whole number.

Copper atoms exist as two naturally occurring isotopes, copper‑63 and copper‑65, which have the same number of protons but different numbers of neutrons. The mass numbers of the two isotopes are 63 and 65, so their masses differ by two atomic mass units. The relative atomic mass of an element is calculated as a weighted average of the masses of all its naturally occurring isotopes, with the weighting factor being the relative abundance of each isotope. Because the average is taken over isotopes that have different masses, the resulting value is not a whole number, giving copper a relative atomic mass of 63.5.