Explain why the atomic mass of copper is not exactly equal to 64, midway between the mass numbers of copper-6?
the atomic mass takes into account prevalence of naturally occurring isotopes. It is an average of all isotope mass numbers.
In nature, not all copper is the same. Copper has naturally occurring isotopes(different numbers of neutrons). So if its 99% of one isotope and 1% of another, and the average is taken, the atomic mass won't be a whole number.
Because Copper has isotopes. All Coper atoms have exactly the same number of protons, but not all of them have the same number of neutrons. The ones with different numbers of neutrons are known as isotopes. A given sample of Copper will contain various amounts of each of its isotopes. The atomic mass (weight) is an average weight, based on the percentages of the isotopes.
