The atomic mass of copper is 63.546 amu. Which of copper's two isotopes is more abundant: copper-63 or copper-65?

Copper-63

Explanation:

Since atomic masses are averages of a certain element's mass from all of its isotopes, the average of the atomic mass will be closest to the most abundant isotope. Knowing that the mass of copper-63 is 63 amu and the average atomic mass of copper is 63.546, its most abundant isotope must be copper-63

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the atomic number will stay the same no matter how many electrons it has, so it doesn’t decrease nor increase. it’ll only change with a different number of protons