Suppose that there were two natural isotopes of Copper. 80% of the atoms had a mass of 63, and 20% of the atoms had a mass of 65. What would that average atomic weight of copper be?

The isotope with atomic mass 63u has mass percentage = 80%

the isotope with atomic mass 65u has mass percentage = 20%

average atomic mass = 63 x 80/100 + 65 x 20/100

= 50.4 + 13

= 63.4u

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