Isotopes and Average Atomic Mass

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Multiple Choice

Gallium has two stable isotopes. The mass of gallium-69 is 68.926 amu and has an abundance of 60.11%. The mass of gallium-71 is 70.925 amu and has an abundance of 39.89%. Which is the average atomic mass of gallium?

• 69.93 amu

• 69.72 amu

• 292.47 amu

• 139.85 amu

Multiple Choice

Bromine has two stable isotopes. With its average atomic mass on the periodic table being 79.90 amu, which isotope is more abundant?

• Bromine-79

• Bromine-81