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QUESTION
What is a rule for determining the mass number of an atom?
Mass number is the sum of the protons and neutrons in the atomic nucleus.
The (##"A"##) is the sum of the (##"Z"##), which is the number of protons, and the number of neutrons (##"N"##) in the atomic nucleus of an isotope of an element. ##"A=Z+N"##
