Final answer:
The mass number of an oxygen atom is determined by the total number of protons and neutrons in its nucleus, with oxygen-16 having 8 protons and 8 neutrons for a total mass number of 16.
Explanation:
The mass number of an oxygen atom, represented as ^16O, is 16 because it is the sum of the number of protons and neutrons in the nucleus of the atom. In this case, oxygen-16 (^16O) has 8 protons and 8 neutrons, totaling 16 nucleons.
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