Final answer:
The mass number of an oxygen atom represented as 16/8 O is 16 because it has 8 protons and 8 neutrons, totaling 16 nucleons in the nucleus.
Step-by-step explanation:
An oxygen atom can be represented as 16/8 O, where 8 is the atomic number indicating the number of protons, and 16 is the mass number. The mass number is the total number of protons and neutrons in the nucleus of an atom.
For oxygen, with an atomic number of 8, it always has 8 protons.
The most common isotope of oxygen, oxygen-16, has 8 neutrons in addition to the 8 protons, totaling a mass number of 16. Hence, oxygen is represented as 16/8 to reflect its mass number of 16 due to its composition of protons and neutrons.