The atomic number of an element represents the number of protons in the nucleus of an atom. In the case of oxygen, with an atomic number of 8, it means there are 8 protons in the nucleus.
The mass number of an atom, on the other hand, represents the total number of protons and neutrons in the nucleus. To find the mass number of an oxygen atom with 7 neutrons, we need to add the number of protons (8) and neutrons (7).
Therefore, the mass number of an oxygen atom with 7 neutrons is 8 + 7 = 15.