The mass number of an atom is the sum of its protons and neutrons. Since oxygen (O) has an atomic number of 8, it means it normally has 8 protons. If an oxygen atom has 7 neutrons, we can calculate its mass number by adding the number of protons and neutrons:
Mass number = Number of protons + Number of neutrons
In this case:
Mass number = 8 protons + 7 neutrons = 15
Therefore, the mass number of an oxygen atom with 7 neutrons is 15.