The mass number of an atom is the sum of its protons and neutrons. Since the atomic number of oxygen is 8, it means an oxygen atom normally has 8 protons. If an oxygen atom has 7 neutrons, we can calculate its mass number as follows:
Mass number = Protons + Neutrons Mass number = 8 protons + 7 neutrons Mass number = 15
Therefore, an oxygen atom with 7 neutrons has a mass number of 15.