Answer: 15
Step-by-step explanation:
The mass of the ion is the sum of the protons and neutrons inside its nucleus.
i.e Mass number = protons + neutrons
Mass number = 8 protons + 7 neutrons
= 15
Furthermore, since the number of protons differ from the number of electrons by 2, the ion was formed when its atom gained 2 electrons. Thus, it is a negatively charged ion (anion) with a mass of 15