Answer:
18
Step-by-step explanation:
The mass of an isotope is the sum of all the nucleons in that isotope. Nucleons are the protons and neutrons found in the nucleus of that atom.
So the mass = 11 + 7 = 18
It is worth noting that even though isotopes have electrons, we do not ad them them to the mass because their mass is so small and insignificant that we normally ignore it in calculations