How Do You Calculate the Mass Defect of O-15?

[atomicpedals]

Homework Statement

Find the mass defect of O-15.

2. The attempt at a solution

I've see a couple different suggested ways for computing the mass defect and I want to know if I'm doing this correctly.

Mass of O-15 = 13,975 MeV/c^2
Proton mass = 938.3 MeV/c^2
Neutron mass = 939.6 MeV/c^2

D = (8Mp + 7Mn - M) = (8(938.3) + 7(939.6) - (13,975)) = 108.6 MeV/c^2

 

[dikmikkel]

You probably need to add 8 electron masses also, it means something at this scale unless the Mass of O-15 is without electrons.