What Causes the Mass Defect in a 15N Nucleus?

[pveclipse96]

What is the mass defect of the 15N nucleus?

My attempt:

15-7=8
7(1.008665)+8(1.0078225)-14.003074
=1.120161

the answer needs to be in u so i don't know if this makes a difference or not but for some reason it is not working

 

[alxm]

Why are you using the number 14.003074? That's the mass of 14N. The mass defect is the real mass (of 15N) minus the mass of the constituent neutrons and protons.