mass defect is defined as the difference or discrepancy between the mass of the nucleons and the mass of the nucleus. the nucleons involved are the protons and neutrons.
mp = 1.6727 x 10-24 g = 1.007316 amumn = 1.6750 x 10-24 g = 1.008701 amu
mTh 234 = 234 g/mol = 234 amu
Th234 (protons = 90; neutrons = 144)
mass defect = mp*90 + mn *144 - 234
mass defect = 1.911384 amu