What is the mass defect (in amu) for the 5828Ni nucleus? neutrons = 1.008665 amu and proton = 1.00728 amu. Hint: 58Ni isotopic mass = 57.935342907 amu. Mass defect = total nucleon mass − nuclear mass. total nucleon mass = sum of masses of neutrons and protons. nuclear mass = isotopic mass - summed mass of electrons.