The mass of each proton is about 1.00728 microns, and the mass of each neutron is 1.00866 microns. Therefore, the sum of the masses of 2 protons and 2 neutrons will be 4.03188 u. Therefore, this must be the mass of one helium nucleus. But the mass of the helium nucleus is slightly less than about 4.00153 u. The difference between these two values is about 0.03035 u, which is called a mass defect. The mass defect can be used to calculate the binding energy for a helium nucleus using the Einstein relation.
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