If you measure protons and neutrons and electrons in atomic mass units the weights are:
protons = 1.007277
neutrons = 1.008665
electrons = 0.000548
Normally atoms are less than the sum of their parts because of binding energy in the nucleus. Hydrogen has no binding energy in the nucleus so it should be the sum of its parts. Its actually higher at 1.00794 (1.007277 + 0.000548 = 1.007825)
Why is this?