Weigh a helium-4 atom very carefully and then weigh its parts separately. You will find that they do not add up.

Using atomic masses, which keep the electrons on both sides so they cancel:

• 2 hydrogen-1 atoms (2 protons + 2 electrons): 2 × 1.007825 u = 2.015650 u
• 2 neutrons: 2 × 1.008665 u = 2.017330 u
• Total: 4.032980 u
• Measured mass of one helium-4 atom: 4.002603 u

The difference, 0.030378 u, is the mass defect. The assembled nucleus is lighter than the pieces.