Bound nucleons lose weight
The English physicist Francis Aston was the first to measure the mass of nuclei. Dividing this mass by the number of nucleons present allows for a calculation of the average mass of a bound nucleon. Aston’s curve shows that belonging to a nucleus makes a nucleon lose just under 1% of its mass. This loss, when multiplied by the speed of light squared, comes out an energy loss which can surpass 8 million electronvolts. This is the energy that on average needs to be spent to rip a nucleon away from the nucleus: the binding energy. The proportionality between mass loss and binding energy is general: in the case of a nucleus, the mass lost is low but detectable, whereas in chemical phenomena involving energies of a few electronvolts the masses defects are too small to measure.