Question

in nuclear reactions there is often a measurable difference in the mass of the reactants and the mass of the products that is not the result of errors in massing. what is the reason for the difference in mass value?

Answer 1

One theory is that a neutron decays to a proton + an electron. The electron is ejected and the proton and neutron have slightly different masses. Adding up over a few billion billion atoms might make a small difference.