Hi, I was wondering about something in my notes that I don't quite understand. In positive beta decay in nuclear physics, we have a proton becoming a neutron and kicking out a positron and an electron neutrino (which is assumed massless here). In the expression for the energy released, Q = [M(A,Z) - M(A,Z-1) - 2me]c2, we have the kinetic energy of the products is equal to the mass-energy of the original nucleus minus the mass energy of the new nucleus minus TWO electron masses. This factor of two is what confuses me. All my notes really say is that this is due to an excess of electrons. Now I assume what this means is that originally the atom is electrically neutral, with equal numbers of protons and electrons. After this decay, we are one proton short, so there is on excessive electron in an atomic electron shell. I still do not see where this extra electron mass-energy comes from though, because the excess electron in question is not being created, it was always there. All that has been created is one positron, and we are assuming the neutrino to be massless. Can anyone explain this? Thanks.