When a quark changes flavour, as it does in Beta decay, it releases a W boson. On average, W bosons only last for 3x10 seconds before themselves decaying into other particles, which is why we had not discovered them until less than half a century ago. Surprisingly, W bosons have a mass of about 80 times that of a proton. Keep in mind that the neutron that it came from has almost the same weight as the proton. In the quantum world, it is not uncommon for a more massive particle to come from a less massive particle; the extra mass comes from stored energy via Einstein's famous formula, formula_1. After the 3x10 seconds has passed, a W boson decays into one electron and one neutrino. Since neutrinos rarely interact with matter, we can ignore them from now on. The electron is propelled out of the atom at a high speed. The proton that was produced by the Beta decay stays in the atom nucleus, and raises the atomic number by one.