[neutralino]

Originally Posted by theunify I've read something on this I believe it's because the Neutron produces a proton when (one) electron is given off. Some radiation in the form of photons are given off and that is why one electron plus one proton does not equal one neutron.

You're along the right lines, but this is not correct. When the neutron decays to the proton a photon is not given off, but an anti-electron-neutrino is. One of the reasons that it is an antineutrino and not a photon is due to something called lepton number conservation. A lepton is assigned a lepton number of +1 and an antilepton a value of -1. A neutron is a baryon, and thus has a lepton number of zero, so the total lepton number after the decay must also be zero. A proton is a baryon (lepton number 0) and a photon is a boson (lepton number 0) so, if the interaction went , lepton number would not be conserved. The correct decay, does indeed conserve lepton number.

This still doesn't really answer Pat's question, though, since a neutron is not the same as a proton, electron and an antineutrino. Yes, it can decay to these particles, but that is not the same as saying that it is made up of these particles. The real "reason" is that the proton and neutron are made up of different multiples of different quarks, which have different masses. They are both made up of multiples of the "up" and "down" quark, but the proton is "uud" and the neutron is "udd." The slightly different mass of the quarks causes a difference in mass between the proton and the neutron.