There are actually "three" quarks in a proton -- two up-type
quarks and one down-type quark. These are held together by the strong
nuclear force. Quarks have electrical charge (up-type quarks have
charge 2/3, and down-type quakrs have charge -1/3. In these units,
electrons have charge -1), and also a different kind of charge,
whimsically called "color". Electrically charged particles interact
electromagnetically (attracting or repelling) by exchanging photons.
Color-charged particles interact with each other by exchanging gluons.
Gluons are a lot like photons (they're massless, they carry a
fundamental force), and also a bit unlike photons, in that gluons
themselves have color charge. This last property means that gluons
interact with each other, while photons, being neutral, do not. Gluons,
because they interact with each other, make a sticky mess, holding the
quarks together. In fact, no one has ever observed any particle with a
net color charge -- the three quarks in a proton have colors which add
up to a net color charge of zero. Try to pull one of the quarks out by
hitting it, and you end up stretching the gluon mess, and by stretching
it enough you give it enough energy to make new particles, instead of
just pulling out one of the quarks.
If left alone, neutrons will decay, with an average lifetime of
886 seconds. They decay into a proton, an electron, and an electron
antineutrino. Neutrons are just a very tiny bit heavier than protons,
and so this reaction is allowed. This reaction proceeds via the weak
nuclear force, by the exchange of a charged W boson.
Neutrons bound up in atomic nuclei sit in energy levels determined
by quantum mechanics (just like the electrons in orbit around the
nucleus). In stable nuclei, like Carbon 12, the neutrons cannot decay,
because to do so, the resulting proton would have to be put into a
higher energy level than the neutron was in that it came from. The
Pauli exclusion principle says that energy levels "fill up", and the
lowest unfilled one gets the proton from the neutron decay. If the
energy difference between the neutron's level and the proton's level,
plus the energy needed to make the electron and the antineutrino is
bigger than the mass difference between the neutron and the proton
(E=mc^2 after all), then the decay won't happen.
Carbon 14 has more neutrons than protons, and so the state the
proton goes into isn't much different in energy than the state the
neutron was in in the first place. This decay proceeds at a very slow
rate, though -- C14 it has a lifetime measured in thousands of years.
(republished on 07/21/06)