hehe, well I don't think we're meant to understand it.
it says identify, so all you have to say is that protons and neutrons spin...
who knows why. its like asking why do you exist
but if someone can say why, please, tell. I'd be most interested to know why!
ahh but net spin, that's a relatively simple concept. protons,and neutrons, are all spinning, and a certain artificial value is placed on how much spin there is. like a single proton, such as hydrogen ion, has a spin of 1/2. the consequence of the spin is that it has a magnetic field around it, since any spinning/moving charge generates a magnetic field.
however, they're either spinning one way or the other...say clockwise and anti-clockwise to make it simple. The book uses parallel and anti-parallel, or north and south, but anyway, if you have two protons, spinning in opposite directions, then the magnetic field produced by either one, is cancelled out by the other.
and so you get a zero net magnetic field, or magnetic dipole movement.
they're called paired protons, cause they're two protons, spinning in opposite directions close to each other, such that they cancel each others magnetic field.
say you had a set of paired protons, and an extra proton, spinning. the paired protons will cancel each other's magnetic field out, but because the extra proton, or unpaired proton, is spinning by itself, it produced a magnetic field. thus, the net magnetic field is produced by the unpaired proton.
protons produce a spin of either 1/2 or -1/2, which just indicates which way they're spinning, or what direction the magnetic field is moving.
for larger nuclei or atoms, your looking at the whole nuclei spinning, so they have different spin. say 3/2 spin. your looking at the interaction of that nuclei spinning, with another nuclei of the same property, and which way the second nuclei is spinning.
