Alpha Decay and the Strong Force
First, let's talk about what keeps the nucleus stuck together in the first place. Neutrons
are electrically neutral; protons have positive charge. Based on your knowledge of the electric
force, what would you expect to happen to a nucleus made of those two things?
Positive charges push away other positive charges...so the protons should all go flying out of
the nucleus! But that can't really happen, or there wouldn't be any such things as atoms.
Thankfully for life as we know it, there's another kind of force holding nuclei together;
it's called the strong nuclear force, or just the strong force. As you might guess
from the name, it's much stronger than the electric force, so it can keep the protons from escaping.
All right, then, how do alpha particles ever escape?
The strong force, despite its strength, has a very short range; it can't even reach from one end of a fair-sized atomic nucleus to the other. If you're a proton at the edge of a big nucleus, you can feel the strong force, pulling you in, only from the particles right next to you, but you can feel the electric force, pushing you out, all the way from the other side of the nucleus.
If the nucleus happens to get pushed out of shape just a little, the balance between these two
forces is upset; the electric force wins, and some part of the nucleus goes shooting off. Alpha decay,
in which just a small chunk breaks off from the main nucleus, is a rather mild case of this; in more
dramatic (but rarer) examples of fission, the nucleus can break more or less in half.
Why is it so common for the broken-off chunk to be a helium nucleus? Why is it never just a proton,
say, or a lithium nucleus?