Question

The reason most atoms undergo nuclear decay is because:

Answer 1

I'm not sure what you mean by "most." One possibility is that you mean "most conceivable atoms," such as an atom with 100 billion protons and 1 neutron, which would of course be completely unstable. There are an infinite number of possibilities for atoms, because you can have any number of protons from 1 to infinity, and any number of neutrons from 0 to infinity. So you have an infinity squared number of possibilities, of which infinity squared minus a few hundred are completely unstable. So, in that sense, most atoms are unstable.

If this is what you mean, the reason all those possible atoms are unstable is because they have too many protons and not enough neutrons. Nucleons (protons or neutrons) stick together because of the strong nuclear force, which is very strong, but has a very short range. Beyond a few femtometers its strength sinks to zero. But protons also repel each other, because they have like charges, through the electromagnetic force, which is much weaker than the strong force, but which has infinite range. If you start clumping protons together, at first the strong nuclear force is plenty strong enough to hold them together against the repulsion of the electromagnetic force. But as the clump starts to get bigger and bigger, the protons on opposite sides of the clump get far enough apart that the strong force they feel attracting them to each other gets too weak -- and can't compensate for the electromagnetic repulsion. Sooner or later, if the clump gets big enough, the strong force is too weak to bind the protons on opposite sides of the clump, because they're too far apart, and that's about as big as you can make a clump of protons.

It helps to add in neutrons, because neutrons increase the strength of the strong force without adding any electromagnetic repulsion. In fact, you can probably always make any clump of protons stable by adding enough neutrons to it. However, an important fact is that atoms are of different elements only by virtue of the number of protons (because this determines the number of electrons, and the number and arrangement of electrons is what determines the chemistry of the atom). So adding neutrons doesn't actually give you a NEW atom -- it just gives you a new isotope of an existing atom.

Furthermore, at some point, when you add ridiculous numbers of neutrons, you aren't really dealing with an atom any more -- you've got a tiny neutron star, with some smattering of positive charge on it, although theoretically this is as far as I know uncharted territory. That is, so far as I know, nobody has thought about the nature and properties of objects in some sense intermediate between atomic nuclei and tiny neutron stars.

Answer 2

But what you might mean, instead, is "Of the atoms that are only moderately unstable -- with half-lives of at least seconds, if not hours, days and years -- how do they typically decay?"

The answer depends on why they are unstable. Light atoms with too many protons typically decay initially by positron (beta+) emission, which turns a proton into a neutron:

12N -> 12C + e+

Light atoms with too many neutrons typically decay initially by electron (beta-) emission, which turns a neutron into a proton:

12B -> 12C + e-

Heavy atoms with too many of both often decay initially by alpha emission, which throws off two protons and two neutrons:

238U -> 234Th + a

Unless the result of the first decay is stable, there is then an additional decay, and another, and so on, until a stable nuclide is reached. Some chains are quite long, and most have "branches" so that at various points the decay might go down one branch or another. For example, uranium-238 decays by a chain of 20 decays:

238U -> 234Th -> 234mPa -> 234Pa -> 234U -> 230Th -> 226Ra -> 222Rn -> 218Po -> 218At -> 218Rn -> 214Pb -> 214Bi -> 214Po -> 210Tl -> 210Pb -> 210Bi -> 210Po -> 206Tl -> 206Pb

The last nuclide, lead 206, is stable. The first step in the chain is longest, with a half-life of 4 billion years, and the next longest is the decay of uranium-234, with a half-life of 250,000 years. Some of the shorter steps take only milliseconds.