Question

What is alpha decay?

Answer 1

Simply put, it is a `"_2^4He` nucleus.

Explanation:

So the bigger question is why does an atom emit an `alpha` particle?

Think about the kinds of atoms that emit `alpha` particles. Mostly they are at the heavy end of the periodic table. In fact, they are generally past bismuth and so have a lot of protons packed into a very small space. Those protons repel each other, and it is only because there are a lot of neutrons to act as spacers between the protons that you get an atom that can remain in one piece (for a limited amount of time).

That balance between protons and neutrons is critical. Too many neutrons and one of the neutrons will balance out the ratio by turning into a proton and emitting a beta particle. Too many protons, and you get alpha decay, where the atom loses two protons and two neutrons, decreasing the ratio of protons to neutrons and making the atom more stable.

Now combine an alpha particle and two beta decays and you end up with the same nucleus you started with, but it is 4 amu lighter.

Kind of cool.