system
Given the following post, respond as if you're commenting on Reddit:
human
Why are free neutrons unstable?

Neutrons that are not part of a nucleus but just kind of floating around by themselves are unstable and undergo beta decay with a half life around 15 minutes.  Do we know why they are unstable?  What is making them decay?  

What keeps them from decaying when they are in the nucleus of an atom?  Why are free protons, by contrast, (essentially) stable?

Thanks.
gpt
Protons are slightly less massive (and therefore, energetic) then neutrons, and a neutron can turn into a proton by beta decay. Because the higher energy neutron can turn into the lower energy proton, it eventually does.

A proton can't spontaneously decay into a neutron by inverse beta decay because the neutron has higher mass. In an atom, however, the mass is also determined by the energy that binds the protons and neutrons together, so in this case neutrons can be stable or protons unstable.