system
Given the following post, respond as if you're commenting on Reddit:
human
Question about nuclear decay and atomic bombs

I was wondering if someone could tell me the basics on how an atomic bomb work? does it have anything to do with nuclear decay and the lack of stability? sorry if this seems like a stupid question I'm hopelessly clueless when it comes to science!
gpt
Certain isotopes, mainly actinides with odd A numbers, have special material properties to them: a big probability of absorbing a neutron, fissioning, and releasing two to three more neutrons.  These two or three neutrons can go on to re-react with the material and release 2-3 more neutrons.  This cycle, if it sustains itself is called "critical".  If it multiplies over time it is called "supercritical".

So if you have the right material how to you get it to go critical?  Well, first you have to put it into a critical mass.  A critical mass the amount of mass in a certain geometric configuration which provides the material with the right neutron absorbing geometry to self-sustain itself.

However, a critical mass needs one last thing to go critical: a neutron source to provide it with a start up.  Put that in there and it will all come together.

A bomb needed to be put in a super critical mass very quickly.  There were two designs for this, a bullet of uranium firing into a receiver uranium cup (upun combination becomes supercritical)  and a sphere of sub-critical material with a shell of explosives which would detonate and shrink the sphere, causing it to go supercritical.  Those were the two, original, atomic bomb designs.

Things get a little more complicated with neutron energies, but this is the general idea.