Question: why is Uranium radioctive?

Duncan McNicholl answered on 4 Nov 2017:

Because of its big fat lardy nucleus.

There are two forces that are important inside the nucleus of an atom (there are actually three, but you don’t need to know about the third one, which is good, because I don’t really understand it). The two forces are called the electromagnetic force (which you might have already heard of), and the strong force (which you probably haven’t already heard of). In the nucleus, the electromagnetic force is trying to push all of the particles away from each other, and the strong force is trying to hold them together.

As you can probably guess, the strong force is stronger, but here’s the weird bit: it only works over really short distances, and the electromagnetic force works over longer distances. that means that if you have a really fat nucleus, you can end up with one side being so far away from the other that the strong force won’t reach, but the electromagnetic force still does, so there’s a force pushing things apart, but not one holding things together any more, and that’s when bits start to fly away.

The bits are the radiation.