I know that other things are used like hydrogen, but as far as I know, uranium is more common. If all you need is to split an atom to create a nuclear explosion, why use a rare earth metal instead of something more abundant since everything is made of atoms?
In: Physics
To make a nuclear explosion you need an element that, upon absorbing one neutron (a subatomic particle) of any energy level, will split, releasing energy and more neutrons that can continue the reaction. It’s not about just splitting “an” atom, it’s creating a chain reaction so that you can split a trillion trillion atoms in a millisecond or so.
There are only a small number of types of atoms that meet the above criteria (the technical term for this is “fissile”), and of those, only two are “easy” to produce in bulk: uranium-235 (a specific type of uranium that can be separated from normal uranium in special facilities), and plutonium-239 (and artificial element that is made in nuclear reactors). Even these are not “easy” to make by any reasonable standard, but they are a lot easier than the other kind.
So there are real physical constraints on what the options are. Uranium-238, the most common form of uranium, cannot work in a bomb like this, because even though it _can_ be split by neutrons, it can only be split by high-energy neutrons, and the neutrons produced by splitting atoms are not high-enough energy to have a chain reaction in U-238. (This is why you have to separate the U-235 from the U-238.)
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