system
Given the following post, respond as if you're commenting on Reddit:
human
How are uranium atomic bombs and plutonium atomic bombs different and similar chemically?

I know one has uranium-235 isotopes and the other has plutonium-239 isotopes but that's about it.
gpt
The Little Boy bomb used high-enriched uranium. That means that the uranium fuel was composed of, on average, 80% U-235, the rest being U-238. It had a very simple method of "assembly," that is, the formation of a supercritical mass at precisely the moment desired (detonation). The goal was to make a solid block of about 64 kilograms of U-235, which is more than enough to explode. To accomplish this, they shot a cylinder composing about 60% of the total mass into a target "spike" that was the other 40%. It was literally just shot through a large gun barrel, so they call this a "gun-type" design. It was a very crude use of material, and only about 1% of the uranium in the weapon underwent fission. Yet it still released some 15,000 tons of TNT worth of energy.

The Fat Man bomb, the plutonium bomb, worked very differently. It used plutonium-239 as the fuel, as you know. Plutonium-239 cannot be used in a gun-type configuration because of impurities in the fuel that make it likely to "pre-detonate." In other words, if you tried to shoot two pieces of plutonium together, they would have a tiny explosion that would just blow the fuel apart; it wouldn't really undergo a full nuclear blast. (The impurities are the isotope plutonium-240, which is formed when breeding plutonium-239 in a nuclear reactor. Plutonium-240 has a relatively high spontaneous fission rate so it increases the number of neutrons floating around.) Instead, they had a small (subcritical) amount of plutonium fuel in the shape of a sphere. Using specially designed explosives, they compressed the ball into several times its original density. This caused it to be a supercritical mass. Using explosives to make a internal, compressive force is known as implosion, so this is the "implosion" design. The Fat Man bomb contained only 6.2 kg of plutonium — over 10 times less material than the Little Boy bomb. It had almost the same yield at the Little Boy bomb, though (20,000 tons of TNT), so you can see how much more efficient it was (about 20% of its material fissioned). 

There are more details, but those are the chief differences. Note that while you can't use plutonium in a gun-type design, you _can_ use uranium-235 in an implosion design. For this reason, the Little Boy design was basically a one-off weapon, developed for the war and never used again. (A few other gun-type design weapons were later built for very specific purposes, like making nuclear artillery shells, where the diameter of the weapon is very constrained.) The implosion design became the basis for future fission weapons, with the goal being to make the weapons smaller, use less fissile material, have bigger booms, and so forth. When you put U-235 into an implosion weapon, it has slightly different engineering properties (it has a slightly higher critical mass), so it's not just a case of swapping it in or out, but it's not so different enough as to be considered a very interesting distinction.

As for the chemical differences, they are less interesting and less important than the physical differences between the weapons. Uranium and plutonium have very different metallurgies and chemistries (plutonium is the odd duck here — it has unusually complicated metallurgical properties), but for the purpose of the bomb that is not as relevant, because the plutonium is alloyed in a way that makes it relatively stable (in Fat Man, it was a plutonium-gallium alloy that kept the plutonium stabilized in the delta phase — that isn't very interesting, or very meaningful, unless you're really curious about plutonium metallurgy). Both types of bomb derive nearly all of their energy from nuclear reactions, even though they were initiated by different types of chemical reactions (the assembling explosives, which were just conventional high explosives).