The vast majority of nuclear weapons use Plutonium as at least part if not all the fission fuel. No specific scientist first suggested it, it was just obvious once the element was discovered to in cyclotron experiments.
plutonium + weapon
Plutonium 239
There are many different elements involved in the construction of an atomic bomb, but the elements that actually cause an atomic explosion are either uranium (specifically the U235 isotope) or plutonium.
In general, a fusion bomb (hydrogen bomb) is more powerful than a fission (atomic) bomb. Fusion bombs use an atomic bomb to begin the fusion reaction.
Of the two dropped on Japan, one was a Uranium bomb and the other a Plutonium bomb. Both Uranium and Plutonium are elements, and are radioactive. The radioactivity makes these elements suitable as sources of energy, for power generation or explosions.
They understood the power of the atomic bomb and feared its use.
An atomic bomb uses a nuclear fission reaction. This involves splitting the nucleus of a heavy atom, such as uranium or plutonium, into smaller nuclei, releasing a large amount of energy in the process.
There's no plutonium here. I haven't even seen any plutonium, recently.
That depends, an atomic weapon from WWII was armed with plutonium 239, bombs now use a more dangerous Uranium armament. In order to use the bomb, a smaller explosion much trigger a chain reaction explosion in order to create, for lack of better terms, a "big bang".
The atomic bomb dropped on Nagasaki was codenamed "Fat Man." It was detonated on August 9, 1945, and was a plutonium-based bomb, differing from the uranium-based bomb dropped on Hiroshima, known as "Little Boy." The use of Fat Man contributed to Japan's eventual surrender and the end of World War II.
The atomic bomb is sphere shaped and weird. An atomic explosion is made by taking two halves of a critical mass of fissile material (plutonium or U235) and pushing them together very rapidly. Atomic bombs use conventional explosives to move the fissile material fast enough. If it does not move fast enough it just overheats and melts, rather than exploding. That's the key to designing an atomic bomb. Good luck!
Oppenheimer was the scientist/manager who directed the team that built the tool and made it work. The final decision on what to do with it and when was made by the Commander in Chief of the Armed Forces ... President Truman.