The basic unit of matter in the universe is the atom. They’re extremely tiny, each made of a central nucleus of protons and neutrons surrounded by orbiting electrons. Atoms of different elements have different numbers of nucleons—roughly half are protons and half are neutrons—and each contains the same number of negatively-charged electrons as it has protons, which are positively charged and attract the electrons. Neutrons have no charge, but sometimes they can be released to fly away until they strike, and sometimes join or split, another atom’s nucleus.

A gun assembly is an atomic bomb shaped like a short artillery gun with a clump of uranium at one end and a smaller amount at the other end that’s fired into the big clump, causing it to explode in a chain reaction. “Little Boy,” the weapon that the US detonated over Hiroshima, was this kind of weapon. A more powerful weapon, made of plutonium, required a different system to detonate but became the weapon of choice thereafter. Some years later, this too was superseded by more powerful fusion bombs. Thus, the gun assembly bomb, the first atomic weapon ever used, was retired immediately following its successful use.