"Basically, the sheer amount of empty space in atoms means that the conditions for fission or fusion are very, very specific. Atoms are not packed tightly. The necessary domino effect of the chain reaction is like mousetraps loaded with ping-pong balls. The chain reaction only happens if you have an assload of traps close together, and lots of ping-pong balls flying as a result. Normal atoms are like two or three loaded traps in a room, hard to get that reaction going. But if you throw a thousand traps loaded with ping-pong balls into a small room really quickly, kaboom."