Be sure to check out the answer to a highly similar question: What would happen if you split an atom?
In order for atoms to explode when splitting, youhave to get the right kinds of heavy atoms, like Uranium-235. The atoms have to start a chain reaction when they split, otherwise, they won't gain enough energy to make a big explosion! For example, some elements, like Francium, will split so easily you can never get enough of that kind of atom in one place fast enough for francium to "go nuclear." Also, if you have very light atoms, those will go nuclear if you fuse them together instead of split them apart.