Energy of Nuclear Reactions
A note of warning: what is found here is a VERY simple explanation and, as such, is incomplete. It is however enough to understand why nuclear reactions are so very important (and dramatic).
If you were to weigh the bomb dropped on Hiroshima VERY precisely, and then, after the explosion, collect all of the parts of the bomb and weigh them you would find that the material after the explosion weighed less than the material that made up the bomb.
The easy explanation would be that you didn't manage to find all of the material that had comprised the bomb initially (and that certainly would be true if you actually tried to do this) but that is not the point. The point is that EVEN IF you could find every bit of the bomb, the "products" of the nuclear reaction that occurred in the explosion would weigh less than the material that made the bomb.
This "problem" was known as the missing mass problem and cuts a the very heart of chemistry (the conservation of mass).
The solution was derived by Albert Einstein and was offered as a "afterthought" to his special theory of relativity during what was called his "miracle year." (During that year he wrote papers that essentially proved the existence of atoms, created quantum theory and developed relativity. Not bad for a guy who didn't have a doctorate yet and who couldn't get a job teaching physics.)
The solution was the equation that everyone knows:
The mass that is lost, is actually converted to energy (it turns out that energy and mass are different version of the same thing). This lost mass (turned to energy) is what devestated the city of Hiroshima.