Atoms don't so much have energy as they represent energy levels that the components of atoms (protons and neutrons) can have. Think, for example, about chemistry: oxygen is at a lower energy state when it is bonded with hydrogen to make water or carbon to make carbon dioxide than when bonded to other oxygen. When oxygen is combined with either of these elements, energy is released - the resulting chemical reaction is called "fire". Nuclear reactions follow the same basic principle. The bottom of the energy well is iron, element 26. Elements heavier than iron, release energy when broken into smaller chunks. Elements lighter than iron, release energy when combined to form heavier atoms. The differences in energy levels between nuclear energy states is much larger than the difference in energy levels between chemical energy states, however, which is why nuclear reactions are so much more powerful.