In fission, energy is gained by splitting apart heavy atoms, for example, uranium, into smaller atoms such as iodine, caesium, strontium, xenon and barium, to name just a few. However, fusion is combining light atoms, for example, two hydrogen isotopes, deuterium and tritium to form the heavier helium. Both reactions release energy which, in a power plant, would be used to boil water to drive a steam generator, thus producing electricity.