http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.html

- Provides a technical explanation of fusion and fission production of energy. Simplistically, if you fuse elements below iron in the periodic table, the resulting nucleus has less mass than the ones that fused, and the missing mass is converted to energy. You cannot produce energy by fissioning the resulting nucleus. In the case of fissioning nuclei like uranium, the resulting smaller nuclei have a combined mass less than the uranium, and again the missing mass is converted to energy - E=MxCsquared