The Atom
. Fission and Fusion Nuclear energy can be released in two different ways: fission, the splitting of a large nucleus, and fusion, the combining of two small nuclei. In both cases energy—measured in millions of electron volts (MeV)—is released because the products are more stable (have a higher binding energy) than the reactants. Fusion reactions are difficult to maintain because the nuclei repel each other, but fusion creates much less radioactive waste than does fission. The atom consists of a small, massive, positively charged core (nucleus) surrounded by electrons (see Atom ). The nucleus, containing most of the mass of the atom, is itself composed of neutrons and protons bound together by very strong nuclear forces, much greater than the electrical forces that bind the electrons to the nucleus. The mass number A of a nucleus is the number of nucleons, or protons and neutrons, it contains; the atomic number Z is the number of positively charged protons. A specific nucleus is designa