All electrons, nucleons, and nuclei alike are subatomic particles. The nuclear force is nearly independent of whether the nucleons are neutrons or protons. Its nucleus contains 17 protons and 20 neutrons for a total of 37 nucleons. All nuclei containing odd numbers of nucleons have an intrinsic magnetic moment and angular momentum. For example, denotes an atom of uranium with 235 nucleons, 92 of which are protons. The mass of nuclei is significantly different from the masses of the individual nucleons. Isobars are atoms (nuclides) of different chemical elements that have the same number of nucleons. A heavy nucleus can contain hundreds of nucleons. In the case of interacting nucleons, the dotted line would be a virtual pion. Furthermore, for odd-odd nuclei, one has to consider the two "last" nucleons, as in deuterium. Technetium also has numerous nuclear isomers, which are isotopes with one or more excited nucleons. Another typical example is that of the nucleons in the nucleus of an atom. The atomic mass number is the total number of nucleons. Heisenberg introduced the first theory of nuclear exchange forces that bind the nucleons. The nuclear magnetic moment also includes contributions from the orbital motion of the nucleons.