Even and odd atomic nuclei abundant isotopes of their element, the former only by a small margin, and the latter only because the expected beryllium-8 has lower binding energy than two alpha particles and therefore immediately alpha decays. Actinides with odd neutron number are generally fissile (with thermal neutrons), while those with even neutron number are generally not, though they are fissionable with fast neutrons. Only , and have odd neutron number and are the most naturally abundant isotope of their element. Even and odd atomic nuclei In nuclear physics, properties of a nucleus depend on evenness or oddness of its atomic number "Z", neutron
Atomic number Atomic number The atomic number or proton number (symbol "Z") of a chemical element is the number of protons found in the nucleus of an atom. It is identical to the charge number of the nucleus. The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons. The sum of the atomic number "Z" and the number of neutrons, "N", gives the mass number "A" of an atom. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes)