The neutron number, symbol N, is the number of neutrons in a nuclide. Atomic numberplus neutron number equals mass number: Z + N = A. The difference between the neutron number and the atomic number is known as the neutron excess: D = N - Z = A - 2Z. Neutron number is rarely written explicitly in nuclide symbol notation, but appears as a subscript to the right of the element symbol. In order of increasing explicitness and decreasing frequency of usage:
Element
Isotope/Nuclide
With atomic number
With neutron number
8
Nuclides that have the same neutron number but different proton numbers are called isotones. This word was formed by replacing the p in isotope with n for neutron. Nuclides that have the same mass number are called isobars. Nuclides that have the same neutron excess are called isodiaphers. Chemical properties are primarily determined by proton number, which determines which chemical element the nuclide is a member of; neutron number has only a slight influence. Neutron number is primarily of interest for nuclear properties. For example, actinides with odd neutron number are usually fissile while actinides with even neutron number are usually not fissile. Only 58 stable nuclides have an odd neutron number, compared to 194 with an even neutron number. No odd-neutron-number isotope is the most naturally abundant isotope in its element, except for beryllium-9, nitrogen-14, and platinum-195. No stable nuclides have neutron number 19, 21, 35, 39, 45, 61, 89, 115, 123, and ≥ 127. There are 6 stable nuclides and one radioactive primordial nuclide with neutron number 82 : barium-138, lanthanum-139, cerium-140, praseodymium-141, neodymium-142, and samarium-144, as well as the radioactive primordial nuclide xenon-136. Except 20, 50 and 82, all other neutron numbers have at most 4stable isotopes. Most odd neutron numbers have at most one stable isotope, 5, 7, 55 and 107. However, some even neutron numbers also have only one stable isotope; these numbers are 2, 4, 84, 86 and 126. Only two stable nuclides have fewer neutrons than protons: hydrogen-1 and helium-3. Hydrogen-1 has the smallest neutron number, 0.
