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This diagram shows the half-life (T½) of various isotopes with Z protons and neutron number N.

The neutron number, symbol N, is the number of neutrons in a nuclide.

Atomic number (proton number) plus 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
C
Isotope/Nuclide 14
C
With atomic number 14
6
C

Nuclides that have the same neutron number but a different proton number 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.[1]

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 (fissionable with slow neutrons) while actinides with even neutron number are usually not fissile (but are fissionable with fast neutrons).

Only 57 stable nuclides have an odd neutron number, compared to 200 with an even neutron number. No odd-neutron-number isotope is the most naturally abundant isotope in its element, except for beryllium-9 which is the only stable beryllium isotope, nitrogen-14, and platinum-195.

No stable nuclides have neutron number 19, 21, 35, 39, 45, 61, 89, 115, 123, and ≥ 127 (and possible 71, 84, 85, 87, 109 and 126). Besides, there are 6 stable nuclides and one radioactive primordial nuclide have neutron number 82 (82 is the neutron number with the most stable nuclides, since it is a magic number): caesium-137, barium-138, lanthanum-139, cerium-140, praseodymium-141, and neodymium-142, as well as the radioactive primordial nuclide xenon-136. Except 20, 50 and 82 (all these three numbers are magic numbers), all other neutron numbers have at most 4 stable isotopes (in the case of 20, there are 5 stable isotopes S36, Cl37, Ar38, K39 and Ca40 (Ca40 may be unstable), and in the case for 50, there are 5 stable nuclides: Kr86, Sr88, Y89, Zr90 and Mo92 (Mo92 may be unstable) and 1 radioactive primordial nuclide Rb187). All odd neutron numbers have at most one stable isotope (except 1 (H2 and He3), 5 (Be9 and B10), 7 (C13 and N14), 55 (Mo97 and Ru99) and 107 (Hf179 and Ta180m). However, some even neutron numbers also have only one stable isotope (these numbers are 2 (He4), 4 (Li7), 84 (Ce142, possible unstable), 86 (Nd146) and 126 (Pb208, possible unstable), and possible 26 (Ti48), 66 (Sn116), 68 (Sn118), 76 (Xe130), 80 (Ba136), 88 (Sm150), 102 (Yb172), 106 (Hf178), 110 (Re185), 112 (Os188) and 120 (Hg200)). (In theory, no stable nuclides have neutron number 19, 21, 35, 39, 45, 61, 71, 83-91, 95, 96, and ≥ 99) Besides, no nuclides with neutron number 19, 21, 35, 39, 45, 61, 71, 89, 115, 123, 147, 161, ... are stable to beta decay (see Beta-decay stable isobars).

Only two stable nuclides have fewer neutrons than protons: hydrogen-1 and helium-3.

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