Mass number number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons ("N") in a given nucleus: . The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon is carbon-12, or , which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number ("Z") as a subscript to the left of the element symbol directly below the mass number:
Even and odd atomic nuclei 162, and 165+ can also alpha decay.) This gives a total of 101 stable nuclides with odd mass numbers. There are another 9 radioactive primordial nuclides (which by definition all have relatively long half lives, greater than 80 million years) with odd mass numbers. Odd-mass-number nuclides are fermions, i.e. have half-integer spin. Generally speaking, since odd-mass-number nuclides always have an even number of either neutrons or protons, the even-numbered particles usually form part of a "core" in the nucleus with a spin of zero. The nucleon with the odd number (whether protons or neutrons) then form a second core with