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:
Isotopes of rutherfordium Isotopes of rutherfordium Rutherfordium (Rf) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was either Rf in 1966 or Rf in 1969. There are 16 known radioisotopes from Rf to Rf (3 of which, Rf, Rf, and Rf are unconfirmed) and 4 isomers. The longest-lived isotope is Rf with an estimated half-life of 5 hours. The longest directly measured half-life is Rf at 11 minutes, and the longest-lived isomer is Rf with a half-life of 81 seconds. Super-heavy elements such