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Isotopes of nobelium 54 s activity was actually due to No and the isotope No was retracted. Isotopes of nobelium Nobelium (No) 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 (and correctly identified) was No in 1966. There are 12 known radioisotopes, which are No to No and No, and 3 isomers, No, No, and No. The longest-lived isotope is No with a half-life of 58 minutes. The longest-lived isomer is No with a half-life of 1.7 seconds. This cold fusion reaction was