High Quality Content by WIKIPEDIA articles! Nobelium (No) is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized (and correctly identified) was 254No in 1966. There are 16 known radioisotopes which are 248No and 250No to 264No, and 3 isomers, 251mNo, 253mNo, and 254mNo. The longest-lived isotope is 259No with a half-life of 58 minutes. The longest-lived isomer is 251mNo with a half-life of 1.7 seconds.