Isotopes of einsteinium
Einsteinium 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 discovered was 253Es in 1952. There are 18 known radioisotopes, from 240Es to 257Es, and 8 nuclear isomers. The longest-lived isotope is 252Es with a half-life of 471.7 days or 1.291 years; the second longest, 254Es with half-life of 275.7 days is more available.