Isotopes of einsteinium

Einsteinium (₉₉Es) 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 (in nuclear fallout from the Ivy Mike H-bomb test) 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.