the chemical symbol for einsteinium a transuranic element with atomic number 99 The atomic weight of the longest lived isotope with a half life of 276 days is 254 The first isotope discovered having atomic weight 253 and a half life of 20 days was recognized in 1952 in the debris from a hydrogen bomb test As much as 3 micrograms of einsteinium were produced by a complex process involving long irradiation of plutonium isotopes in nuclear reactors Its chemical properties are those of a trivalent actinide element