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 4 nuclear isomers.
The longest-lived isotope is 252Es with a half-life of 471.7 days, or around 1.293 years.