Maria Theresa reigned over Spain.
Einsteinium -LRB- 99Es -RRB- is a synthetic element , and thus a standard atomic weight can not be given .. Einsteinium. Einsteinium. synthetic element. synthetic element. standard atomic weight. standard atomic weight. Like all artificial elements , it has no stable isotopes .. The first isotope to be discovered -LRB- in nuclear fallout from an H-bomb test -RRB- was 253Es in 1952 .. isotope. isotope. fallout. fallout. H-bomb. H-bomb. test. nuclear testing. There are 19 known radioisotopes from 240Es to 258Es , and 3 nuclear isomers -LRB- 250mEs , 254mEs , and 256mEs -RRB- .. The longest-lived isotope is 252Es with a half-life of 471.7 days , or around 1.293 years .. isotope. isotope. half-life. half-life