Scientists measure bond distance in rare, radioactive element einsteinium By (0) Because einsteinium has such a short half-life, researchers at the University of California, Berkeley had to work fast when conducting experiments with element 99. Photo by Marilyn Sargent/Berkeley Lab Feb. 3 (UPI) -- Scientists have, for the first time, measured the bond distance of einsteinium, one of the most radioactive and difficult to make elements on the periodic table. Researchers detailed rare experiments on the element, which carries the atomic number 99, in a paper published Wednesday in the journal Nature. Advertisement With little known about the chemical properties of einsteinium, bond distance -- the average distance between the nuclei of two bonded atoms in a molecule -- is key to understanding how an element will interact with other atoms and molecules.