Scientists predicted the existence of Einsteinium which otherwise would have left a gap at element 99 in the periodic table of elements, and predicted the properties it would have. It was first discovered in 1952 in the debris of the first hydrogen bomb. But since it doesn't occur in nature in any meaningful amounts it has been hard to actually measure its properties.
Now scientists have done that. And, while its properties were mostly what chemists would have expected, there have been some surprises.
The researchers were able to measure a bond distance with einsteinium and also discovered some physical chemistry behavior that was different from what would be expected from the actinide series, which are the elements on the bottom row of the periodic table."Determining the bond distance may not sound interesting, but it's the first thing you would want to know about how a metal binds to other molecules. What kind of chemical interaction is this element going to have with other atoms and molecules?" [Berkeley Lab scientist Rebecca] Abergel said. . . .
"Similar to the latest elements that were discovered in the past 10 years, like tennessine, which used a berkelium target, if you were to be able to isolate enough pure einsteinium to make a target, you could start looking for other elements and get closer to the (theorized)island of stability," where nuclear physicists have predicted isotopes may have half-lives of minutes or even days, instead of the microsecond or less half-lives that are common in the superheavy elements.
The paper and its abstract are as follows:
The transplutonium elements (atomic numbers 95–103) are a group of metals that lie at the edge of the periodic table. As a result, the patterns and trends used to predict and control the physics and chemistry for transition metals, main-group elements and lanthanides are less applicable to transplutonium elements. Furthermore, understanding the properties of these heavy elements has been restricted by their scarcity and radioactivity. This is especially true for einsteinium (Es), the heaviest element on the periodic table that can currently be generated in quantities sufficient to enable classical macroscale studies.
Here we characterize a coordination complex of einsteinium, using less than 200 nanograms of 254Es (with half-life of 275.7(5) days), with an organic hydroxypyridinone-based chelating ligand. X-ray absorption spectroscopic and structural studies are used to determine the energy of the L3-edge and a bond distance of einsteinium. Photophysical measurements show antenna sensitization of EsIII luminescence; they also reveal a hypsochromic shift on metal complexation, which had not previously been observed in lower-atomic-number actinide elements. These findings are indicative of an intermediate spin–orbit coupling scheme in which j–j coupling (whereby single-electron orbital angular momentum and spin are first coupled to form a total angular momentum, j) prevails over Russell–Saunders coupling. Together with previous actinide complexation studies, our results highlight the need to continue studying the unusual behaviour of the actinide elements, especially those that are scarce and short-lived.