Copernicium

Copernicium sits under mercury, and mercury is already a liquid because relativity contracts its 6s pair; run the same argument one row down, where relativistic effects are far stronger, and copernicium's 7s electrons should be locked away so tightly that the element behaves almost like a noble gas. Adsorption experiments at Dubna in 2006–2008, using a few atoms on a gold surface, found it did stick — but far more weakly than mercury does, sitting somewhere between a metal and a noble gas and not clearly either. That ambiguity is why the category here is left as unknown rather than called a transition metal. The name honours Nicolaus Copernicus and was formally adopted on the 537th anniversary of his birth.