Roentgenium is a synthetic radioactive chemical element with the symbol Rg and atomic number 111. It is placed as the heaviest member of the group 11 elements, although a sufficiently stable isotope is not known at this time that would allow its position as a heavier homologue of gold to be confirmed. Roentgenium was called unununium before it was formally discovered. Roentgenium was first observed in 1994 and several isotopes have been synthesized since its first discovery. The most stable known isotope is 281 Rg with a half-life of ~20 seconds, which decays by spontaneous fission, like many other N=170 isotones. Roentgenium was officially...more »