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 (IB) 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 first observed in 1994 and several isotopes have been synthesized since its first discovery. The most stable known isotope is 280Rg with a half-life of ~4 seconds.

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