Roentgenium (Rg) is a synthetic element with the atomic number 111. It is highly unstable and radioactive, and as of my last update, only a few isotopes of roentgenium have been produced. The known isotopes of roentgenium include: 1. **Roentgenium-282 (Rg-282)**: This isotope has a half-life of approximately 2.1 milliseconds.
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