Unbinilium
This element has not been synthesized (obtained or discovered) yet. A German team at the GSI Helmholtz Centre for Heavy Ion Research attempted a research on 2011. It suggested a possible production of  299Ubn. But it couldn't be done. Attempts were planned by the Russian, Japanese, and French teams. Attempts were made in 2017-2020 and the experiments from the attempts showed that 8th period elements were difficult to synthesize. Unbinilium is expected to be the last element that can be synthesized through current technology.