Xenon is a chemical element with symbol Xe and atomic number 54. It is a colorless, dense, odorless noble gas found in the Earth's atmosphere in trace amounts. Although generally unreactive, xenon can undergo a few chemical reactions such as the formation of xenon hexafluoroplatinate, the first noble gas compound to be synthesized. The first excimer laser design used a xenon dimer molecule (Xe2) as the lasing medium, and as the propellant for ion thrusters in spacecraft.