Radon is a colorless and odorless gas, and therefore not detectable by human senses alone. At standard temperature and pressure, radon forms a monatomic gas with a density of 9.73 kg/m3, about 8 times the surface density of the Earth’s atmosphere, 1.217 kg/m3. It is one of the heaviest gases at room temperature and the heaviest of the noble gases, excluding ununoctium. Although colorless at standard temperature and pressure, when cooled down below its freezing point of 202 K (−71 °C; −96 °F), radon has a brilliant phosphorescence which turns yellow as the temperature is lowered, and becomes orange-red as the air liquefies at temperatures below 93 K (−180.1 °C; −292.3 °F). Upon condensation, radon also glows because of the intense radiation it produces.

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