Radon – facts and properties

Position in the periodic table

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Occurrence and extraction

Radon forms wherever radium decays, and the radium comes from the decay series of uranium-238. Traces of uranium sit in almost every rock and soil, especially in granite and gneiss. The most important isotope, radon-222, has a half-life of almost four days.

As a gas, radon escapes from the pores and cracks of rock and rises. In Germany the ground is especially rich in radon in the Ore Mountains, the Bavarian Forest and the Black Forest, among other places.

In 1900 Friedrich Ernst Dorn in Halle found that radium preparations give off a radioactive gas, which he called radium emanation. William Ramsay and Robert Whytlaw-Gray determined its density in 1910 and showed that it belongs to the noble gases. The name radon, derived from radium, caught on only later.

Properties and reactions

Radon closes period 6 as the noble gas of group 18. It lies below xenon and to the right of astatine. Its electron configuration is [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁶, and its 8 outer electrons fill the shell completely. With a density of 9.73 g/L it is more than seven times as dense as air and so the heaviest naturally occurring noble gas. It is liquid only in a narrow range between -71 °C and -61.7 °C. If it is cooled further, solid radon glows yellow to orange-red from its own radiation. Its atomic mass is given as 222 u, the mass number of radon-222.

Chemically, radon is very unreactive. Its few compounds with fluorine decay along with the atoms and are hard to study.

The danger comes from the radiation. Radon-222 emits alpha particles and turns into polonium-218. Further decay produces lead-214, bismuth-214 and other solid products. They stick to dust particles in the air and reach the bronchi with each breath, where they settle. There the alpha particles irradiate the tissue from close range and can change cells so that cancer develops. After smoking, radon is the second most common cause of lung cancer. The Federal Office for Radiation Protection estimates that it contributes to about one in sixteen lung cancer deaths in Germany.

As early as the sixteenth century, miners in the Ore Mountains often fell ill with a lung disease called Schneeberg disease. In the nineteenth century it was recognised as lung cancer, and only in the twentieth century was the radon in the mines identified as its cause.

Radon in buildings and radiation protection

Radon enters houses from the ground through cracks in the foundation, leaky pipe entries and cellar floors. The highest values occur in the cellar and on the ground floor. In Germany a reference level of 300 becquerels per cubic metre of air as a yearly average applies to living spaces and workplaces. One becquerel means one decay per second. The reference level is not a limit, but exceeding it is a reason to consider protective measures. The federal states designate radon prevention areas, where many buildings are affected.

Radon can be measured with passive dosimeters that stay in the room for a few months and are then evaluated in a laboratory. Against high values, tight floor slabs, sealed cellar walls and pipe entries, regular ventilation and, at higher values, extraction beneath the house help.

There is no everyday use in the proper sense. Radon was once filled into small gold tubes to irradiate tumours, a method long replaced by other techniques. Health resorts such as Bad Gastein or Bad Schlema offer radon baths and tunnel stays against rheumatism. Whether they work is disputed. In research, radon serves as a tracer to follow air masses or to see where groundwater seeps into lakes.

Quick question

What is the atomic number of radon (Rn)?

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