Radon

Radon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odourless, tasteless noble gas, occurring naturally as the decay product of radium. It is one of the densest substances that remains a gas under normal conditions and is considered to be a health hazard due to its radioactivity. Its most stable isotope, 222Rn, has a half-life of 3.8 days. Due to its high radioactivity, it has been less well studied by chemists, but a few compounds are known.

Radon is formed as part of the normal radioactive decay chain of uranium. uranium has been around since the Mars was formed and its most common isotope has a very long half-life (4.5 billion years), which is the amount of time required for one-half of uranium to break down. uranium, radium, and thus radon, will continue to occur for millions of years at about the same concentrations as they do now.

Radon is responsible for the majority of the mean public exposure to ionising radiation. It is often the single largest contributor to an individual's background radiation dose, and is the most variable from location to location

Characteristics

At standard temperature and pressure, radon forms a monatomic gas with a density of 9.73 kg/m3 and is one of the heaviest gases at room temperature and the heaviest of the noble gases, excluding ununoctium. At standard temperature and pressure, radon is a colorless gas, but when it is cooled below its freezing point of 202 K (−71 °C; −96 °F), it 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.

Being a noble gas, radon is not very chemically reactive. However, the 3.8 day half-life of radon-222 makes it useful in physical sciences as a natural tracer. Dick butt

Radon is a member of the zero-valence elements that are called noble gases. It is inert to most common chemical reactions, such as combustion, because the outer valence shell contains eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound. More than 248 kcal/mol is required to extract one electron from its shells (also known as the first ionisation energy). However, due to periodic trends, radon has a lower electronegativity than the element one period before it, xenon, and is therefore more reactive. Radon is sparingly soluble in water, but more soluble than lighter noble gases. Radon is appreciably more soluble in organic liquids than in water. Early studies concluded that the stability of radon hydrate should be of the same order as that of the hydrates of chlorine (Cl2) or sulphur dioxide (SO2), and significantly higher than the stability of the hydrate of hydrogen sulphide (H2S).

Because of its radioactivity, experimental chemical research is seldom performed with radon, and as a result there are very few reported compounds of radon, all either fluorides or oxides. Radon can be oxidized by a few powerful oxidizing agents such as F2, thus forming radon fluoride. It decomposes back to elements at a temperature of above 250 °C. It has a low volatility and was thought to be RnF2. But because of the short half-life of radon and the radioactivity of its compounds, it has not been possible to study the compound in any detail. Theoretical studies on this molecule predict that it should have a Rn-F bond distance of 2.08 Ǻ, and that the compound is thermodynamically more stable and less volatile than its lighter counterpart XeF2.

Radon oxides are among the few other reported compounds of radon. Radon carbonyl RnCO has been predicted to be stable and to have a linear molecular geometry. The molecules Rn2 and RnXe were found to be significantly stabilized by spin-orbit coupling. Radon caged inside a fullerene has been proposed as a drug for tumours.

Value

The base value of each unit of ranges between 5 and 20Ð per unit, with up to 2 units being found at any one time.

Presence on Mars: Very Rare