Radon

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86 astatineradonfrancium
Xe

Rn

Uuo
Rn-TableImage.png
General
Name, Symbol, Number radon, Rn, 86
Chemical series noble gases
Group, Period, Block 18, 6, p
Appearance colorless
Atomic mass (222) g/mol
Electron configuration [Xe] 4f14 5d10 6s2 6p6
Electrons per shell 2, 8, 18, 32, 18, 8
Physical properties
Phase gas
Melting point 202 K
(-71 °C, -96 °F)
Boiling point 211.3 K
(-61.7 °C, -79.1 °F)
Critical point 377 K, 6.28 MPa
Heat of fusion 3.247 kJ/mol
Heat of vaporization 18.10 kJ/mol
Heat capacity (25 °C) 20.786 J/(mol·K)
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 110 121 134 152 176 211
Atomic properties
Crystal structure cubic face centered
Oxidation states 0
Electronegativity no data (Pauling scale)
Ionization energies 1st: 1037 kJ/mol
Atomic radius (calc.) 120 pm
Covalent radius 145 pm
Miscellaneous
Magnetic ordering non-magnetic
Thermal conductivity (300 K) 3.61 mW/(m·K)
CAS registry number 10043-92-2
Notable isotopes
Main article: Isotopes of radon
iso NA half-life DM DE (MeV) DP
211Rn syn 14.6 h Epsilon 2.892 211At
Alpha 5.965 207Po
222Rn 100% 3.824 d Alpha 5.590 218Po

Radon (chemical symbol Rn, atomic number 86) is a chemically inert but highly radioactive noble gas. It is formed by the disintegration of radium and is one of the densest gases known. In some places, it accumulates in buildings, drinking water, and underground mines and caves. At significant concentrations, it is a health hazard, as it can cause cancer. It can, however, be used in controlled doses to treat cancer by radiation therapy. In addition, radon concentrations in streams and rivers can serve to indicate the inflow of groundwater, and concentrations in the soil can help map subsurface geological faults.

Contents

Discovery

Radon (named after radium) was discovered in 1900 by Friedrich Ernst Dorn, who called it radium emanation. William Ramsay and Robert Whytlaw-Gray isolated it in 1908 and named it niton (from the Latin word nitens, meaning "shining"). When they determined its density, they found that it was the heaviest gas known as of then. It has been called radon since 1923.

Occurrence

The concentration of radon in the Earth's atmosphere is extremely low: On average, there is one atom of radon in 1 x 1021 molecules of air. This gas can, however, be found at significant concentrations in some spring waters and hot springs. For example, the towns of Misasa, Tottori prefecture, Japan, and Bad Kreuznach, Germany, have radium-rich springs that emit radon.

In certain regions, radon exhausts naturally from the ground. Many of these regions have granitic soils, but not all granitic regions are prone to high emissions of radon. Depending on how houses are built and ventilated, radon may accumulate in basements of dwellings.

Radon emitted from the ground has been shown to accumulate in the air if there is a meteorological inversion and little wind. Concentrations may exceed legal guidelines for short periods. It is not clear that any health effects would be epidemiologically detectable.

In the United States, the National Council on Radiation Protection and Measurement (NCRP) recommends action for any house with a radon concentration higher than eight picocuries per liter (pCi/L). The U.S. Environmental Protection Agency (EPA) recommends action for any house with a radon concentration higher than 148 becquerels per cubic meter (Bq/m3) (given as four pCi/L), and encourages action starting at 74 Bq/m3.[1] According to their statistics, nearly one in 15 homes in the United States has a high level of indoor radon. The U.S. Surgeon General and EPA recommend that all homes be tested for radon. Since 1985, millions of homes have been tested for radon in the United States.

Health Canada has a 1988 guideline from 1988 that recommends action when the annual average concentration in a normal living area exceeds 800 Bq/m3, but they are proposing a new guideline that lowers the action level to 200 Bq/m3.[2] The European Union recommends that action should be taken starting from concentrations of 400 Bq/m3 for old houses and 200 Bq/m3 for new ones.

Notable characteristics

In the periodic table, radon is at the bottom of the noble gas group, that is, group 18 (former group 8A or group 0). It lies in period six, after astatine (At). Thus it is the heaviest noble gas (has the highest density among noble gases) and one of the heaviest gases at room temperature. (Currently, the densest known gas is uranium hexafluoride, UF6.)

At ordinary temperatures and pressures, radon is colorless. It is essentially chemically inert, but all its isotopes are radioactive. When cooled below its freezing point, it produces a bright phosphorescence that turns yellow as the temperature is reduced, and it then becomes orange-red at the temperature when air liquefies.

Natural radon concentrations in the Earth's atmosphere are so low that radon-rich water in contact with the atmosphere will continually lose the gas by volatilization. Consequently, groundwater has a higher concentration of Rn-222 than surface water. Likewise, the saturated zone of a soil frequently has a higher radon content than the unsaturated zone, as a result of diffusional losses to the atmosphere.

Isotopes

There are 20 known isotopes of radon, of which the most stable one is radon-222. It is a decay product (daughter product) of radium-226 and has a half-life of 3.823 days. As it decays, it emits alpha particles. Radon-220 is a natural decay product of thorium and is called thoron. It has a half-life of 55.6 seconds and also emits alpha rays. Radon-219 is derived from actinium and is called actinon. It emits alpha particles and has a half-life of 3.96 seconds.

Compounds

As an inert gas, radon forms few compounds. Some researchers have found that radon reacts with fluorine to form radon fluoride. Others have reported the production of radon clathrates, which are composed of cage-like molecules containing radon atoms. Nonetheless, the rapid radioactive decay of radon limits the usefulness of its compounds.

Applications

As radon is highly radioactive and its isotopes have short half-lives, it is useful for treating cancer by radiotherapy, under carefully controlled conditions.

Radon is used in hydrologic research, when studying the interactions between groundwater, streams, and rivers. This is based on the knowledge that radon in surface waters is rapidly lost to air, while radon remains in groundwater for longer periods. Any significant concentration of radon in a stream or river is a good indicator that there are local inputs of groundwater.

The concentration of radon in the soil has been used experimentally to map subsurface geological faults, because concentrations are generally higher over the faults. Similarly it has found limited use in geothermal prospecting.

Some researchers have checked to see if rapid changes in soil radon concentrations or elevated levels of radon in the soil can be used as predictors for earthquakes. Their results have been unconvincing but may have some limited usefulness in specific locations.

Radon emanation from the soil varies with soil type and surface uranium content, so outdoor radon concentrations can be used to track air masses to a limited degree. This fact has been put to use by some atmospheric scientists.

Controversy about "radon therapy"

Medically unsupervised "radon therapy," involving exposure to ionizing radiation from radon, is a controversial activity. At some "radon spas" in the United States and Europe, people sit for minutes or hours in a high-radon atmosphere in the belief that airborne radiation will invigorate or energize them. Likewise, there are hot water spas in places like Misasa, Tottori prefecture, Japan, where the water is naturally rich in radium and exhales radon. Drinking therapy is applied in Bad Brambach, Germany. Inhalation therapy is carried out in Gasteiner-Heilstollen, Austria, in Kowary, Poland, and in Boulder, Montana, United States. The benefits of this type of radiation exposure have been disputed, in the absence of rigorous scientific testing. Support for this activity is based on the theory of "radiation hormesis," which postulates that low doses of ionizing radiation may be beneficial, possibly by activating genes that help repair damage.

Toxicity

Radon is a radiological poison and carcinogen. Some of the daughter products from the radioactive decay of radon (such as polonium) are also toxic. As radon is a gas, its decay products form a fine dust that is both toxic and radioactive. This dust can potentially stick in the lungs and do far more damage than the radon itself. Based on studies carried out by the National Academy of Sciences in the United States, radon is the second leading cause of lung cancer after cigarette smoking, accounting for 15,000 to 22,000 cancer deaths per year in the US alone according to the National Cancer Institute (USA).

Moreover, radon can also be present in tobacco smoke, in cases where a phosphate matrix containing substantial concentrations of uranium is used for fertilizing the source tobacco. Radon is a daughter product of the decay of uranium. Many phosphate deposits have 30 to 100 times the concentrations of uranium as typical soils. The exposure to radioactivity from inhaled radon and its daughter products is thought to be the source of malignant changes.

Radon accumulates in underground mines and caves. Good ventilation should therefore be maintained in mines. In some countries, guides in tourist caves are classified as radiation workers and their time of exposure is monitored. Nonetheless, tourism of caves is generally not considered a significant hazard for the relatively brief visits by members of the general public.

Radon is a known pollutant emitted from geothermal power stations, but it disperses rapidly, and no radiological hazard has been demonstrated in various investigations. The trend in geothermal plants is to reinject all emissions by pumping deep underground, and this seems likely to decrease such radon hazards further.

The first major studies of the health effects of radon were carried out in the context of uranium mining, first in the Joachimsthal region of Bohemia and then in the American Southwest during the early years of the Cold War. Given that radon is a daughter-product of uranium, uranium mines have high concentrations of radon and its highly radioactive decay products. Many miners—including Native Americans, Mormons, and others in the Four Corners region—contracted lung cancer and other pathologies after exposure to high levels of radon gas while mining uranium for the U.S. Atomic Energy Commission in the mid-1950s. Safety standards required expensive ventilation and were not widely implemented or policed.

The danger of radon exposure in homes was discovered in 1984, with the case of Stanley Watras, an employee at the Limerick nuclear power plant in Pennsylvania. Watras set off radiation detectors on his way into work for two weeks straight, while authorities searched for the source of the contamination. They were shocked to find that the source was astonishingly high levels of radon in the basement of his house, and it was unrelated to the nuclear plant. The risks associated with living in his house were estimated to be equivalent to smoking 135 packs of cigarettes every day. Following this discovery, which was highly publicized, national radon safety standards were set, and radon detection and ventilation became a standard concern for homeowners.

The exact danger posed by indoor radon is debated by experts. Though radon is cited as the second leading cause of lung cancer (after cigarette smoking), the numbers are still relatively low and are often entangled with those of smoking. (The combination of radon gas exposure and cigarette smoking drastically increases lung cancer rates.) It is unknown why indoor radon seems to have been a problem only since the 1980s. Some experts have hypothesized that it is the result of better housing construction, which allows radon to accumulate rather than seep back into the natural environment.

Notes

  1. EPA radon risk level tables, including comparisons to other risks encountered in life Retrieved December 20, 2007.
  2. Health Canada Retrieved December 20, 2007.

References

External links

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