radon

Description

Radon, Rn, is a gaseous radioactive element from the noble gases in family eight on the periodic table. There are 18 radioactive isotopes of radon, all of which have short half-lives. For example, radon 222 has a half-life of 3.8 days. Radon is a colorless gas that is soluble in water. It can be condensed to a colorless transparent liquid and to an opaque, glowing solid. Radon is the heaviest gas known, with a density of 9.72 g/L at 32°F.

Chemical Properties

colourless gas

Physical properties

Radon gas fits the criteria to be classed as a noble element located in group 18(VIIIA) orgroup 0. It is the only noble “inert” gas that is naturally radioactive. It is the heaviest of thegases in group 18.
Radon gas is easily converted to a liquid and will become solid at the relatively hightemperature of –71°C. As a solid, it glows with a yellow light. Its melting point is –71°C, itsboiling point is –62°C, and its density is 0.00973g/cm³.

Isotopes

There are 37 isotopes of radon. All are radioactive. None are stable. They rangein mass numbers from Rn-196 to Rn-228. Their half-lives range from a few microsecondsto 3.8235 days for Rn-222, which is the most common. It is a gas that is the resultof alpha decay of radium, thorium, or uranium ores and underground rocks.

Origin of Name

Originally named “niton” after the Latin word for “shining,” it was given the name “radon” in 1923 because it is the radioactive decay gas of the element radium.

Occurrence

Radon’s source is a step in the transmutation of several elements: uranium → thorium →radium → radon → polonium → lead. (There are a number of intermediate decay productsand steps involved in this process.) Radon-222 forms and collects just a few inches below thesurface of the ground and is often found in trapped pockets of air. It escapes through poroussoils and crevices.

Characteristics

Radon is the heaviest of the noble gases and is the only one that is radioactive. It is thedecay product of radium, thorium, and uranium ores and rocks found underground. As itdecays, it emits alpha particles (helium nuclei) and is then transmuted to polonium andfinally lead. The Earth’s atmosphere is just 0.0000000000000000001% radon, but becauseradon is 7.5 times heavier than air, it can collect in basements and low places in buildingsand homes.

History

Radon was discovered in 1900 by Dorn, who called it radium emanation. In 1908 Ramsay and Gray, who named it niton, isolated the element and determined its density, finding it to be the heaviest known gas. It is essentially inert and occupies the last place in the zero group of gases in the Periodic Table. Since 1923, it has been called radon. Thirty-seven isotopes and isomers are known. Radon-222, coming from radium, has a half-life of 3.823 days and is an alpha emitter; Radon-220, emanating naturally from thorium and called thoron, has a half-life of 55.6 s and is also an alpha emitter. Radon-219 emanates from actinium and is called actinon. It has a half-life of 3.9 s and is also an alpha emitter. It is estimated that every square mile of soil to a depth of 6 inches contains about 1 g of radium, which releases radon in tiny amounts to the atmosphere. Radon is present in some spring waters, such as those at Hot Springs, Arkansas. On the average, one part of radon is present to 1 × 1021 part of air. At ordinary temperatures radon is a colorless gas; when cooled below the freezing point, radon exhibits a brilliant phosphorescence which becomes yellow as the temperature is lowered and orange-red at the temperature of liquid air. It has been reported that fluorine reacts with radon, forming radon fluoride. Radon clathrates have also been reported. Radon is still produced for therapeutic use by a few hospitals by pumping it from a radium source and sealing it in minute tubes, called seeds or needles, for application to patients. This practice has now been largely discontinued as hospitals can order the seeds directly from suppliers, who make up the seeds with the desired activity for the day of use. Care must be taken in handling radon, as with other radioactive materials. The main hazard is from inhalation of the element and its solid daughters, which are collected on dust in the air. Good ventilation should be provided where radium, thorium, or actinium is stored to prevent build-up of this element. Radon build-up is a health consideration in uranium mines. Recently radon build-up in homes has been a concern. Many deaths from lung cancer are caused by radon exposure. In the U.S. it is recommended that remedial action be taken if the air from radon in homes exceeds 4 pCi/L.

Uses

To initiate and influence chemical reactions, as a surface label in the study of surface reactions; in the determination of radium or thorium; in the study of the behavior of filters; in combination with Be or other light materials as a source of neutrons.

Uses

Radon’s main use is as a short-lived source of radioactivity for medical purposes. It iscollected from the decay of radium as a gas and sealed in small glass capsules that are theninserted at the site of the cancer. It is also used to trace leaks in gas and liquid pipelines and tomeasure their rate of flow. The rate at which radon gas escapes from the Earth is one measurementthat helps scientists predict earthquakes.

Definition

A colorless monatomic radioactive element of the rare-gas group, now known to form unstable compounds. It has 19 short-lived radioisotopes; the most stable, radon-222, is a decay product of radium-226 and itself disintegrates into an isotope of polonium with a half-life of 3.82 days. ²²²Rn is sometimes used in radiotherapy. Radon occurs in uranium mines and is also detectable in houses built in certain areas of the country. Symbol: Rn; m.p. –71°C; b.p. –61.8°C; d. 9.73 kg m^–3 (0°C); p.n. 86.

Definition

radon: Symbol Rn. A colourlessradioactive gaseous element belongingto group 18 of the periodic table(the noble gases); a.n. 86; r.a.m. 222;d. 9.73 g dm–3; m.p. –71°C; b.p.–61.8°C. At least 20 isotopes areknown, the most stable beingradon–222 (half-life 3.8 days). It isformed by decay of radium–226 and undergoes alpha decay. It is used inradiotherapy. Radon occurs naturally,particularly in areas underlain bygranite, where it is thought to be ahealth hazard. As a noble gas, radonis practically inert, although a fewcompounds, e.g. radon fluoride,can be made. It was first isolatedby William Ramsey and RobertWhytlaw-Gray (1877–1958) in 1908.

Production Methods

Radon can be isolated from radium by several methods. An aqueous solution of radium salt such as radium bromide is heated, liberating radon. Radioactive bombardment then decomposes water to oxygen and hydrogen. Radon is separated from the gaseous mixture by condensation in tiny tubes placed in liquid air. The tubes then are sealed by melting. A gold or platinum coating is applied to form the “radon seeds” used in radiation therapy.
Alternatively, a slightly acid solution of a soluble radium salt such as chloride or bromide is placed in a soft-glass vessel behind lead shielding. The solution is boiled. Radon is pumped out as needed and frozen into a cold trap at -95°C. Hydrogen and oxygen are the main impurities generated from radiolytic decomposition of water. They are recombined by sparking or applying a hot wire. Carbon dioxide, water vapor, acid vapor, and hydrocarbon impurities are removed by various chemical methods.
Radon can be obtained from radium salts in the solid phase too. At ordinary temperatures, certain mixtures containing radium salts such as radium mixed with barium, radium palmitates, or gels of radium mixed with iron(III) hydroxide or aluminum(III) hydroxide efficiently release radon. Although anyradium salt would emit radon, the latter can diffuse very slowly at room temperature through the walls of the container vessels. However, when a radium salt is heated above 600°C radon diffuses rapidly through the solid container vessels and escapes.
Radon also may be separated from gas streams by adsorption on activated charcoal or silica gel. At temperatures colder than dry ice, charcoal is an excellent adsorbent. Radon may be desorbed by heating the adsorbent in vacuum at 350°C.

Definition

Gaseous radioactive element. Atomic number 86; noble gas group of periodic table; aw 222; valences = 2, 4, (6); 18 radioactive isotopes, all short-lived. The radon-222 isotope has a half-life of 3.8 days, emits α-radiation.

Hazard

As for radium.

Hazard

The major hazard from radon stems from its radiation of alpha particles, even though alphaparticles (helium nuclei) can be stopped by a sheet of cardboard. When Rn-222 is inhaled, itdecays into lead-210, which is also radioactive, and because it not easily exhaled, it remainsin the lungs for long periods, causing lung cancer. It is estimated that about 10% of all lungcancers are cause by radon.
Homes with cracked concrete slabs or dirt basements are at risk for radon contamination.Radon is generated just under the surface of the Earth and can seep through the floors andwalls. If the ventilation is poor in the basement, the gas can accumulate to a dangerous level.Inexpensive kits that measure the levels of radon in homes and businesses are available commercially.
Tobacco plants accumulate radon from the soil. Uranium from the phosphate fertilizerused on the plants is also another source of radiation. Small amounts of lead-210 are spread onthe tobacco leaves. Thus, smokers are exposed to levels of radiation that is about 1,000 timeshigher than the radiation exposure of workers in nuclear power plants.
Some authorities attribute the increase in lung cancer in nonsmokers to radon gas poisoning.Several studies found no positive correlation between the rate of cancer and the numberof homes that had been exposed to radon for large groups of individuals in several countries.A seeming incongruity is the fact that radon does have some usefulness as a radiation sourcefor some cancer treatments.

Safety Profile

A carcinogen. A common air contaminant. Radon is a noble gas and thus is relatively unreactive. Radiation Hazard: Natural isotope *zORn (Thoron, Thorium Series), T1/2 = 55 seconds, decays to radoactive 2'6Po by alphas of 6.3 MeV. Natural isotope 222Rn (Uranium Series), T1/2 = 3.8 days, decays to radioactive permissible levels are gven for 222Rn in equilibrium with its daughters. The chief hazard from this isotope is inhalation of the gaseous element and its solid daughters, whch are collected on the normal dust of the air. Ths material is deposited in the lungs and has been considered to be a major causative agent in the hgh incidence of lung cancer found in uranium miners. Radon and its daughters bdd up to an equhbrium value in about a month from radum compounds, whde the bdd-up from uranium compounds is negligble. Good ventilation of areas where radium is handled or stored is recommended to prevent accumulation of hazardous concentration of Rn and its daughters. Accumulation of radon in homes has been implicated in increased incidence of lung cancers. This accumulation is found in wellinsulated buildtngs located over land that has concentrations of uranium

Carcinogenicity

Radon and its isotopic forms radon-222 and radon-220 are known to be human carcinogens based on sufficient evidence of carcinogenicity from studies in humans.

Source

Uranium-238 is present in small amounts in most rocks and soil. Uranium has a half-life of 4.5 billion years.1 It decays to other elements such as radium, which breaks down to radon. Some of the radon moves to the soil surface and enters the air, whereas some remains below the soil surface and enters the groundwater.
Radon-222 also undergoes radioactive decay and has a radioactive half-life of 3.8 days. Radon-220 and -219 have half-lives measured in seconds and are not nearly as abundant as Radon-222. Thus the discussion of radon health effects here centers on Radon-222. Radon-222 decays into radon daughters or progeny, which are radioactive elements. Two of these (polonium-218 and polonium-214) emit alpha particles (high-energy, high-mass particles, each consisting of two protons and two neutrons), which are highly effective in damaging lung tissues. The decay rate of radioactive elements has traditionally been specified in curies (Ci). The curie is approximately 37 billion disintegrations (37 ￥ 109 disintegrations) per second. In discussing radon, the picocurie (pCi) is used, where 1 pCi is equal to 1 × 10^-12 Ci.

Raw materials

Preparation Products