Chemical Properties
colourless gas
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.
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.
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.
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.
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.
