Radon (Rn): Properties & Uses

Radon is a naturally occurring radioactive element symbolized by Rn and atomic no. 86 in the periodic table. It is the heaviest known noble gas in the periodic table, nine times denser than air. It is obtained from the natural radioactive decay of uranium and other radioactive elements that are present in soil and rocks. After escaping from the earth’s crust, radon decays further into other radioactive elements. Radon poses a major threat to human health, and therefore, it is a matter of great concern to all human beings. Being a monoatomic gas, radon can easily penetrate the walls and windows of a building. As we breathe, radon particles get accumulated inside our body and directly alters our DNA, thereby causing lung cancer. After smoking, radon is the largest known cause of lung cancer around the world.

 

Discovery and Naming

The discovery of radon presents an interesting case in the literal history of chemical elements, as to who should be credited for its discovery. An internet search shows that over 90% of sites represent Fredrich Ernst Dorn, a German physicist, as the discoverer of Radon. Some of the remaining websites suggest Ernst Rutherford as the discoverer of radon, while others assert that Perie Curie and Marie Curie should be given credit for its discovery. In 1899, Perie Curie and Marie Curie, while studying the properties of radium, noticed that a gas emitted by radium remained radioactive for a month. Later that year, Ernst Rutherford and Robert B Owens were trying to measure the radiations from thorium oxide at McGill University in Montreal, when they noticed that thorium emits a gas that remains radioactive for several minutes. They called it thorium emanation (“Th Em”). A year later, in 1900, Fredrich Ernst Dorn noticed the emanation of radium and named it (“Ra Em”). Another year later, Rutherford and Harriet Brooks successfully demonstrated that the radium emanations were radioactive, but they credited Curie’s study for their discovery. In the year 1903, Andre-Louis Debierne discovered similar emanation for actinium and named it (“Ac Em”). Struggle for a shorthand notation for these emanations continued for more than a decade until, eventually, in 1920, they were named “radon,” “thoron,” and “actinon,” respectively.

When William Ramsay studied the spectrum of these emanations, he noticed the similarity between the noble gas family and these radiations. So, he positioned radon in the 18th group of the periodic table with atomic no. 86. On further studies, it was found that thoron and actinon are isotopes of radon. However, the name actinon is rarely encountered today due to its relatively short half-life. Certainly, the credit for the discovery of the element no. 86 in the periodic table doesn’t go to one person but to many including Fredrich Ernst Dorn, Ernst Rutherford, Robert B Owens, Harriet Brooks, Andre-Louis Debierne, William Ramsay, Perie Curie, and Marie Curie

Occurrence

Radon is present everywhere in our environment in distinctive concentrations. It is produced naturally by the radioactive decay of uranium and radium which are mainly found in uranium ores, phosphate rock, shales, igneous and metamorphic rocks, such as granite, gneiss, and schist. Relatively minute traces can also be found in common rock such as limestone. Uranium 238 is an unstable isotope and decays to produce thorium that further goes under radioactive decay to produce radium, and finally, radium decays to produce radon

_{92 }^{ 238 }{ U }→ _{ 90 }^{ 234 }{Kr } + _{ 2 }^{ 4 }{ He }^{ 2+ }

_{90 }^{ 232 }{Kr }→ _{ 88 }^{ 228 }{Ra } + _{ 2 }^{ 4 }{ He }^{ 2+ }

_{ 88}^{ 226 }{Ra }→ _{ 86 }^{ 222 }{Rn } + _{ 2 }^{ 4 }{ He }^{ 2+ }

_{ 86 }^{ 222 }{Rn } has a half-life of 3.8 days, which is sufficient enough for it to escape into the atmosphere through the soil. Radon is found in natural sources only because of its continuous accumulation from the radioactive decay of its longer-lived predecessors in minerals containing uranium or thorium. When it comes to general nomenclature, the word “radon” usually refers to _{  }^{ 222 }{Rn }. Although _{  }^{ 222 }{Rn } and _{  }^{ 220 }{Rn } have same average rate of production from their parent nuclei thorium,  _{  }^{ 220 }{Rn } is present in much less concentrations than _{  }^{ 222 }{Rn } in the atmosphere due to its shorter half-life period of 55 seconds. The average concentration of radon in the earth’s atmosphere is very less, about 150 atoms per millilitre of air. However, it is by far the largest contributor to radiation pollution in the atmosphere. Radon is a monoatomic volatile gas; therefore, naturally occurring groundwater repositories have a higher concentration of radon content than the surface water. Hot springs and spring waters are found rich in radon content in countries like Germany and Japan. All other minerals and resources extracted out of the earth’s surface, such as petroleum, water-wells, oil, and natural gas, contain minute amounts of radon.

 

Detection

Radon is a prime culprit when it comes to radiation exposure of populations around the world. Therefore, a majority of techniques and instruments have been designed to measure human exposure to radon gas. Radon, present in our atmosphere, is too low in concentration to be detected chemically. Moreover, radon accumulation can also depend on meteorological interferences, i.e., a long time flow of wind in one direction can increase the radon concentration to a concerning level. Based on these observations, radon measurement methods are divided into two categories, active and passive methods. These methods are further categorized into four variations based on the duration of the measurement.

• Grab sample – seconds or minutes.

• Short-term average – days.

• Long-term average – weeks or months.

• Continuous – integration over a period of minutes or hours, repeated indefinitely.

Some of these methods are mentioned below.

1. Scintillation cells

Scintillation cells are a widespread technology for measuring radon concentration in the gas (typically air) samples. This instrument was developed by Sir Lucas in 1957. Scintillators are the devices used to change the kinetic energy of a charged particle into a light signal. Scintillation cells is a cylinder with a cap at one end. This cap acts as a filter to the air that enters the cell, thus, preventing radon decay products to enter. Once filled, radon inside the cell starts decaying to form decay products. For every decay, there are three alpha particles emitted, which interact with the walls of the cylinder covered with alpha scintillation material ZnS(Ag). The light produced by these interactions then goes into a photomultiplier tube for amplification, and this amplified signal is then fed into associated electronics. Data from these electronics is then analyzed as counts over a period of time. Based on this data, the radon concentration in an air sample can be determined.

2. Ionization Chambers

The ionization chamber is a cheaper alternative to detect radon in a gas sample. High energy alpha particles emitted from the radioactive decay of radon in a filtered sample of air can cause ionization of the gas inside the chamber. When a potential difference is applied with the help of two electrodes attached at both ends of the chamber, ionized atoms start to flow in one direction, thereby forming an electric current. The value of this current will then be measured by associative electronic devices and will show us the presence of radon in the sample. This method is not as efficient as the scintillation cell because gas molecules present inside the gas chamber can be ionized by other high energy particles such as beta particles and gamma rays. Though with the help of proper electronics, a threshold value can be set to precisely determine the type of radiation.

3. Electrostatic Collection of Decay Products

This method of radon detection is based widely upon the physics of semiconductor devices. They are mainly used for a long-term analysis of radon concentration in a particular region. Semiconductor devices can be used to track the electron-hole pairs generated along the path of a charged particle. For Radon detection, alpha particles produced as the progeny of radon decay are collected on a semiconductor-based screen, which can give us readings on how much increase in radon concentration is there over a period of time.

 

Isotopes

There are 39 isotopes of Radium, ranging from mass no. 193 to 231, out of which _{ }^{ 219 }{ Ra }, _{ }^{ 220 }{ Ra }, and _{ }^{ 222 }{ Ra } are naturally occurring. _{ }^{ 222 }{ Ra }, commonly addressed as radon, is a natural decay product of the most long-lived isotope of uranium ( _{ }^{ 238 }{ U }). Whereas, _{ }^{ 220 }{ Ra }, commonly referred to as thoron,  is a natural decay product of the most long-lived isotope of thorium ( _{ }^{ 232 }{ Th }). The half-life of _{ }^{ 222 }{ Ra } and _{ }^{ 220 }{ Ra } is 3.8 days and 55 secs, respectively. _{ }^{ 222 }{ Ra } decays via alpha decay to form a radioactive isotope of polonium,_{ }^{ 218 }{ Po }, whereas _{ }^{ 220 }{ Ra } decays to another radioisotope of polonium _{ }^{ 216 }{ Po }. Third naturally occurring radioisotope of radon, _{ }^{ 219 }{ Ra }, generally referred to as actinon, is a decay product of the most stable isotope of actinium (_{ }^{ 227 }{ Ac }). There are three other synthetic isotopes of radon that have a half-life of over an hour: _{ }^{ 210 }{ Ra },_{ }^{ 211 }{ Ra }, and _{ }^{ 224 }{ Ra }. Other isotopes of radon are too short-lived to have any real significance.

Properties of Radon

 

The most interesting fact about radon is that although it’s gas, all its decay-products are metals. Radon (Rn) is a colourless, tasteless, and odourless gas at standard pressure and temperature conditions, and it is the densest noble gas known. At normal temperature and pressure conditions, radon generally exists in a gaseous state.

Physical Properties

At normal temperature and pressure conditions, radon generally exists as a monoatomic gas with a gas density of about 9.73 kilograms per cubic meter, about 8 times the density of air. Although radon is a colourless gas at room temperature, once it gets below its freezing point (−71°C), it shows radioluminescence by emitting yellow coloured light. As we lower the temperature further, the yellow colour changes to a bright orange-red colour. The boiling point of radon is -61°C, which makes it highly volatile. Radon is compactly soluble in water as compared to its solubility in other noble gases. Furthermore, radon can dissolve easily in most of the organic compounds.

Chemical Properties

Radon belongs to the noble gas group of the periodic table with atomic no. 84 and has an electronic configuration of [Xe] 4{ f }^{ 14 }5{ d }^{ 10 }6{ s}^{ 2 }6{ p }^{ 6 }, and it is chemically inert but radioactive. The problem scientists encounter while studying the chemistry of radon is that it has a very short half-life, and hence, it is explored by tracer methods. The outermost shell of radon consists of eight tightly bound electrons, which provides it with the first ionization energy of 1034Kj/mol. For a long time, radon was found unreactive in many attempts, but after the discovery of the first noble gas compound, xenonplatino fluoride, and other subsequent noble gas compounds, it has been found that radon can also form compounds with fluorine and other complex salts.

1. Clathrates

A clathrate is a structure in which water molecules under certain conditions bond to form complex networks of molecules forming cage-like structures that encapsulate a guest molecule, which is a gas. Radon forms a series of clathrates when mixed in trace amounts with relatively larger amounts of other substances such as sulphur dioxide, hydrogen sulphide, phenol, p-chlorophenol, and water after its phase transition. This method of selective formation of radon hydrates can also be used to separate radon from a mixture of noble gases.

2. Radon Fluorides

The radioactive nature and a short-lived half-life of radon make it harder for scientists to conduct research on radon compounds. However, scientists have managed to form difluorides of radon in very minute quantities, under a very isolated and precise procedure. The products of these fluorination reactions have not been analyzed yet because of their small mass and intense radioactivity. Nevertheless, by comparing reactions of radon with those of krypton and xenon, it is safe to deduce that radon forms a difluoride, RnF2, and derivatives of the difluoride.

{ Rn } + { F }_{ 2 } →  { RnF }_{ 2 }

Theoretically, radon can form other compounds also with halogens and organic compounds, but due to its intense radioactivity, scientists can not prove those predictions to be true.

Uses of Radon

Ironically, the radioactive nature of radon made it employable also. All the applications of radon in the society are based on its radioactivity only. Let’s discuss a few of its uses.

1. Medical

In the early 20th century, people were not aware of the carcinogenic properties of radioactive elements, so they used to practice a pseudoscientific therapy known as “radiotorium” in which they used to take a radioactive spa to cure their illness. Later, this quackery led a foundation for cancer treatment with the help of radioactive elements. For some time, the ionization property of alpha particles emitted by radon’s radioactive decay is used to kill cancerous cells in humans. Limited exposure of radon in a controlled environment was also used to treat autoimmune diseases such as arthritis. This process was known as radiation hormesis. Later on, better and safe alternatives replaced the use of radon for such applications.

2.Scientific

Due to their intense radioactivity, it is not wise to employ radioactive elements for general public use. However, in the realm of science and technology, radioactive elements have made revolutionary breakthroughs. In meteorology, scientists study the variations in radon emanations from the soil to gather data about the uranium content in the soil and then use that data to track air masses. The study of the movement of water makes use of radon content in the water to track that if there is are any interaction between the stream and the groundwater flow.

It has always been a prime interest for human beings to predict any natural disaster so that they can take preventive measures before such clamities. The closest approach to detect earthquakes has been made possible only because of radon. Scientists study the changes in groundwater radon concentrations and geothermal gradients associated with them to predict the time and the location coordinates of the epicentre of an earthquake. However, scientists are not completely successful in making exact predictions, but ongoing research shows promising potential.

Health Aspects

The relation between radon exposure and lung cancer is well-known across the globe. The first reported casualties due to radon exposure go back to the 16th century when miners in central Europe were dying in the prime years of their life. Radioactivity was not discovered at that time, and for the subsequent few hundred years, it remained unclear as to what was the cause of those deaths. In 1879, Harting and Hesse performed an autopsy on a miner who had died of mala metal forum and identified mala metal forum as lung cancer. Few years after the First World War, significant no. of deaths were reported in Czechoslovakia miners due to lung cancer. On inspection of the mine, it was found that the mine has a high radon concentration. Since then, radon exposure has become a major concern for people all around the world. According to international agency on research for cancer, _{ }^{ 220 }{ Rn } is classified as a carcinogenic element. Epidemiological studies regarding residential radon exposure and the risk of lung cancer in the general population usually use the average concentration of radon gas per cubic meter (Bq/m3 or pCi/L, where 1 pCi/L is equal to 37 Bq/m3) of indoor air throughout individual residence.

In addition to lung cancer, scientists also think that radon exposure can also be the potential reason for leukaemia, but there are no practical pieces of evidence to support these theories.