Polonium is a chemical element; it has symbol Po and atomic number 84. A rare and highly radioactive metal (although sometimes classified as a metalloid) with no stable isotopes, polonium is a chalcogen and chemically similar to selenium and tellurium, though its metallic character resembles that of its horizontal neighbours in the periodic table: thallium, lead, and bismuth. Due to the short half-life of all its isotopes, its natural occurrence is limited to tiny traces of the fleeting polonium-210 (with a half-life of 138 days) in uranium ores, as it is the penultimate daughter of natural uranium-238. Though two longer-lived isotopes exist (polonium-209 with a half-life of 124 years and polonium-208 with a half-life of 2.898 years), they are much more difficult to produce. Today, polonium is usually produced in milligram quantities by the neutron irradiation of bismuth. Due to its intense radioactivity, which results in the radiolysis of chemical bonds and radioactive self-heating, its chemistry has mostly been investigated on the trace scale only.
Polonium was discovered on 18 August 1898 by Marie Skłodowska-Curie and Pierre Curie, when it was extracted from the uranium ore pitchblende and identified solely by its strong radioactivity: it was the first element to be discovered in this way. Polonium was named after Marie Skłodowska-Curie's homeland of Poland, which at the time was partitioned between three countries. Polonium has few applications, and those are related to its radioactivity: heaters in space probes, antistatic devices, sources of neutrons and alpha particles, and poison (e.g., poisoning of Alexander Litvinenko). It is extremely dangerous to humans.
Characteristics Po is an alpha emitter that has a half-life of 138.4 days; it decays directly to its stable daughter isotope, Pb. A milligram (5 curies) of Po emits about as many alpha particles per second as 5 grams of Ra, which means it is 5,000 times more radioactive than radium. A few curies (1 curie equals 37 gigabecquerels, 1 Ci = 37 GBq) of Po emit a blue glow which is caused by ionisation of the surrounding air.
About one in 100,000 alpha emissions causes an excitation in the nucleus which then results in the emission of a gamma ray with a maximum energy of 803 keV.Greenwood, p. 250
Solid state form
Polonium is a radioactive element that exists in two metallic allotropes. The alpha form is the only known example of a simple cubic crystal structure in a single atom basis at STP (space group Pmm, no. 221). The unit cell has an edge length of 335.2 picometers; the beta form is rhombohedral.Greenwood, p. 753 The structure of polonium has been characterized by X-ray diffraction and electron diffraction.
Po has the ability to become airborne with ease: if a sample is heated in air to , 50% of it is vaporized in 45 hours to form diatomic molecules, even though the melting point of polonium is and its boiling point is . More than one hypothesis exists for how polonium does this; one suggestion is that small clusters of polonium atoms are spalled off by the alpha decay.
Chemistry The chemistry of polonium is similar to that of tellurium, although it also shows some similarities to its neighbor bismuth due to its metallic character. Polonium dissolves readily in dilute acids but is only slightly soluble in alkalis. Polonium solutions are first colored in pink by the ions, but then rapidly become yellow because alpha radiation from polonium ionizes the solvent and converts into . As polonium also emits alpha-particles after disintegration, this process is accompanied by bubbling and emission of heat and light by glassware due to the absorbed alpha particles; as a result, polonium solutions are volatile and will evaporate within days unless sealed.Bagnall, p. 206 At pH about 1, polonium ions are readily hydrolyzed and complexed by acids such as oxalic acid, citric acid, and tartaric acid.
Compounds Polonium has no common compounds, and almost all of its compounds are synthetically created; more than 50 of those are known.Bagnall, p. 199 The most stable class of polonium compounds are polonides, which are prepared by direct reaction of two elements. has the antifluorite structure, the polonides of Ca, Ba, Hg, Pb and lanthanides form a NaCl lattice, BePo and CdPo have the wurtzite and MgPo the nickel arsenide structure. Most polonides decompose upon heating to about 600 °C, except for HgPo that decomposes at ~300 °C and the lanthanide polonides, which do not decompose but melt at temperatures above 1000 °C. For example, the polonide of praseodymium (PrPo) melts at 1250 °C, and that of thulium (TmPo) melts at 2200 °C.Greenwood, p. 766 PbPo is one of the very few naturally occurring polonium compounds, as polonium alpha decays to form lead.
Polonium hydride () is a volatile liquid at room temperature prone to dissociation; it is thermally unstable. Water is the only other known hydrogen chalcogenide which is a liquid at room temperature; however, this is due to hydrogen bonding. The three oxides, PoO, and , are the products of oxidation of polonium.
Halides of the structure , and are known. They are soluble in the corresponding hydrogen halides, i.e., in HCl, in HBr and in HI. Polonium dihalides are formed by direct reaction of the elements or by reduction of with and with with at room temperature. Tetrahalides can be obtained by reacting polonium dioxide with HCl, HBr or HI.Greenwood, pp. 765, 771, 775
Other polonium compounds include the polonite, potassium polonite; various polonate solutions; and the acetate, bromate, carbonate, citrate, chromate, cyanide, formate, (II) or (IV) hydroxide, nitrate, selenate, selenite, monosulfide, sulfate, disulfate or sulfite salts.Figgins, P. E. (1961) The Radiochemistry of Polonium, National Academy of Sciences, US Atomic Energy Commission, pp. 13–14 Google BooksBagnall, pp. 212–226
A limited organopolonium chemistry is known, mostly restricted to dialkyl and diaryl polonides (), triarylpolonium halides (), and diarylpolonium dihalides (). Polonium also forms soluble compounds with some ligands, such as 2,3-butanediol and thiourea.
+Polonium compoundsWiberg, Egon; Holleman, A. F. and Wiberg, Nils Inorganic Chemistry, Academic Press, 2001, p. 594, .FormulaColor m.p. (°C) Sublimation temp. (°C) SymmetryPearson symbol Space group Noa (pm) b(pm) c(pm) Z ρ (g/cm3) refPoO black pale yellow 500 (dec.) 885 fcc cF12Fmm 225 563.7563.7563.74 8.94 -35.5 dark ruby red 355 130 orthorhombic oP3Pmmm47 3674354501 6.47 purple-brown 270 (dec.) yellow300 200 monoclinic red330 (dec.) fcc cF100 Fmm 2255605605604 black
Oxides PoO
Hydrides
Halides (except )
(salmon pink)
Isotopes
All nuclear data not otherwise stated is from the standard source:
There are 42 known isotopes of polonium (Po), all radioactive, stretching from Po to Po. The isotopes 210 through 218 occur naturally in the four principal decay chains. In terms of curies they are mostly of even mass number, coming from uranium-238 and thorium-232, with smaller amounts of 211 and 215 from uranium-235, and very little with mass number 213 coming from neptunium-237, and much less with mass number 217 coming from a rare branch of the neptunium decay chain. Po has the longest half-life by far (138.376 days) and is, therefore, the most abundant by far in terms of moles or mass. It is also the most easily synthesized isotope, by neutron capture on natural bismuth, and so by far the most abundant artificial isotope as well.
Two other isotopes have longer lives: Po with a half-life of 124 years and Po with a half-life of 2.898 years. Both are made by using a cyclotron to bombard bismuth with protons.
History Tentatively called "radium F", polonium was discovered by Marie and Pierre Curie in July 1898, English translation. and was named after Marie Curie's native land of Poland (). Poland at the time was under Russian, German, and Austro-Hungarian partition, and did not exist as an independent country. It was Curie's hope that naming the element after her native land would publicize its lack of independence. Polonium may be the first element named to highlight a political controversy.
This element was the first one discovered by the Curies while they were investigating the cause of pitchblende radioactivity. Pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than the uranium and thorium combined. This spurred the Curies to search for additional radioactive elements. They first separated out polonium from pitchblende in July 1898, and five months later, also isolated radium. English translation German scientist Willy Marckwald successfully isolated 3 milligrams of polonium in 1902, though at the time he believed it was a new element, which he dubbed "radio-tellurium", and it was not until 1905 that it was demonstrated to be the same as polonium.
In the United States, polonium was produced as part of the Manhattan Project's Dayton Project during World War II. Polonium and beryllium were the key ingredients of the 'Urchin' initiator at the center of the bomb's spherical pit.Nuclear Weapons FAQ, Section 4.1, Version 2.04: 20 February 1999. Nuclearweaponarchive.org. Retrieved on 2013-04-28. 'Urchin' initiated the nuclear chain reaction at the moment of prompt-criticality to ensure that the weapon did not fizzle. 'Urchin' was used in early U.S. weapons; subsequent U.S. weapons utilized a pulse neutron generator for the same purpose.
Much of the basic physics of polonium was classified until after the war. The fact that a polonium-beryllium (Po-Be) initiator was used in the gun-type nuclear weapons was classified until the 1960s.
The Atomic Energy Commission and the Manhattan Project funded human experiments using polonium on five people at the University of Rochester between 1943 and 1947. The people were administered between of polonium to study its excretion.American nuclear guinea pigs: three decades of radiation experiments on U.S. citizens . United States. Congress. House. of the Committee on Energy and Commerce. Subcommittee on Energy Conservation and Power, published by U.S. Government Printing Office, 1986, Identifier Y 4.En 2/3:99-NN, Electronic Publication Date 2010, at the University of Nevada, Reno, unr.edu"Studies of polonium metabolism in human subjects", Chapter 3 in Biological Studies with Polonium, Radium, and Plutonium, National, Nuclear Energy Series, Volume VI-3, McGraw-Hill, New York, 1950, cited in "American Nuclear Guinea Pigs ...", 1986 House Energy and Commerce committee reportMoss, William and Eckhardt, Roger (1995) "The Human Plutonium Injection Experiments" , Los Alamos Science, Number 23.
Occurrence and production Polonium is a very rare element in nature because of the short half-lives of all its isotopes. Nine isotopes, from 210 to 218 inclusive, occur in traces as decay products: Po, Po, and Po occur in the decay chain of U; Po and Po occur in the decay chain of U; Po and Po occur in the decay chain of Th; and Po and Po occur in the decay chain of Np. (No primordial Np survives, but traces of it are continuously regenerated through (n,2n) knockout reactions in natural U. The ratio of neptunium to uranium is around 1 to 10, but having a shorter half-life the ratio in terms of radioactivity is around 1 to 10. Po only occurs if Ra decays to Rn, with a probability of 0.000026, and then the Rn decays to Po with probability 0.22.) Of these, Po is the only isotope with a half-life longer than 3 minutes.
Polonium can be found in uranium ores at about 0.1 mg per metric ton (1 part in 1010),Greenwood, p. 746Bagnall, p. 198 which is approximately 0.2% of the abundance of radium. The amounts in the Earth's crust are not harmful. Polonium has been found in tobacco smoke from tobacco leaves grown with phosphate fertilizers.