# Polonium

 84 bismuth ← polonium → astatine Te↑Po↓Uuh
General
Name, Symbol, Number polonium, Po, 84
Chemical series metalloids
Group, Period, Block 16, 6, p
Appearance silvery
Atomic mass (209) g/mol
Electron configuration [Xe] 4f14 5d10 6s2 6p4
Electrons per shell 2, 8, 18, 32, 18, 6
Physical properties
Phase solid
Density (near r.t.) (alpha) 9.196 g·cm−3
Density (near r.t.) (beta) 9.398 g·cm−3
Melting point 527 K
(254 °C, 489 °F)
Boiling point 1235 K
(962 °C, 1764 °F)
Heat of fusion ca. 13 kJ·mol−1
Heat of vaporization 102.91 kJ·mol−1
Heat capacity (25 °C) 26.4 J·mol−1·K−1
 P/Pa 1 10 100 1 k 10 k 100 k at T/K (846) 1003 1236
Atomic properties
Crystal structure cubic
Oxidation states 4, 2
(amphoteric oxide)
Electronegativity 2.0 (Pauling scale)
Ionization energies 1st: 812.1 kJ/mol
Miscellaneous
Magnetic ordering nonmagnetic
Electrical resistivity (0 °C) (α) 0.40 µΩ·m
Thermal conductivity (300 K)  ? 20 W·m−1·K−1
Thermal expansion (25 °C) 23.5 µm·m−1·K−1
CAS registry number 7440-08-6
Selected isotopes
Main article: Isotopes of polonium
iso NA half-life DM DE (MeV) DP
208Po syn 2.898 y α 5.215 204Pb
ε, β+ 1.401 208Bi
209Po syn 103 y α 4.979 205Pb
ε, β+ 1.893 209Bi
210Po syn 138.376 d α 5.407 206Pb
References

Polonium (IPA: /pə(ʊ)ˈləʊniəm/) is a chemical element in the periodic table that has the symbol Po and atomic number 84. A rare and highly radioactive metalloid, polonium is chemically similar to tellurium and bismuth, and it occurs in uranium ores. Polonium has been studied for possible use in heating spacecraft. It exists as a number of radio-isotopes.

##  Applications

When it is mixed or alloyed with beryllium, polonium can be a neutron source: beryllium releases a neutron upon absorption of an alpha particle that is supplied by 210Po. It has been used in this capacity as a neutron trigger for nuclear weapons. Other uses include:

##  History

Also called tentatively "Radium F", polonium was discovered by Marie Curie and her husband Pierre Curie in 1898<ref>Template:Cite journal</ref> and was later named after Marie's native land of Poland (Latin: Polonia).<ref>Template:Cite journal</ref> <ref>Template:Cite journal</ref> Poland at the time was under Russian, Prussian, and Austrian partition, and not existing as an independent country. It was Marie'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.<ref>Template:Cite journal</ref> Poland became an independent country again in 1918, following WW1.

This element was the first one discovered by the Curies while they were investigating the cause of pitchblende radioactivity. The pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than both the uranium and thorium put together. This spurred the Curies on to find additional radioactive elements. The Curies first separated out polonium from the pitchblende, and then within a few years, also isolated radium.

##  Occurrence

A very rare element in nature, because of the very short half-life of all its isotopes, polonium is found in uranium ores at about 100 micrograms per metric ton (1 part in 1010). Its natural abundance is approximately 0.2% of the abundance of radium. Polonium has been found in tobacco smoke from tobacco leaves grown with phosphate fertilizers.<ref>Template:Cite journal</ref><ref>Alpha Radioactivity (210 Polonium) and Tobacco Smoke</ref>

###  Synthesis by (n,$\gamma$) reaction

In 1934 an experiment showed that when natural 209Bi is bombarded with neutrons, 210Bi is created, which then decays to 210Po via β decay. Polonium may now be made in milligram amounts in this procedure which uses high neutron fluxes found in nuclear reactors. Only about 100 grams is produced each year, making polonium exceedingly rare. <ref>http://www.rsc.org/chemistryworld/News/2006/November/27110601.asp RSC Chemistry World Q&A</ref>

###  Synthesis by (p,n) and (p,2n) reactions

It has been found that by proton bombardment of bismuth using a cyclotron that the longer lived isotopes of polonium can be formed. Other more proton rich isotopes can be formed by the irradation of platinum with carbon nuclei.<ref>Template:Cite journal</ref>

##  Isotopes

Polonium has 25 known isotopes, all of which are radioactive. They have atomic masses that range from 194 amu to 218 amu. 210Po is the most widely available. 209Po (half-life 103 years) and 208Po (half-life 2.9 years) can be made through the alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron. However these isotopes are expensive to produce.

All elements containing 83 or more protons are radioactive, if only slightly. [Bi-209 has a half-life of more than 1019 years.] Alpha decay is a common form of decay for these nuclei. The most stable isotopes, with half-lives of more than four billion years, with more than 82 protons are thorium-232 and uranium-238; which form an "island of stability" which renders them stable enough to be found in large quantities in nature, but heavier nuclei are more and more susceptable to spontaneous fission or rapid beta-decay.

###  210Po

Polonium-210 is an alpha emitter that has a half-life of 138.376 days. A milligram of 210Po emits as many alpha particles as 5 grams of radium. A few curies (1 curie equals 37 gigabecquerels) of 210Po emit a blue glow which is caused by excitation of surrounding air. A single gram of 210Po generates 140 watts of power.<ref>Polonium, Argonne National Laboratory</ref> Because it emits many alpha particles, which are stopped within a very short distance in dense media and release their energy, 210Po has been used as a lightweight heat source to power thermoelectric cells in artificial satellites. A 210Po heat source was also used in each of the Lunokhod rovers deployed on the surface of the Moon, to keep their internal components warm during the lunar nights. Some anti-static brushes contain up to 500 microcuries of 210Po as a source of charged particles for neutralizing static electricity in materials like photographic film.<ref>http://www.amstat.com/solutions/staticmaster.html</ref>.

Another use of Polonium-210 has come to light recently is its effectiveness as a radioactive poison. Former KGB spy Alexander Litvinenko was poisoned with Polonium-210, probably on November 1, 2006, resulting in his death on November 23, 2006.

The majority of the time 210Po decays only by emission of an alpha particle, not by emission of an alpha particle and a gamma ray. About one in a 100,000 decays results in the emission of a gamma ray<ref>http://atom.kaeri.re.kr/cgi-bin/decay?Po-210%20A</ref>. This low gamma ray production rate makes it more difficult to find and identify this isotope. Rather than gamma ray spectroscopy, alpha spectroscopy will be the best method of measuring this isotope.

##  Chemical characteristics

Polonium dissolves readily in dilute acids, but is only slightly soluble in alkalis. It is closely related chemically to bismuth and tellurium. 210Po (in common with 238Pu) has the ability to become airborne with ease: if a sample is heated in air to 328 K (55°C, 131°F), 50% of it is vaporized in 45 hours, even though the melting point of polonium is 527 K (254°C, 489°F) and its boiling point is 1235 K (962°C, 1763°F).<ref>Template:Cite journal</ref> 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.

It has been reported that microbes can methylate polonium by the action of methylcobalamin.<ref> Template:Cite journal </ref><ref> Template:Cite journal</ref>This is similar to the way in which mercury, selenium and tellurium are methylated in living things to create organometallic compounds. As a result when considering the biochemistry of polonium one should consider the possibility that the polonium will follow the same biochemical pathways as selenium/tellurium.

Image:Alpha po lattice.jpg
The alpha form of solid polonium.

##  Solid state form

The alpha form of solid polonium is cubic with a distance of 3.352 Å between atoms. It is a simple cubic solid which is not interpenetrated.

The beta form of polonium is rhombohedral; it has been reported in the chemical literature, along with the alpha form, several times. A picture of it is present on the web.[2]

Two papers report X-ray diffraction experiments on polonium metal.<ref>R.J. Desando and R.C Lange, Journal of Inorganic and Nuclear Chemistry, 1966, 28, 1837-1846.</ref> <ref>W.H Beamer and C.R. Maxwell, Journal of Chemical Physics, 1946, 14, 569-569.</ref> The first report of the crystal structure of polonium was done using electron diffraction.<ref>M.A. Rollier, S.B. Hendricks and L.R. Maxwell, Journal of Chemical Physics, 1936, 4, 648-652.</ref>

##  Tests

Image:Gammaspectrscopy.png
Intensity against photon energy for three isotopes

### Gamma counting

By means of radiometric methods such as gamma spectroscopy (or a method using a chemical separation followed by an activity measurement with a non-energy-dispersive counter), it is possible to measure the concentrations of radioisotopes and to distinguish one from another. In practice, background noise would be present and depending on the detector, the line width would be larger which would make it harder to identify and measure the isotope. In biological/medical work it is common to use the natural 40K present in all tissues/body fluids as a check of the equipment and as an internal standard.

Image:Alpha1spec.png
Intensity against alpha energy for four isotopes, note that the line width is narrow and the fine details can be seen
Image:Alpha5spec.png
Intensity against alpha energy for four isotopes, note that the line width is wide and some of the fine details can not be seen. This is for liquid sintillation counting where random effects cause a variation in the number of visible photons generated per alpha decay

### Alpha counting

The best way to test for (and measure) many alpha emitters is to use alpha-particle spectroscopy as it is common to place a drop of the test solution on a metal disk which is then dried out to give a uniform coating on the disk. This is then used as the test sample. If the thickness of the layer formed on the disk is too thick then the lines of the spectrum are broadened, this is because some of the energy of the alpha particles is lost during their movement through the layer of active material. An alternative method is to use internal liquid scintillation where the sample is mixed with a scintillation cocktail. When the light emitted is then counted, some machines will record the amount of light energy per radioactive decay event. Due to the imperfections of the liquid scintillation method (such as a failure for all the photons to be detected, cloudy or coloured samples can be difficult to count) and the fact that random quenching can reduce the number of photons generated per radioactive decay it is possible to get a broadening of the alpha spectra obtained through liquid scintillation. It is likely that these liquid scintillation spectra will be subject to a Gaussian broadening rather than the distortion exhibited when the layer of a active material on a disk is too thick.

A third energy dispersive method for counting alpha particles is to use a semiconductor detector.

From left to right the peaks are due to 209Po, 210Po, 239Pu and 241Am. The fact that isotopes such as 239Pu and 241Am have more than one alpha line indicates that the nucleus has the ability to be in different discrete energy levels (like a molecule can).

##  Toxicity

###  Overview

Polonium is a highly-radioactive and chemically-toxic element and it is very difficult to handle safely. Even in microgram amounts, handling 210Po is extremely dangerous, requiring specialized equipment and strict handling procedures. Alpha particles emitted by polonium will damage organic tissue easily if polonium is ingested, inhaled, or absorbed (though they do not penetrate the epidermis and hence are not hazardous if the polonium is outside the body).

###  Acute effects

The lethal dose (LD50) for acute radiation exposure is generally about 4 Sv <ref name=pnl>http://www.pnl.gov/main/publications/external/technical_reports/PNNL-14424.pdf</ref>. One Bq of 210Po (i.e., an amount that produces one decay per second) causes a radiation dose of 0.51 µSv if ingested, and 2.5 µSv if inhaled <ref name=nsds>Nuclide Safety Data Sheet: Polonium–210</ref>. Since 210Po has an activity of 166 TBq per gram<ref name=nsds/>, a fatal 4-Sv dose can be caused by ingesting 8 MBq (200 microcurie), about 50 nanograms (ng), or inhaling 1.6 MBq (40 microcurie), about 10 ng. One gram of 210Po could thus in theory poison 100 million people.

###  Long term (Chronic effects)

In addition to the acute effects, radiation exposure (both internal and external) carries a long-term risk of death from cancer of approximately 5% per Sv. <ref name=pnl/>

###  Body burden limit

The maximum allowable body burden for ingested polonium is only 1,100 Bq (0.03 microcurie), which is equivalent to a particle weighing only 6.8 picograms. The maximum permissible concentration for airborne soluble polonium compounds is about 7,500 Bq/m3 (2 × 10-11 µCi/cm3). The biological half-life of polonium in humans is 30 to 50 days.<ref>Effective half-life of polonium in the human</ref> The target organs for polonium in humans are the spleen and liver.[3] As the spleen (150 g) and the liver (1.3 to 3 Kg) are much smaller than the rest of the body, if the polonium is concentrated in these vital organs, it is a greater threat to life than the dose which would be suffered (on average) by the whole body if it were spread evenly throughout the body, in the same way as cesium or tritium (as T2O). A review of biological effects of fission products and actinides can be read here: [4]

###  Famous polonium poisoning cases

Notably, the death in 2006 of Alexander Litvinenko has been announced as due to 210Po poisoning. <ref>"The mystery of Litvinenko's death", BBC News, 24 November 2006.</ref> Generally, Po is most lethal when it is ingested. According to Nick Priest, a radiation expert speaking on Sky News on December 2, Litvinenko was probably the first person ever to die of the acute α-radiation effects of Polonium, although Irene Joliot-Curie was actually the first person ever to die from the radiation effects of Polonium (due to a single intake) in the late 1950's. It is reasonable to assume that many people have died as a result of lung cancer caused by the alpha emission of polonium present in their lungs, either as a radon daughter or from tobacco smoke.

###  Treatment

It has been suggested that Chelation therapy using dimercaprol can be used to decontaminate humans.

##  References

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References and External links verified 2006-11-25 unless noted.