Polonium

Polonium 209 +2 +4   Po 84 [Xe] 6s2 4f14 5d10 6p4 16,p,6 [ ? ] Metalloid: Properties: Metallic, radioactive Uses: studied for possible use in heating spacecraft Hazard: extremely hazardous radiation

Polonium is a chemical element in the periodic table that has the symbol Po and atomic number 84. A rare radioactive metalloid, polonium is chemically similar to tellurium and bismuth and occurs in uranium ores. Polonium had been studied for possible use in heating spacecraft.

Notable characteristics

Polonium has no stable isotopes and has over 50 potential isotopes. It is extremely toxic and highly radioactive. Polonium has been found in tobacco smoke from tobacco leaves grown at some specific places, as a contaminant [1][2] and in uranium ores. All elements from polonium onward are radioactive. The great radioactivity of polonium and its immediate neighbors to the right on the periodic table, and its stark contrast with lead and bismuth, is due to the great instability of nuclei containing 84 or more protons. Nuclei that contain 128 neutrons along with 84 or more protons are especially unstable. Curiously, thorium-232 and uranium-238 are in an "island of stability" which renders them stable enough to be found in large quantities in nature, but heavier nuclei are more and more affected by spontaneous fission. Even in milligram or microgram amounts, handling ²¹⁰Po is difficult and requires special equipment because of the ability of Po to enter the vapour state. Internal exposure can expose tissues from alpha particles. This radioactive element dissolves readily in dilute acids, but is only slightly soluble in alkalis. It is closely related chemically to bismuth and tellurium. Polonium (in common with ²³⁸Pu) has the ability to become airborne with ease. More than one hypothesis exists for how polonium does this; one suggestion that that small clusters of polonium atoms are spalled off by the alpha decay.

The maximum allowable body burden for ingested polonium is only 1100 becquerels (0.03 microcurie), which is equivalent to a particle weighing only 6.8 × 10⁻¹² gram. Weight for weight polonium is approximately 2.5 × 10¹¹ times as toxic as hydrocyanic acid. The maximum permissible concentration for airborne soluble polonium compounds is about 7,500 Bq/m³ (2 × 10⁻¹¹ µCi/cm³).

N. Momoshima, L.X. Song, S. Osaki and Y. Maeda, Environ Sci Technol. 2001, 35, 2956-2960 [3] report that microbes can methylate polonium. They also claim that methylcobalamin can also methylate polonium.

Solid state form

The alpha form of the Po solid is cubic (Po-Po distance is 3.352 Å), it is a simple cubic solid which is not interpenetrated. A myth has grown up from a single sentence in one of the original papers on the crystal structure as determined by X-ray powder diffraction that describes Polonium's structure as an interpenetrated cubic solid. While this is not the structure, a reasonable number of real examples of such an interlocking network have been found.

The beta form of polonium is hexagonal; it has been reported along with the alpha form several times in chemical literature.

Two papers report X-ray diffraction experiments on Po metal.

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 are produced each year, making polonium exceedingly rare.

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

It has been found that the longer lived isotopes of polonium can be formed by proton bombardment of bismuth using a cyclotron. Other more neutron rich isotopes can be formed by the irradiation of platinum with carbon nuclei.

Testing humans

The case of the poisoning of the former Soviet operative Alexander Litvinenko has resulted in the need for a large number of humans to be tested for contamination with polonium. The method which is publicly explained by the British health protection agency [4] to to take 1000 ml of urine in a 2L beaker, add a known activity of either ²⁰⁸Po or ²⁰⁹Po (circa 0.2 Bq) before the addition of nitric acid. The urine sample is then evapourated to dryness, before dissolution in 6 M hydrochloric acid (after treatment with concentrated hydrochloric acid), the mixture is then treated with hydroxylamine HCl salt before adjustment of the pH. The polonium is then plated out onto silver discs before counting with an alpha spectrometer for at least 12 hours. The health protection agency claim that this method can detect and measure ²¹⁰Po activitys as low as 0.02 Bq.[5]

References

• Los Alamos National Laboratory – Polonium

External links

• WebElements.com – Polonium
• History of Polonium