84
  Po  
210.000000
Polonium

Name: Polonium
Symbol: Po
Atomic Number: 84
Atomic Weight: 210.000000
Family: Metalloids
CAS RN: 7440-08-6
Description: A silvery metal, very rare in nature.
State (25C): Solid
Oxidation states: +2, +4

Molar Volume: 22.73 cm3/mole
Valence Electrons: 6p4

Boiling Point:  1235K, 962C, 1764F
Melting Point:
527K, 254C, 489F
Electrons Energy Level: 2, 8, 18, 32, 18, 6
Isotopes: 29
Heat of Vaporization: unknown
Heat of Fusion: 60.1 kJ/mol
Density: 9.3 g/cm3 @ 300K
Specific Heat: 0.12 J/gK
Atomic Radius: 1.53
Ionic Radius: 2.3
Electronegativity: 2 (Pauling); 1.76 (Allrod Rochow)
Vapor Pressure: 0.0176 Pa @ 254C
Also called tentatively "Radium F", Polonium was discovered by Marie Sklodowska-Curie and her husband Pierre Curie in 1893 and was later named after Marie's native land of Poland (Latin: Polonia).  Poland at the time was under Russian, Prussian, and Austrian partition, and did not exist 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.  Poland became an independent country again in 1918, following World War I.

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.

The discovery was made by extraction of the remaining radioactive components of pitchblende following the removal of uranium.  There is only about 10-6 g per ton of ore.  Current production for research purposes involves the synthesis of the element in the lab rather than its recovery from minerals.  This is accomplished by producing Bi-210 from the abundant Bi-209. The new isotope of bismuth is then allowed to decay naturally into Po-210.

Although radioactive, Polonium has a few commercial uses. You can buy your own sample of Polonium at a photography store.  It is part of the special anti-static brushes for dusting off negatives and prints.

8
O
15.99
16
S
32.06
34
Se
78.96
52
Te
127.6
84
Po
210.0
116
Uuh
289.0

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10 6s2p4

Characteristics

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 radioisotope.

mpcurie.jpg (53047 bytes)

Marie Sklodowska-Curie & Pierre Curie

The chemistry of Polonium is similar to that of Tellurium and Bismuth.  Polonium dissolves readily in dilute acids, but is only slightly Soluble in alkalis.  The Hydrogen compound PoH2 is liquid at room temperature (M.P.  -36.1C to B.P. 35.3C).  Halides of the structure PoX2, PoX4 and PoX6 are known. The two oxides PoO2 and PoO3 are the products of oxidation of polonium.

210Po (in common with 238Pu) has the ability to become airborne with ease: if a sample is heated in air to 328 K (55C, 131F), 50% of it is vaporized in 45 hours, even though the melting point of Polonium is 527 K (254C, 489F) and its boiling point is 1235 K (962C, 1763F).  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 some microbes can methylate Polonium by the action of Methylcobalamin.  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 and Tellurium.

150px-Alpha_po_lattice.jpg (6640 bytes)

The alpha form of solid Polonium

Solid State Form

The alpha form of solid Polonium is cubic with a distance of 3.352 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.

Two papers report X-ray diffraction experiments on Polonium metal.  The first report of the crystal structure of Polonium was done using electron diffraction. 

Occurrence

Polonium is a very rare element in nature because of the short half-life of all its isotopes.  It is found in Uranium ores at about 100 micrograms per metric ton (1 part in 1010), 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.

Synthesis by (n, g) Reaction

In 1934 an experiment showed that when natural 209Bi is bombarded with neutrons, 210Bi is created, which then decays to 210Po via 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.

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 e that is supplied by 210Po.   It has been used in this capacity as a neutron trigger (a.k.a. initiator) for nuclear weapons.  Other uses include:

Isotopes

Polonium has 29 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.

210Po

210Po is an alpha emitter that has a half-life of 138.376 days; it decays directly to its daughter isotope 206Pb.  A milligram of 210Po emits as many alpha particles per second as 5 grams of 226Ra.  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.  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; for instance, 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.

The majority of the time 210Po decays by emission of an alpha particle only, 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.  This low gamma ray production rate makes it more difficult to find and identify this isotope.  Rather than gamma ray spectroscopy, alpha spectroscopy is the best method of measuring this isotope.

atom.gif (700 bytes)

Isotope Atomic Mass Half-Life
Po190 189.955 2 ms
Po191 190.995 15.5 ms
Po192 191.992 0.0332 seconds
Po193 192.991 0.42 seconds
Po194 193.988 0.392 seconds
Po195 194.988 4.64 seconds
Po196 195.986 5.8 seconds
Po197 196.986 53.6 seconds
Po198 197.983 1.77 minutes
Po199 198.984 5.48 minutes
Po200 199.982 11.5 minutes
Po201 200.982 15.3 minutes
Po202 201.981 44.7 minutes
Po203 202.9814 36.7 minutes
Po204 203.9803 3.53 hours
Po205 204.9812 1.66 hours
Po206 205.9805 8.8 days
Po207 207.9816 5.80 hours
Po208 207.9812 2.898 years
Po209 208.9824 102 years
Po210 209.9829 138.376 days
Po211 210.9866 0.516 seconds
Po212 211.9888 0.299 us
Po213 212.9928 4.2 us
Po214 213.9952 164.3 us
Po215 214.9994 1.781 ms
Po216 216.0019 0.145 seconds
Po217   <10 seconds
Po218 218.009 3.10 minutes

Tests

600px-Gammaspectrscopy.jpg (17287 bytes)

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.

400px-Alpha1spec.jpg (15552 bytes)

Intensity against alpha energy for four isotopes, note that the line width is narrow and the fine details can be seen

400px-Alpha5spec.jpg (14293 bytes)

 

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 scintillation 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 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).

Precautions

Overview

40px-Skull_and_crossbones.svg.jpg (1420 bytes) Weight-for-weight, polonium is around 5 million times more toxic than Hydrogen Cyanide, HCN, (the oral LD50 for 210Po is about 50 ng compared to about 250 mg for Hydrogen Cyanide.  The main hazard is its intense radioactivity (as an alpha emitter), which makes it very difficult to handle safely - one gram of Po will self-heat to a temperature of around 500C.

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 median lethal dose, LD50, for acute radiation exposure is generally about 4.5 Sv.   The committed effective dose equivalent 210Po is 0.51 Sv/Bq if ingested, and 2.5 Sv/Bq if inhaled.  Since 210Po has an activity of 166 TBq per gram (1 gram produces 1661012 decays per second), a fatal 4-Sv dose can be caused by ingesting 8.8 MBq (238 microcurie), about 50 nanograms (ng), or inhaling 1.8 MBq (48 microcurie), about 10 ng.  One gram of 210Po could thus in theory poison 100 million people of whom 50 million would die.  The actual toxicity of 210Po is lower than these estimates, because radiation exposure that is spread out over several weeks (the biological half-life of Polonium in humans is 30 to 50 days is somewhat less damaging than an instantaneous dose. It has been estimated that a minimal lethal dose of 210Po for an 80 kg person is 4 millicuries, or 0.89 micrograms, still an extremely small amount.

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 5–10% per Sv.  The general population is exposed to small amounts of Polonium as a Radon daughter in indoor air; the isotopes 214Po and 218Po are thought to cause the majority of the estimated 15,000-22,000 lung cancer deaths in the US every year that have been attributed to indoor radon.  Tobacco smoking causes significant additional exposure to Po.

Regulatory Exposure Limits

The maximum allowable body burden for ingested 210Po is only 1,100 Bq (0.03 microcurie), which is equivalent to a particle weighing only 6.8 picograms. The maximum permissible workplace concentration of airborne 210Po is about 10 Bq/m3 (3 10-10 Ci/cm3).  The target organs for Polonium in humans are the spleen and liver.  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).

210Po is widely used in industry, and readily available with little regulation or restriction.  In the US, a tracking system run by the Nuclear Regulatory Commission will be implemented in 2007 to register purchases of more than 16 curies of Polonium 210 (enough to make up 5,000 lethal doses).  The IAEA "is said to be considering tighter regulations... There is talk that it might tighten the Polonium reporting requirement by a factor of 10, to 1.6 curies."

Famous Polonium Poisoning Cases

Notably, the murder of Alexander Litvinenko in 2006 was announced as due to 210Po poisoning.  Generally, 210Po 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 a-radiation effects of 210Po , although Irene Joliot-Curie was actually the first person ever to die from the radiation effects of Polonium (due to a single intake) in 1956.

According to the book The Bomb in the Basement, several death cases in Israel during 1957-1969 were caused by 210Po.  A leak was discovered at a Weizmann Institute laboratory in 1957. Traces of 210Po were found on the hands of Prof. Dror Sadeh, a physicist who researched radioactive materials.  Medical tests indicated no harm, but the tests did not include bone marrow.  Sadeh died prematurely from cancer.  One of his students died of leukemia, and two colleagues died after a few years, both from cancer.  The issue was investigated secretly, and there was never any formal admission that a connection between the leak and the deaths had existed.

Irene Joliot-Curie, daughter of Marie (Sklodowska) Curie and Pierre Curie and the wife of Frederic Joliot-Curie was accidentally exposed to Polonium when a sealed capsule of the element exploded on her laboratory bench.  A decade later, on Maarch 17, 1956, she died in Paris from leukemia.

Treatment

It has been suggested that chelation agents such as British Anti-Lewisite (Dimercaprol) can be used to decontaminate humans.  In one experiment, rats were given a fatal dose of 1.45 MBq/kg (8.7 ng/kg) of 210Po; all untreated rats were dead after 44 days, but 90% of the rats treated with the chelation agent HOEtTTC remained alive after 5 months.

Handling

210Po is relatively safe to transport but can be difficult to handle. It has the ability to vapourize with ease at temperatures well below its boiling point.  The precise mechanism for this is not known.

atom.gif (700 bytes)

Polonium Data

Atomic Radius (): 1.53
Atomic Volume cm3/mol : 22.23cm3/mol
Covalent Radius: 1.46
Crystal Structure: Monoclinic
Ionic Radius: 2.3

Chemical Properties

Electrochemical Equivalents: 3.8986 g/amp-hr
Electron Work Function: unknown
Electronegativity: 2 (Pauling); 1.76 (Allrod Rochow)
Heat of Fusion: 60.1 kJ/mol
Incompatibilities: unknown
First Ionization Potential: 8.42
Second Ionization Potential: unknown
Third Ionization Potential: unknown
Valence Electron Potential: -12.5
Ionization Energy (eV): unknown

Physical Properties

Atomic Mass Average: 208.9824
Boiling Point: 1235K, 962C, 1764F
Melting Point: 527K, 254C, 489F
Heat of Vaporization: unknown
Coefficient of Lineal Thermal Expansion/K-1: 23E-6
Electrical Conductivity: 0.0219 106/cm
Thermal Conductivity: 0.2 W/cmK
Density: 9.3 g/cm3 @ 300K
Enthalpy of Atomization: 134 kJ/mole @ 25C
Enthalpy of Fusion: unknown
Enthalpy of Vaporization: 60.3 kJ/mole
Flammability Class: unknown
Molar Volume: 22.73 cm3/mole
Optical Refractive Index: unknown
Relative Gas Density (Air=1): unknown
Specific Heat: 0.12 J/gK
Vapor Pressure: 0.0176 Pa @ 254C
Estimated Crustal Abundance: unknown
Estimated Oceanic Abundance: unknown


(Poland, native country of Mme. Curie) Polonium was the first element discovered by Mme. Curie in 1898 while seeking the cause of radioactivity of pitchblende from Joachimsthal, Bohemia.  The electroscope showed it separating with Bismuth. Polonium is also called Radium F.  Polonium is a very rare natural element.  Uranium ores contain only about 100 micrograms of the element per ton.  Its abundance is only about 0.2% of that of Radium.  In 1934, it was found that when natural Bismuth (209Bi) was bombarded by neutrons, 210Bi, the parent of Polonium, was obtained.   Milligram amounts of Polonium may now be prepared this way, by using the high neutron fluxes of nuclear reactors.  Polonium-210 is a low-melting, fairly volatile metal, 50% of which is vaporized in air in 45 hours at 55oC.  It is an alpha emitter with a half-life of 138.39 days.  A milligram emits as many alpha particles as 5 g of radium.  The energy released by its decay is so large (140W/g) that a capsule containing about half a gram reaches a temperature above 500oC.   The capsule also presents a contact gamma-ray does rate of 0.012 Gy/h. A few curies (1 curie = 3.7 x 1010Bq) of Polonium exhibit a blue glow, caused by excitation of the surrounding gas. Because almost all alpha radiation is stopped within the solid source and its container, giving up its energy, Polonium has attracted attention for uses as a lightweight heat source for thermoelectric power in space satellites.  Twenty nine isotopes of Polonium are known, with atomic masses ranging from 194 to 218.  Polonium-210 is the most readily available.  Isotopes of mass 209 (half-life 103 years) and mass 208 (half-life 2.9 years) can be prepared by alpha, proton, or deuteron bombardment of Lead or Bismuth in a cyclotron, but these are expensive to produce.  Metallic Polonium has been prepared from Polonium Hydroxide and some other Polonium compounds in the presence of concentrated aqueous or anhydrous liquid Ammonia.  Two allotropic modifications are known to exist.  Polonium is readily dissolved in dilute acids, but is only slightly soluble in alkali.  Polonium slats of organic acids char rapidly; halide amines are reduced to the metal.   Polonium can be mixed or alloyed with Beryllium to provide a source of neutrons.   It has been used in devices for eliminating static charges in textile mills, etc.; however, beta sources are more commonly used and are less dangerous.  It is also used on brushes for removing dust from photographic films.  The Polonium for these is carefully sealed and controlled, minimizing hazards to the user.  Polonium-210 is very dangerous to handle in even milligram or microgram amounts, and special equipment and strict control is necessary. Damage arises from the complete absorption of the energy of the alpha particle into tissue.  The maximum permissible body burden for ingested Polonium is only 0.03 microcuries, which represents a particle weighing only 6.8 x 10-12 g.  Weight for weight it is about 2.5 x 1011 times as toxic as Hydrocyanic Acid.  The maximum allowable concentration for soluble Polonium compounds in air is about 2 x 10-11 microcuries/cm3.  Polonium is available commercially on special order from the Oak Ridge National Laboratory.

Source: CRC Handbook of Chemistry and Physics, 1913-1995. David R. Lide, Editor in Chief. Author: C.R. Hammond