   #copyright

Polonium

2007 Schools Wikipedia Selection. Related subjects: Chemical elements


               84               bismuth ← polonium → astatine
               Te
               ↑
               Po
               ↓
               Uuh

                                  Periodic Table - Extended Periodic Table

                                                                   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] 4f^14 5d^10 6s^2 6p^4
                                   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

   CAPTION: Vapor pressure

                                           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
                                                      Atomic radius 190 pm
                                              Atomic radius (calc.) 135 pm
                                                             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

                 CAPTION: 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 radioactive
   metalloid, polonium is chemically similar to tellurium and bismuth and
   occurs in uranium ores. Polonium has been studied for possible use in
   heating spacecraft. It exists as a number of 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 polonium-210. It has been used in this
   capacity as a neutron trigger for nuclear weapons. Other uses include:
     * Devices that eliminate static charges in textile mills and other
       places. However, beta sources are more commonly used and are less
       dangerous.
     * Brushes that remove accumulated dust from photographic films. The
       polonium used in these brushes is sealed and controlled thus
       minimizing radiation hazards.
     * As ^210Po, a lightweight heat source to power thermoelectric cells.

History

   Also called "Radium F", polonium was discovered by Marie Curie and her
   husband Pierre Curie in 1897 and was later named after Marie's homeland
   of Poland (Latin: Polonia). Poland at the time was under Russian,
   Prussian and Austrian domination, and not recognized as an independent
   country. It was Marie's hope that naming the element after her home
   land would add notoriety to its plight. 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. The pitchblende,
   after removal of uranium and radium, was more radioactive than both
   radium and uranium put together. This spurred them on to find the
   element. The electroscope showed it separating with bismuth.

Occurrence

   A very rare element in nature, polonium is found in uranium ores at
   about 100 micrograms per metric ton (1:10^10). Its natural abundance is
   approximately 0.2% of the abundance of radium.

   In 1934 an experiment showed that when natural ^209Bi is bombarded with
   neutrons, ^210Bi, which is the parent of polonium, was created.
   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 believed to be produced each year, making polonium exceedingly
   rare.

   Polonium has been found in tobacco smoke from tobacco leaves grown with
   phosphate fertilizers.

Isotopes

   Polonium has 25 known isotopes all of which are radioactive. They have
   atomic masses that range from 194 u to 218 u. ^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 84 or more protons are radioactive. Alpha decay
   is a common form of decay for these nuclei. The most stable isotopes
   with more than 84 protons are ^232Th and ^238U; 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 affected by
   spontaneous fission.

Polonium-210

   This isotope of polonium 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 great deal of energy is released by its decay with
   half a gram quickly reaching a temperature above 750 K. A few curies (
   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. 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.. Polonium-210 has very rare
   properties as an unstable isotope, as it decays only by emission of an
   alpha particle, not by emission of an alpha particle and a gamma ray.

Chemical characteristics

   Polonium dissolves readily in dilute acids, but is only slightly
   soluble in alkalis. It is closely related chemically to bismuth and
   tellurium. Polonium-210 (in common with ^238Pu) has the ability to
   become airborne with ease ( volatilize), 50% of a sample is vaporized
   in air in 45 hours at 328K (55°C, 131°F) even though its melting point
   is 527K (254°C, 489°F) and its boiling point is 1235K (962°C, 1763°F).
   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.

Solid state form

   The alpha form of solid polonium.
   Enlarge
   The alpha form of solid polonium.

   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 hexagonal; it has been reported in the
   chemical literature, along with the alpha form, several times.

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

Tests

   Intensity against photon energy for three isotopes
   Enlarge
   Intensity against photon energy for three isotopes

   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.

Toxicity

   Polonium is a highly radioactive and toxic element and is very
   difficult to handle. Even in milligram or microgram amounts, handling
   polonium-210 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).

   To produce a potentially lethal radiation dose of 10 sieverts, if
   ingested, requires just 0.12 micrograms (millionths of a gram) of
   polonium-210 (about 525 microcuries of radioactivity). A cube of pure
   polonium-210 about the size of a written period (0.35 mm wide, or 400
   micrograms) would still be 3400 times the lethal dose. These
   calculations are based on a committed effective dose equivalent (CEDE)
   of 5.14×10^−7 sieverts per becquerel (1.9×10^3 mrem/microcurie) for
   ingested ^210Po and a specific activity of 1.66×10^14 Bq/gram
   (4.49×10^3 curies/gram). If the polonium is inhaled, the CEDE is even
   higher, 2.54×10^-6 Sv/Bq or 9.43×10^3 mrem/microcurie, making the
   lethal dose just 106 microcuries or 0.026 micrograms.

   The maximum allowable body burden for ingested polonium is only 1,100
   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 × 10^11 (250 billion) times as toxic as hydrogen
   cyanide. The maximum permissible concentration for airborne soluble
   polonium compounds is about 7,500 Bq/m^3 (2 × 10^-11 µCi/cm^3). The
   biological halflife of polonium in humans is 30 to 50 days.

Use as a poison

   Polonium-210 was allegedly used to murder Alexander Litvinenko, a
   former Russian FSB counterintelligence officer, administered around
   November 1, 2006 with death occuring November 23, 2006. According to
   Pat Troop, chief executive of Britain's Health Protection Agency,
   traces of polonium were found in Litvenenko's urine, as well as in
   several locations he had visited shortly before becoming ill.

   Retrieved from " http://en.wikipedia.org/wiki/Polonium"
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   with only minor checks and changes (see www.wikipedia.org for details
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