Polonium

Polonium,  84Po
Polonium.jpg
Polonium
Pronunciationm/ (LOH-nee-əm)
Allotropesα, β
Appearancesilvery
Mass number209 (most stable isotope)
Polonium in the periodic table
HydrogenHelium
LithiumBerylliumBoronCarbonNitrogenOxygenFluorineNeon
SodiumMagnesiumAluminiumSiliconPhosphorusSulfurChlorineArgon
PotassiumCalciumScandiumTitaniumVanadiumChromiumManganeseIronCobaltNickelCopperZincGalliumGermaniumArsenicSeleniumBromineKrypton
RubidiumStrontiumYttriumZirconiumNiobiumMolybdenumTechnetiumRutheniumRhodiumPalladiumSilverCadmiumIndiumTinAntimonyTelluriumIodineXenon
CaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumGoldMercury (element)ThalliumLeadBismuthPoloniumAstatineRadon
FranciumRadiumActiniumThoriumProtactiniumUraniumNeptuniumPlutoniumAmericiumCuriumBerkeliumCaliforniumEinsteiniumFermiumMendeleviumNobeliumLawrenciumRutherfordiumDubniumSeaborgiumBohriumHassiumMeitneriumDarmstadtiumRoentgeniumCoperniciumNihoniumFleroviumMoscoviumLivermoriumTennessineOganesson
Te

Po

Lv
bismuthpoloniumastatine
Atomic number (Z)84
Groupgroup 16 (chalcogens)
Periodperiod 6
Blockp-block
Element category  Post-transition metal, but this status is disputed
Electron configuration[Xe] 4f14 5d10 6s2 6p4
Electrons per shell
2, 8, 18, 32, 18, 6
Physical properties
Phase at STPsolid
Melting point527 K ​(254 °C, ​489 °F)
Boiling point1235 K ​(962 °C, ​1764 °F)
Density (near r.t.)alpha: 9.196 g/cm3
beta: 9.398 g/cm3
Heat of fusionca. 13 kJ/mol
Heat of vaporization102.91 kJ/mol
Molar heat capacity26.4 J/(mol·K)
Vapor pressure
P (Pa)1101001 k10 k100 k
at T (K)(846)10031236
Atomic properties
Oxidation states−2, +2, +4, +5,[1] +6 (an amphoteric oxide)
ElectronegativityPauling scale: 2.0
Ionization energies
  • 1st: 812.1 kJ/mol
Atomic radiusempirical: 168 pm
Covalent radius140±4 pm
Van der Waals radius197 pm
Color lines in a spectral range
Spectral lines of polonium
Other properties
Natural occurrencefrom decay
Crystal structurecubic
Cubic crystal structure for polonium

α-Po
Crystal structurerhombohedral
Rhombohedral crystal structure for polonium

β-Po
Thermal expansion23.5 µm/(m·K) (at 25 °C)
Thermal conductivity20 W/(m·K) (?)
Electrical resistivityα: 0.40 µΩ·m (at 0 °C)
Magnetic orderingnonmagnetic
CAS Number7440-08-6
History
Namingafter Polonia, Latin for Poland, homeland of Marie Curie
DiscoveryPierre and Marie Curie (1898)
First isolationWilly Marckwald (1902)
Main isotopes of polonium
Iso­topeAbun­danceHalf-life (t1/2)Decay modePro­duct
208Posyn2.898 yα204Pb
β+208Bi
209Posyn125.2 y[2]α205Pb
β+209Bi
210Potrace138.376 dα206Pb
| references

Polonium is a chemical element with the symbol Po and atomic number 84. A rare and highly radioactive metal with no stable isotopes, polonium is chemically similar to selenium and tellurium, though its metallic character resembles that of its horizontal neighbors 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 slightly longer-lived isotopes exist, 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 in 1898 by Marie 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 so discovered. Polonium was named after Marie Curie's homeland of Poland. 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. It is a radioactive element, and extremely dangerous to humans.

Characteristics

210Po is an alpha emitter that has a half-life of 138.4 days; it decays directly to its stable daughter isotope, 206Pb. A milligram (5 curies) of 210Po emits about as many alpha particles per second as 5 grams of 226Ra.[3] A few curies (1 curie equals 37 gigabecquerels, 1 Ci = 37 GBq) of 210Po 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.[4][5]

Solid state form

The alpha form of solid polonium.

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, with an edge length of 335.2 picometers; the beta form is rhombohedral.[6][7][8] The structure of polonium has been characterized by X-ray diffraction[9][10] and electron diffraction.[11]

210Po (in common with 238Pu[citation needed]) has the ability to become airborne with ease: if a sample is heated in air to 55 °C (131 °F), 50% of it is vaporized in 45 hours to form diatomic Po2 molecules, even though the melting point of polonium is 254 °C (489 °F) and its boiling point is 962 °C (1,764 °F).[12][13][1] 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 Po2+ ions, but then rapidly become yellow because alpha radiation from polonium ionizes the solvent and converts Po2+ into Po4+. 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.[14][15] At pH about 1, polonium ions are readily hydrolyzed and complexed by acids such as oxalic acid, citric acid, and tartaric acid.[16]

Compounds

Polonium has no common compounds, and almost all of its compounds are synthetically created; more than 50 of those are known.[17] The most stable class of polonium compounds are polonides, which are prepared by direct reaction of two elements. Na2Po 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, PrPo melts at 1250 °C and TmPo at 2200 °C.[18] PbPo is one of the very few naturally occurring polonium compounds, as polonium alpha decays to form lead.[19]

Polonium hydride (PoH
2
) is a volatile liquid at room temperature prone to dissociation; it is thermally unstable.[18] Water is the only other known hydrogen chalcogenide which is a liquid at room temperature; however, this is due to hydrogen bonding. The two oxides PoO2 and PoO3 are the products of oxidation of polonium.[20]

Halides of the structure PoX2, PoX4 and PoF6 are known. They are soluble in the corresponding hydrogen halides, i.e., PoClX in HCl, PoBrX in HBr and PoI4 in HI.[21] Polonium dihalides are formed by direct reaction of the elements or by reduction of PoCl4 with SO2 and with PoBr4 with H2S at room temperature. Tetrahalides can be obtained by reacting polonium dioxide with HCl, HBr or HI.[22]

Other polonium compounds include potassium polonite as a polonite, polonate, acetate, bromate, carbonate, citrate, chromate, cyanide, formate, (II) and (IV) hydroxides, nitrate, selenate, selenite, monosulfide, sulfate, disulfate and sulfite.[21][23]

Polonium compounds[22][24]
Formula Color m.p. (°C) Sublimation
temp. (°C)
Symmetry Pearson symbol Space group No a (pm) b(pm) c(pm) Z ρ (g/cm3) ref
PoO2 pale yellow 500 (dec.) 885 fcc cF12 Fm3m 225 563.7 563.7 563.7 4 8.94 [25]
PoCl2 dark ruby red 355 130 orthorhombic oP3 Pmmm 47 367 435 450 1 6.47 [26]
PoBr2 purple-brown 270 (dec.) [27]
PoCl4 yellow 300 200 monoclinic [26]
red 330 (dec.) fcc cF100 Fm3m 225 560 560 560 4 [27]
black [28]

Isotopes

Polonium has 42 known isotopes, all of which are radioactive. They have atomic masses that range from 186 to 227 u. 210Po (half-life 138.376 days) is the most widely available and is made via neutron capture by natural bismuth. The longer-lived 209Po (half-life 125.2±3.3 years, longest-lived of all polonium isotopes)[2] and 208Po (half-life 2.9 years) can be made through the alpha, proton, or deuteron bombardment of lead or bismuth in a cyclotron.[29]