Polonium

bismuth ← polonium → astatine Te ↑ Po ↓ Uuh

Atomic properties Atomic number 84 Standard atomic weight 209.982 8737(13) Electron configuration [Xe] 6s2 4f14 5d10 6p4 Physical properties Melting point 252(2) °C Boiling point 962(2) °C Density 9.196(6) g cm−3 (α-form) Ionization energy 8.42 eV 812.4 kJ mol−1 Thermodynamic properties Enthalpy change of vaporization 102.82(13) kJ mol−1 Miscellaneous CAS number 7440-08-6 Where appropriate, and unless otherwise stated, data are given for 100 kPa (1 bar) and 298.15 K (25 °C).

Polonium (symbol: Po) is a chemical element, one of the chalcogens. It is a silvery-white metal with a unique simple cubic structure, although it is rare that polonium or its compounds are encountered in weighable quantities.

Polonium has no stable isotopes, and the metal and all its compounds are intensely radioactive. The power output due to the radioactive decay of polonium-210 is about 140 W g⁻¹, which leads to considerable self-heating and which complicates the investigation of its properties with macroscopic quantities.

History

The existence of polonium was predicted by Dmitri Mendeleev in 1889. Mendeleev's "dvi-tellurium" was predicted to a low-melting grey solid with a density of about 9.3 times that of water, forming an amphoteric dioxide and a strongly oxidizing trioxide. It was predicted to be more metallic in character than tellurium, but less so than bismuth. These chemical predictions were to prove remarkably accurate.

Polonium was discovered in 1898 by Pierre and Marie Curie while they were investigating the radioactivity of pitchblende, and named after Marie Curie's native Poland. The discovery earned Marie Curie the Nobel Prize for Chemistry in 1911. The Curies were not able to isolate a pure sample of polonium, although they could prove its existence and chemical similarity to bismuth by radiochemical and later spectroscopic means. In one operation, Curie and Debierne started with "several tons" of residue from the processing of high-grade pitchblende (from St. Joachimsthal, Austria) to isolate just two milligrams of polonium concentrate, containing about 0.1 mg of polonium.

Polonium was independently discovered by Willy Marckwald in 1902. Marckwald initially doubted that his new element, which he named "radiotellurium" because of its chemical similarities to tellurium, was the same as that discovered by the Curies, but the identity was later proved by Hofmann and Rutherford.

The first weighable quantities of polonium were prepared in 1944 from the lead residues from the radium refinery in Port Hope, Ontario. This polonium had also originally come from uranium ores: it required the treatment of 37 tons of lead dioxide between 1943 and 1945 to produce just 9 mg (40 Ci) of polonium.

Occurrence and production

The only naturally ocurring isotope of polonium is polonium-210 (t_½ = 138.38 days), the penultimate member of the radium decay series. It occurs in uranium ores at about 0.1 mg per tonne of ore (a mass fraction of about 10⁻¹⁰). The overall abundance of polonium in crustal rocks can be estimated at about 3 × 10⁻¹⁰ ppm.

Extraction of polonium from natural sources is impractical, and it is produced commercially by the neutron irradiation of bismuth in a nuclear reactor: 20983Bi(n,γ)21083Bi. The bismuth-210 produced undergoes β-decay (t_½ = 5.01 days) to give polonium-210. Commercial production is about 85 grams per year, 97% of which is produced by a single facility near Samara in Russia.

Use

Polonium-210 is a vitually pure α-emittor (E_α = 5.30 MeV), with only 0.0011% γ-decay, and is used as an intense source of α-particles.

The main commercial use is in anti-static brushes, which can contain up to 500 µCi (20 MBq) ²¹⁰Po. The α particles ionize molecules from the air, which allows an equalization of charges on the surface being brushed and the helps the mechanical removal of dust particles. Smaller quantities – typically 4–40 kBq (0.1–1.0 µCi) – are occasionally used as α particle sources for teaching and research. In both cases, these are sealed sources.

Polonium-210 is also used in a mixture with beryllium oxide as a portable source of neutrons through the 94Be(α,n)126C reaction. The self-heating of polonium-210 has also been used as a source of heat and/or thermoelectric power in space exploration.

Chemical properties

The chemistry of polonium is similar to that of tellurium, except that the +6 oxidation state is notably less stable due to the inert-pair effect. The principal oxidation states are +2 and +4.

Polonium metal dissolves in dilute acids to give a range of polonium(II) salts. The reaction of polonium with oxygen at 250 °C gives amphoteric polonium(IV) oxide, PoO₂, and the the tetrahalides PoCl₄ and PoBr₄ can also be prepared by the direct reaction of the elements. Polonium(IV) oxide is considerably more basic than its tellurium analogue, and polonium(IV) salts such as the nitrate, sulfate and selenate are known: however, hydrated polonium(IV) oxide, PoO(OH)₂, will dissolve in dilute alkali to give salts which are usually written as containing the trioxopolonate(2−) anion, PoO2−3.

Polonium will react with electropositive metals to form polonides. The polonides of the alkali metals and alkaline earth metals can be considered to contain the Po²⁻ anion, and magnesium polonide (MgPo, nickeline structure) will react with dilute acids to give the unstable, gaseous hydrogen polonide, H₂Po. The stable polonides of the transition metals and the lanthanoids usually have a 1:1 stoichiometry, and are best considered as intermetallic compunds.

Physical properties

Polonium displays metallic conductivity, with an electric resistivity of 1.4(1) µΩ m at 20 °C (slightly higher than that of bismuth) and a thermal coefficient of 4.21 × 10⁻³ K⁻¹.

External links

• WebElements
• Argonne National Lanoratory