Polonium

84 bismuth ← polonium → astatine Te ↑ Po ↓ Uuh 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] 4f14 5d10 6s2 6p4
Electrons per shell	2, 8, 18, 32, 18, 6
Physical properties
Phase	solid
Density (near r.t.)	(alpha) 9.196 g/cm³
Density (near r.t.)	(beta) 9.398 g/cm³
Melting point	527 K (254 °C, 489 °F)
Boiling point	1235 K (962 °C, 1764 °F)
Heat of fusion	ca. 13 kJ/mol
Heat of vaporization	102.91 kJ/mol
Heat capacity	(25 °C) 26.4 J/(mol·K)
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·K)
Thermal expansion	(25 °C) 23.5 µm/(m·K)
CAS registry number	7440-08-6
Notable isotopes
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

Polonium (chemical symbol Po, atomic number 84) is a rare, extremely toxic, and highly radioactive chemical element. It is found in small amounts in uranium ores and is chemically similar to tellurium and bismuth. It has been studied for possible use in heating spacecraft.

Occurrence

A very rare element in nature, polonium is found in uranium ores at about 100 micrograms per metric ton (1:10¹⁰). Its natural abundance is approximately 0.2% that of radium. It has also been identified in tobacco smoke from tobacco leaves grown in certain places.[1][2]

A 1934 experiment showed that when natural bismuth-209 was bombarded with neutrons, it was converted to bismuth-210, the parent of polonium. This procedure can now be applied to produce polonium in milligram amounts, using the high neutron fluxes present in nuclear reactors.

History

Once known as Radium F, polonium was discovered by Maria Skłodowska-Curie (Marie Curie) and her husband Pierre Curie in 1898. Later, the element was named after Marie's homeland of Poland (Latin word Polonia). At the time, Poland was not recognized as an independent country but was partitioned and under Russian, Prussian, and Austrian domination. It was Marie's hope that naming the element after her homeland would call attention to its plight.

This element was the first one discovered by the Curies while they were investigating the cause of the radioactivity of pitchblende, a uranium-rich mineral. The pitchblende, after removal of uranium and radium, was more radioactive than radium and uranium put together. This spurred them on to find the element. The electroscope showed that it separated with bismuth.

Notable characteristics

Polonium is a chemical element that lies in group 16 (former group 6A) of the periodic table, just below tellurium. In addition, it is part of period 6, situated between bismuth and astatine. Like tellurium, polonium is classifed as a metalloid, because its properties are intermediate between those of metals and nonmetals. Polonium is highly radioactive, and all elements from polonium onward are significantly radioactive.

This substance dissolves readily in dilute acids but is only slightly soluble in alkalis. Chemically, it is closely related to bismuth and tellurium. Polonium (in common with plutonium-238) has the ability to become airborne with ease. To explain this phenomenon, one hypothesis suggests that small clusters of polonium atoms may be ejected during the emission of alpha particles.

Some researchers have reported that methyl groups can be attached to polonium by certain microbes or by the chemical compound methylcobalamin.

Solid state form

A model of the alpha form of solid polonium.

Solid polonium can exist in two forms: alpha and beta. In the alpha form, the atoms are arranged as a simple cubic crystal system that is not interpenetrated, as shown in the illustration. In the beta form, the atoms lie in a hexagonal arrangement.

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

Isotopes

Polonium has no stable isotopes and has over 50 potential isotopes. Polonium has many isotopes all of which are radioactive. There are 25 known isotopes of polonium with atomic masses that range from 194 u to 218 u. Polonium-210 is the most widely available. Polonium-209 (half-life 103 years) and polonium-208 (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 thorium-232 and uranium-238; 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 138.39 days. A milligram of polonium-210 emits as many alpha particles as 5 grams of radium. A great deal of energy is released by its decay with a half a gram quickly reaching a temperature above 750 K. A few curies (gigabecquerels) of polonium-210 emit a blue glow which is caused by excitation of surrounding air. A single gram of polonium-210 generates 140 watts of heat energy. Since nearly all alpha radiation can be easily stopped by ordinary containers and upon hitting its surface releases its energy, polonium-210 has been used as a lightweight heat source to power thermoelectric cells in artificial satellites. A polonium-210 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. Because of its short halflife though polonium-210 cannot provide power for long-term space missions and has been phased out of use in this application.

Applications

• When mixed or alloyed with beryllium, polonium can be a source of neutrons.

• Polonium has been used in devices that eliminate static electric charge in textile mills and elsewhere. However, beta sources are more commonly used, being less dangerous.

• Polonium is used in brushes that remove dust from photographic film. It is sealed in these brushes and the radioactive emissions are controlled, thus minimizing radiation hazards.

Precautions

Polonium is a highly radioactive, toxic element and should be handled with extreme care. Handling even milligram or microgram amounts of polonium-210 is very dangerous and requires special equipment with strict procedures. Body tissues can be directly damaged by energy absorbed from alpha particles.

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 × 10¹¹ times as toxic as hydrocyanic acid. The maximum permissible concentration for airborne soluble polonium compounds is about 7,500 becquerels per cubic meter (2 × 10^-11 microcurie per cubic centimeter).

References

ISBN links support NWE through referral fees

• Los Alamos National Laboratory – Polonium

External links

• WebElements.com – Polonium
• History of Polonium

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