Polonium

Polonium is a chemical element; it has symbol Po and atomic number 84. A rare and highly radioactive metal with no stable isotopes, polonium is a chalcogen and 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 in uranium ores, as it is the penultimate daughter of natural uranium-238. Though two 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 on 18 July 1898 by Marie Skłodowska-Curie 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 discovered in this way. Polonium was named after Marie Skłodowska-Curie's homeland of Poland, which at the time was partitioned between three countries. 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 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 of ²¹⁰Po emits about as many alpha particles per second as 5 grams of ²²⁶Ra, which means it is 5,000 times more radioactive than radium. A few curies 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.

Solid state form

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. The unit cell has an edge length of 335.2 picometers; the beta form is rhombohedral. The structure of polonium has been characterized by X-ray diffraction and electron diffraction.
²¹⁰Po has the ability to become airborne with ease: if a sample is heated in air to, 50% of it is vaporized in 45 hours to form diatomic Po₂ molecules, even though the melting point of polonium is and its boiling point is.
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⁺. As polonium also emits alpha-particles after disintegration, 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. At pH about 1, polonium ions are readily hydrolyzed and complexed by acids such as oxalic acid, citric acid, and tartaric acid.

Compounds

Polonium has no common compounds, and almost all of its compounds are synthetically created; more than 50 of those are known. 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, the polonide of praseodymium melts at 1250 °C, and that of thulium melts at 2200 °C. PbPo is one of the very few naturally occurring polonium compounds, as polonium alpha decays to form lead.
Polonium hydride is a volatile liquid at room temperature prone to dissociation; it is thermally unstable. Water is the only other known hydrogen chalcogenide which is a liquid at room temperature; however, this is due to hydrogen bonding. The three oxides, PoO, PoO₂ and PoO₃, are the products of oxidation of polonium.
Halides of the structure PoX₂, PoX₄ and PoF₆ are known. They are soluble in the corresponding hydrogen halides, i.e., PoCl_X in HCl, PoBr_X in HBr and PoI₄ in HI. 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.
Other polonium compounds include the polonite, potassium polonite; various polonate solutions; and the acetate, bromate, carbonate, citrate, chromate, cyanide, formate, or hydroxide, nitrate, selenate, selenite, monosulfide, sulfate, disulfate or sulfite salts.
A limited organopolonium chemistry is known, mostly restricted to dialkyl and diaryl polonides, triarylpolonium halides, and diarylpolonium dihalides. Polonium also forms soluble compounds with some ligands, such as 2,3-butanediol and thiourea.

Formula	Color	m.p.	Sublimation temp.	Symmetry	Pearson symbol	Space group	No	a	b	c	Z	ρ	ref
PoO	black
PoO₂	pale yellow	500	885	fcc	cF12	Fmm	225	563.7	563.7	563.7	4	8.94
PoH₂		-35.5
PoCl₂	dark ruby red	355	130	orthorhombic	oP3	Pmmm	47	367	435	450	1	6.47
PoBr₂	purple-brown	270
PoCl₄	yellow	300	200	monoclinic
PoBr₄	red	330		fcc	cF100	Fmm	225	560	560	560	4
PoI₄	black

Oxides

PoO
PoO₂
PoO₃

Hydrides

PoH₂

Halides

PoX₂
PoX₄
PoF₆
PoBr₂Cl₂
Isotopes

There are 42 known isotopes of polonium, all radioactive, stretching from ¹⁸⁶Po to ²²⁷Po. The isotopes 210 through 218 occur naturally in the four principal decay chains; of these, ²¹⁰Po with a half-life of 138.376 days has the longest half-life and is, therefore, the most abundant by mass. It is also the most easily synthesized isotope, by neutron capture on natural bismuth, and so by far the most abundant artificial isotope as well.
Two other isotopes have longer lives: ²⁰⁹Po with a half-life of 124 years and ²⁰⁸Po with a half-life of 2.898 years. Both are made by using a cyclotron to bombard bismuth with protons.

History

Tentatively called "radium F", polonium was discovered by Marie and Pierre Curie in July 1898, and was named after Marie Curie's native land of Poland. Poland at the time was under Russian, German, and Austro-Hungarian partition, and did not exist as an independent country. It was Curie'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.
This element was the first one discovered by the Curies while they were investigating the cause of pitchblende radioactivity. Pitchblende, after removal of the radioactive elements uranium and thorium, was more radioactive than the uranium and thorium combined. This spurred the Curies to search for additional radioactive elements. They first separated out polonium from pitchblende in July 1898, and five months later, also isolated radium. German scientist Willy Marckwald successfully isolated 3 milligrams of polonium in 1902, though at the time he believed it was a new element, which he dubbed "radio-tellurium", and it was not until 1905 that it was demonstrated to be the same as polonium.
In the United States, polonium was produced as part of the Manhattan Project's Dayton Project during World War II. Polonium and beryllium were the key ingredients of the 'Urchin' initiator at the center of the bomb's spherical pit. 'Urchin' initiated the nuclear chain reaction at the moment of prompt-criticality to ensure that the weapon did not fizzle. 'Urchin' was used in early U.S. weapons; subsequent U.S. weapons utilized a pulse neutron generator for the same purpose.
Much of the basic physics of polonium was classified until after the war. The fact that a polonium-beryllium initiator was used in the gun-type nuclear weapons was classified until the 1960s.
The Atomic Energy Commission and the Manhattan Project funded human experiments using polonium on five people at the University of Rochester between 1943 and 1947. The people were administered between of polonium to study its excretion.