Polonium

In July 1898, Marie and Pierre Curie announced that pitchblende — a uranium-bearing ore — was far more radioactive than its uranium content alone could explain. Something else was there. After processing tonnes of ore in a leaking shed over months of brutal labor, they isolated a new element and named it polonium, after Marie's homeland of Poland, then occupied by Russia, Prussia, and Austria and without a place on any political map. The name was a quiet act of patriotism. Polonium was the first element ever discovered by following radioactivity rather than chemical properties. It is extraordinarily rare in nature — a single gram requires processing about 100 tonnes of uranium ore — and intensely radioactive. Its most accessible isotope, Po-210, emits pure alpha particles with such density that a microgram-scale source generates measurable heat, a fact with both peaceful and lethal applications.

Polonium-210's most benign commercial application is in anti-static brushes used to neutralize static charge on photographic film, vinyl records, and precision optical equipment during manufacturing and cleaning. A tiny Po-210 source ionizes the surrounding air, allowing charge to dissipate without physical contact. Po-210 also found use in anti-static devices in textile mills and paper processing. In nuclear weapons, polonium-beryllium initiators (nicknamed 'urchins') provided the burst of neutrons needed to start the fission chain reaction at precisely the moment of maximum compression; this was the design used in the Fat Man bomb dropped on Nagasaki. The Soviet Luna and early Lunokhod spacecraft used Po-210 heater units to keep electronics warm during the lunar night, taking advantage of the natural heat generated by its dense alpha decay. Polonium has been explored as a power source for thermoelectric generators. Its highest-profile recent use was as a radiological assassination weapon — the deliberate poisoning of Alexander Litvinenko in London in 2006.

Marie Sklodowska Curie and Pierre Curie discovered polonium in July 1898 after systematically tracking the anomalous radioactivity of pitchblende from the Joachimsthal mines in Bohemia. Marie had noted in her doctoral research that the radioactivity of uranium compounds was strictly proportional to uranium content — implying radioactivity was an atomic property — and that pitchblende was four times more active than its uranium alone warranted. The isolation required processing enormous quantities of ore; Marie Curie spent years separating the tiny amounts of polonium and radium present. The Curies announced polonium in a paper to the French Academy of Sciences on July 18, 1898, followed by radium in December of the same year. Marie Curie won the Nobel Prize in Physics in 1903 (shared with Pierre and Henri Becquerel) partly for this work, and the Nobel Prize in Chemistry in 1911 for the isolation of radium and polonium. Pierre was killed by a horse-drawn cart in 1906; Marie Curie died in 1934 of aplastic anemia, almost certainly caused by decades of radiation exposure.

Polonium is one of the rarest naturally occurring elements. In Earth's crust it exists only as an intermediate decay product in the uranium-238 and uranium-235 decay chains — it is produced and immediately consumed as radioactive decay proceeds toward stable lead. The concentration of polonium-210 in equilibrium with uranium in typical crustal rock is on the order of 0.1 parts per quadrillion by mass. Uranium ores like pitchblende contain slightly higher concentrations, but even high-grade ores yield only about 100 micrograms of polonium per tonne of ore. Virtually all polonium used today is produced artificially by bombarding bismuth-209 targets with neutrons in a nuclear reactor; the bismuth captures a neutron to become Bi-210, which then beta-decays to Po-210 with a half-life of about five days. Russia has historically been the primary commercial producer through its reactor network. Trace amounts of Po-210 enter the human body naturally through food, particularly shellfish and leafy vegetables grown in polonium-bearing soils, and tobacco leaves concentrate polonium from phosphate fertilizers.

Polonium behaves chemically like tellurium and selenium, its neighbors in Group 16, but its intense radioactivity dominates all practical chemistry — samples of its compounds are self-heating, self-irradiating, and continuously destroyed by their own radiation. Polonium dissolves in dilute acids to give the Po²⁺ cation and forms a range of salts: polonium chloride (PoCl₂), polonium sulfate (PoSO₄), and polonium nitrate are known. Polonium dioxide (PoO₂) forms when the metal is oxidized in air. Polonium also forms alloys with metals — polonium-nickel alloys have been studied for emitter applications. The most toxicologically significant 'compound' is not a formal chemical compound at all: when Po-210 is used as a poison, it enters the body as the free metal or in solution and distributes into tissues, where its alpha emission destroys cells from within. Because alpha particles do not penetrate skin or paper, Po-210 is safe to handle externally but catastrophically destructive when ingested or inhaled. Litvinenko received an estimated lethal dose of around one microgram.

• Marie Curie's laboratory notebooks from the 1890s are still too radioactive to handle safely without protection — they are kept in lead-lined boxes in the Bibliothèque nationale de France, and researchers who wish to examine them must sign a liability waiver.
• A single gram of polonium-210 generates about 140 watts of heat purely from radioactive decay — enough to glow red-hot — making it one of the most power-dense natural substances and explaining its use as a compact heat source on spacecraft.
• Tobacco leaves accumulate polonium-210 from phosphate fertilizers, and studies estimate that a pack-a-day smoker receives a radiation dose from inhaled Po-210 equivalent to roughly 300 chest X-rays per year, contributing to the link between smoking and lung cancer.
• The assassination of Alexander Litvinenko with polonium-210 in 2006 was identified partly because polonium emits virtually no gamma radiation, making the perpetrators believe it was undetectable — but sensitive alpha-particle detection equipment eventually traced the poison trail across several London locations.
• All 33 known isotopes of polonium are radioactive, with half-lives ranging from microseconds to 103 years — making it one of the elements with no stable isotopes at all, a distinction it shares with every element heavier than bismuth.