Polonium
Polonium is a chemical element with symbol Po and atomic number 84, discovered in 1898 by Marie Curie and Pierre Curie. A rare and highly radioactive element with no stable isotopes, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Applications of polonium are few.
Identity
- Atomic mass
- [209] u
- Category
- Post-transition metals
- Group
- 16
- Period
- 6
- Block
- p
- Oxidation States
- −2, +2, +4, +6
- Phase (25 °C)
- Solid
- Appearance
- silvery
Electronic Structure
- Electron configuration
- 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6 6s2 4f14 5d10 6p4
- Short configuration
- [Xe] 4f14 5d10 6s2 6p4
- Electrons per shell
- 2, 8, 18, 32, 18, 6
- Electronegativity (Pauling)
- 2
- Electron affinity
- 136 kJ/mol
- 1st ionization energy
- 812 kJ/mol
Physical properties
- Melting point
- 253.9 °C (527 K)
- Boiling point
- 961.9 °C (1235 K)
- Density
- 9.196 g/cm³
- Molar heat capacity
- 26.4 J/(mol·K)
Chemical behavior of polonium
Polonium has an electronic configuration [Xe] 4f¹⁴ 5d¹⁰ 6s² 6p⁴. It is a rare, extremely volatile radioactive metalloid belonging to group 16 (chalcogens), located below tellurium. Its oxidation states are -2, +2, +4 and +6, with the +4 state being the most stable and characteristic in aqueous solution. It is a shiny silver-grey metal with a relatively low melting point (254 °C). Its chemistry is dominated by its intense alpha radioactivity: the tremendous ionising auto-irradiation breaks molecular bonds of its own compounds and solvents in minutes, and heats small solid samples to over 500 °C by radiogenic heat.
Reactivity
Po + O₂ → PoO₂
It oxidises spontaneously in contact with air, forming black dioxide of polonium with fluorite-type structure.
Po + 2 HCl → PoCl₂ + H₂
It dissolves easily in dilute mineral acids such as hydrochloric acid, releasing hydrogen gas and forming pink solutions of polonium(II) chloride that then auto-oxidise to yellow tetrachloride of polonium.
Po + HNO₃ → Po(NO₃)₄
It reacts with concentrated nitric acid forming oxidising solutions of polonium(IV) nitrate.
Po + Na₂S → PoS + 2 Na⁺
It precipitates quantitatively in acidic or neutral medium with sulfides, forming black insoluble polonium sulfide.
Notable compounds
| Compound | Formula | Use or importance |
|---|---|---|
| Polonium dioxide | PoO₂ | Dark solid with fluorite-type cubic crystal lattice, the most stable oxide of polonium formed in air |
| Polonium tetrachloride | PoCl₄ | Vivid yellow crystals that sublime and easily decompose by alpha radiolysis of their own chemical bonds |
| Lead polonide | PbPo | Very thermally stable intermetallic compound used to immobilise polonium in neutron sources |
| Polonium hydride | PoH₂ | Very unstable and endothermic inorganic gas that spontaneously decomposes into metallic polonium and hydrogen |
Electron shell distribution
View in the atom builderIsotopes
Polonium has no stable isotopes: all its more than forty isotopes are radioactive. The most famous and commercially available is polonium-210 (²¹⁰Po, with 126 magic neutrons), a nearly pure alpha emitter with a half-life of 138.38 days that is part of the natural decay chain of uranium-238. The longest-lived isotope is polonium-209 (²⁰⁹Po, half-life of 125 years), produced only in cyclotron accelerators by bombarding bismuth with protons.
Occurrence and production
It is one of the rarest natural elements on the planet (about 0.2 parts per trillion, 0.2 ppt), millions of times scarcer than radium. In nature it exists only as an ephemeral product of uranium decay in minerals such as pitchblende (uraninite), where it is found only about a milligram of polonium per 25,000 kilograms of mineral uranium. Today, polonium-210 is artificially manufactured in nuclear reactors by bombarding bismuth-209 with thermal neutrons, a process that produces bismuth-210 which decays beta to polonium-210.
History and origin of the name
It was discovered in Paris in July 1898 by the scientists Marie Skłodowska-Curie and Pierre Curie while investigating the anomalous radioactivity of Joachimsthal pitchblende. It was the first chemical element discovered thanks to the phenomenon of radioactivity. Marie Curie gave it the name 'polonium' in honour of her native land, Poland, which at that time did not exist as a sovereign state and was under the domination of the Russian, Austro-Hungarian and Prussian empires, with the intention of making visible the cause of her independence.
Applications and relevance of polonium
Uses of the element
Polonium-210 is one of the most concentrated alpha sources in the world: a single gram generates 140 watts of continuous thermal power by alpha particle bombardment. It has been used as a thermal energy source in radioisotope thermoelectric generators (RTGs) on spacecraft and in Soviet lunar rovers Lunokhod to keep their instruments warm during the freezing lunar night. It is also mixed with beryllium to make portable neutron sources for starting nuclear reactors and in industrial anti-static brushes to eliminate charges on photographic films.
Uses of its compounds
Given its extreme lethal radioactivity and short half-life, its compounds have no commercial applications outside nuclear physics laboratories and specialised weapons.
Health, safety and environment
Biological role
It plays no biological role in any living thing and is one of the most lethal radiological toxins known to science: its toxicity by weight is billions of times greater than that of hydrogen cyanide. A microdose of less than one microgram is sufficient to cause human death from acute radiation syndrome.
Safety
Its alpha particles are stopped by a simple sheet of paper or the outer layer of dead skin and do not penetrate from outside; but if inhaled, ingested or absorbed through wounds, polonium-210 is distributed to the spleen, liver, kidneys and bone marrow. The intense ionising radiation of alpha rays destroys cellular DNA and internal tissues, causing fulminating multiorgan failure that conventional Geiger detectors may not easily warn about.
Environment
Polonium-210 is found naturally in air and soils by the decay of radon. It is absorbed by the leaves of tobacco plants through their sticky glandular trichomes, converting cigarette smoke into a source of inhalation of radioactive alpha-emitting particles in smokers' lungs.
Frequently asked questions about Polonium
Why does polonium release so much heat spontaneously?
Polonium-210 is so intensely radioactive that a single gram emits billions of alpha particles per second. Being heavy particles, they immediately collide with the very atoms of the piece of metal and transfer all their kinetic energy over a microscopic distance. This radiation self-disintegration turns decay into pure heat, making the metal turn red-hot and easily exceed five hundred degrees Celsius.
What did the radiological poisoning media of Alexander Litvinenko with polonium-210 in London consist of?
In 2006, the Russian former spy Alexander Litvinenko suddenly fell ill in London after drinking tea in a hotel. He rapidly presented rapid hair loss, immune system failure and fulminating multiorgan collapse without doctors finding known toxins in his blood. Hours before dying, advanced gamma spectroscopy analysis revealed that he had ingested micrograms of polonium-210. The alpha radiation tore his digestive tract and bone marrow from the inside without being initially detected by surface Geiger counters.
How do industrial anti-static brushes incorporating polonium-210 work?
In optical lens factories, paper printing presses and cleanrooms for microelectronics, friction generates static electric charges that attract microscopic dust difficult to remove. Anti-static brushes house a microscopic sealed sheet of polonium-210 that emits alpha particles into the surrounding air. The ionising radiation ionises oxygen and nitrogen molecules in air into positive and negative ions, instantly neutralising the static charge on the lens and allowing it to be cleaned without friction.
How is polonium-210 synthesised in nuclear reactors from bismuth?
Given the extreme natural scarcity of polonium, the entire world's commercial polonium is artificially produced in special Russian nuclear reactors. Pure 209-bismuth rods are introduced into the reactor core, where they absorb slow thermal neutrons to transform into bismuth-210. This intermediate isotope beta-decays with a five-day half-life becoming polonium-210. The rods are removed and heated in vacuum distillation furnaces to evaporate and condense the pure polonium.
How did they keep the polonium heat in Soviet lunar vehicles Lunokhod?
During the fourteen consecutive Earth days of the Moon's nights, temperatures drop to freezing −150 °C, which would freeze the batteries and electronics of any exploration vehicle. In the Soviet missions Lunokhod 1 and Lunokhod 2 of the 1970s, engineers installed an isotopic source of polonium-210 in the pressurised compartment. The constant radiogenic heat of polonium kept the scientific equipment warm until lunar dawn without consuming electricity.
Why does tobacco smoke contain radioactive polonium?
Phosphate fertilisers used in intensive tobacco farming contain traces of uranium and radium. These minerals decompose releasing radon gas, whose radioactive decay products — especially polonium-210 — adhere to the sticky glandular hairs of tobacco leaves. When smoking, polonium is inhaled with the smoke and deposits in the lungs, where it emits alpha radiation that damages tissues.
Why are polonium's alpha particles harmless outside but lethal if ingested?
Alpha particles are heavy helium nuclei with low penetrating capacity: they easily collide with matter and cannot cross a sheet of paper or the outer dead corneal layer of human skin. However, if polonium is inhaled or ingested, it comes into intimate and direct contact with living cells of the stomach, lungs and blood. There it releases all its massive ionising energy, tearing cellular DNA helices.
Main sources consulted: PubChem, Royal Society of Chemistry, NIST, IUPAC. Last revision: November 2026.