Polonium Facts – Po or Atomic Number 84


Polonium Facts

Polonium is a rare and highly radioactive element with the symbol Po and atomic number 84. A metal or metalloid in the periodic table, it lies in the p-block, and is a member of group 16, the chalcogens. Its discovery in 1898 marked a significant milestone in the study of radioactivity.

Discovery of Polonium

Polonium was discovered by Marie and Pierre Curie in 1898 in Paris.

The story began in 1896 when Henri Becquerel discovered radioactivity. Inspired by his findings, Marie Curie decided to investigate uranium radiation as a potential field of research for her doctoral thesis. Marie Curie, with her husband Pierre, began by examining the uranium-rich mineral pitchblende, which exhibited stronger radioactive activity than uranium alone. This anomaly suggested the presence of other radioactive elements. They used an electrometer built by Pierre Curie and his brother to measure the ionizing radiation of the samples, a method more quantitative than the photographic approach used by Becquerel. Polonium was the first element discovered using its radioactivity.

Isolation

The isolation of polonium was challenging due to its extreme rarity and radioactivity.

The Curies processed tons of pitchblende, laboriously refining the ore to remove uranium and other elements. This involved multiple steps of dissolving the ore, precipitating out different elements, and examining the radioactive residues left behind.

They employed a fractional crystallization process. By repeatedly dissolving the residues in acid and then adding hydrogen sulfide, different compounds were formed at different rates, allowing for a gradual separation of elements. In July 1898, they identified a substance many times more radioactive than uranium. It was only after several years of further refining and isolating that they gathered enough polonium to confirm its properties and prove it was indeed a new element.

For their groundbreaking work, the Curies, along with Henri Becquerel, were awarded the Nobel Prize in Physics in 1903.

Naming

Polonium takes its name for Poland, the homeland of Marie Curie, as a way of highlighting her country’s political struggle for independence.

Appearance

Polonium is a silvery-gray metal or metalloid solid that is very unstable and highly radioactive. It has a metallic luster that quickly tarnishes to a dull gray when exposed to air. This tarnishing results from the reaction of polonium with atmospheric gases.

The element occurs in two crystal forms or allotropes. Polonium’s alpha form, the most stable under standard conditions, has a simple cubic crystal structure. The beta form has a rhombohedral crystal structure.

Polonium Properties

  • Radioactivity: One of the most notable properties of polonium is its intense radioactivity. It emits alpha particles as it decays, making it highly hazardous. But, around one in 100,000 causes an excitation in the atomic nucleus that produces a gamma ray. It is around 5000 times more radioactive than radium.
  • Blue Glow: Due to its high radioactivity, polonium ionizes molecules in air, producing a faint blue glow surrounding the element.
  • Density: Polonium has a high density of 9.196 grams per cubic centimeter, which is more than that of many common metals.
  • Volatility: Polonium is unusually volatile, adding to its risk factor.
  • Melting and Boiling Points: Polonium’s melting point is at 254 °C, and its boiling point is 962 °C, both of which are relatively low for metals.
  • Thermal Conductivity: It has a low thermal conductivity compared to most metals, but this varies depending on the allotrope and its purity.
  • Chemical Reactivity: Chemically, polonium dissolves readily in dilute acids but only slightly in alkalis.
  • Compounds: In its compounds, polonium usually exhibits the +2 and +4 oxidation states, the latter being more stable. Polonium dihalides and polonium tetrachloride are examples of its chemical compounds.
  • Self-Heating: Due to its radioactivity, polonium displays self-heating. A gram of Po-210 reaches temperatures up to 500 °C as a result of the energy released by its radioactive decay.
  • Anomalous Electrical Conductivity: Polonium’s electrical conductivity behaves anomalously compared to metals, partly due to the lattice damage caused by its own radioactivity.
  • Toxicity: While not actually toxic, polonium’s radioactivity poses a significant risk to human health if ingested or inhaled.

Element Group

Polonium is in group 16 on the periodic table. This makes it a member of the oxygen family or chalcogens. As a member of the chalcogen group, it shares chemical properties in common with selenium and tellurium. This group is known for its variable oxidation states and reactivity. Polonium has metallic character similar to its horizontal neighbors on the periodic table: lead, strontium, and bismuth.

Isotopes

Polonium has 42 known isotopes, ranging from atomic mass 186 to 227. All of the isotopes are radioactive. Po-210 is the most widely known, produced by neutron capture by bismuth. Po-209 (half-life of 124 years and the longest-lived polonium isotope) and Po-208 occur via cyclotron bombardment of lead or bismuth. The isotopes Po-210 to Po-218 are trace decay products of U-238. Decay schemes of thorium and neptunium also produce polonium.

Abundance and Sources

Today, most polonium comes from neutron irradiation of bismuth.

Polonium is extremely rare in nature and occurs in trace amounts in uranium ores. It occurs in uranium ores at concentrations of about 100 micrograms per ton. Its scarcity is due to its short half-life.

Tobacco smoke from plants grown using phosphate fertilizers contains polonium-210. The element also occurs in the food chain, especially from seafood.

Uses of Polonium

Polonium has several niche uses:

  • Radioisotope Thermoelectric Generators (RTGs): Polonium finds use in spacecraft for its ability to generate heat through radioactive decay.
  • Heat Source in Spacecraft: Apart from RTGs, polonium is a heat source in lunar and planetary exploration missions for maintaining instrument temperatures.
  • Anti-static Devices: Useful in industries for eliminating static charges in machinery.
  • Neutron Initiator: Polonium in combination with beryllium initiates nuclear reactions by emitting neutrons. This has applications in nuclear weapons, where the Po-Be combination is a neutron source for triggering the nuclear chain reaction.
  • Research and Radiography: In scientific research, polonium is a radioactive tracer. It is also useful in certain types of radiography, although this use is limited due to safety concerns.
  • Brushes for Removing Dust: The anti-static properties of polonium are useful in brushes that remove dust from photographic films and other delicate surfaces without the risk of static charge damage.
  • Luminescent Paints: In the early 20th century, polonium tested in luminescent paints, although this use was discontinued due to safety concerns over its radioactivity.
  • Spark Plugs: Firestone made spark plugs using polonium from 1940 to 1953, based on the radium spark plug. Radioactive spark plugs were discontinued because buildup quickly blocked the radiation that supposedly improved engine performance by ionizing fuel in the cylinder.
  • Education and Collecting: Tiny amounts of Po-210 are sold as “needle sources.” A metal plating over the polonium permits alpha particle radiation, while prevent polonium vaporization.

Oxidation States and Main States

Polonium exhibits multiple oxidation states, including -2, +2, +4, +5, and +6. The +2 and +4 states are the most stable.

Biological Role and Health Effects

Polonium has no known biological role. It is not chemically toxic. Instead, the risk is a result of its extreme radioactivity. Its volatility poses a significant inhalation hazard. The alpha particles cause intense ionization effects within cells, leading to cellular and molecular damage. Chronic exposure, even at lower levels, increases the risk of cancers and cumulative damage to tissues and organs. Exposure to high doses of polonium causes acute radiation sickness, characterized by symptoms like nausea, fatigue, hair loss, and diminished organ function, leading to potential organ failure and death. Chelation ages, such as dimercaprol, show promise for decontamination. While cases of accidental poisoning are rare, there are cases of intentional poisoning. The most famous case is that of former Russian FSB agent Alexander Litvinenko.

Key Polonium Facts for Scientists

PropertyPolonium (Po) Information
NamePolonium
SymbolPo
Atomic Number84
Atomic Weight[209]
Group16 (Chalcogens)
Period6
Blockp-block
Electron Configuration[Xe] 4f14 5d10 6s2 6p4
Electrons Per Shell2, 8, 18, 32, 18, 6
State at Room Temp.Solid
Melting Point254 °C
Boiling Point962 °C
Density9.196 g/cm³ (α-Po); 9.398 g/cm³ (β-Po)
Heat of Fusion~13 kJ/mol
Heat of Vaporization102.91 kJ/mol
Molar Heat Capacity26.4 J/(mol·K)
Oxidation States-2, +2, +4, +5, +6
ElectronegativityPauling scale: 2.0
Ionization Energies1st: 812.1 kJ/mol
Atomic Radiusempirical: 168 pm
Covalent Radius140±4 pm
Van der Waals Radius197 pm
Crystal StructureSimple cubic (alpha), rhombohedral (beta)
Thermal Conductivity20 W/(m·K)
Electrical Resistivity(20 °C) 0.4 µΩ·m
Magnetic OrderingNon-magnetic

References

  • Curie, P.; Curie, M. (1898). “Sur une substance nouvelle radio-active, contenue dans la pechblende” [On a new radioactive substance contained in pitchblende]. Comptes Rendus (in French). 127: 175–178.
  • Desando, R. J.; Lange, R. C. (1966). “The structures of polonium and its compounds—I α and β polonium metal”. Journal of Inorganic and Nuclear Chemistry. 28 (9): 1837–1846. doi:10.1016/0022-1902(66)80270-1
  • Emsley, John (2001). Nature’s Building Blocks. New York: Oxford University Press. pp. 330–332. ISBN 978-0-19-850341-5.
  • Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5.
  • Radioactive spark plugs“. Oak Ridge Associated Universities.
  • Thayer, John S. (2010). “Relativistic Effects and the Chemistry of the Heavier Main Group Elements”. Relativistic Methods for Chemists. Challenges and Advances in Computational Chemistry and Physics. 10: 78. ISBN 978-1-4020-9974-8. doi:10.1007/978-1-4020-9975-5_2