Polonium is an extremely rare and highly radioactive element known for its intense energy output. This unique substance, a silvery-gray metal in its purified form, exists in trace amounts across the environment and can be produced synthetically in specialized facilities. Its remarkable properties make it both a subject of scientific interest and a substance of notorious danger.
Defining Element 84: Basic Properties and Discovery
Polonium, designated by the chemical symbol Po and having the atomic number 84, is classified as a metalloid element. The most common and studied isotope is Polonium-210 (\(\text{Po-210}\)), defined by its 84 protons and 126 neutrons. This particular isotope is characterized by an extremely short half-life of approximately 138 days, meaning half of any given sample will radioactively decay in just over four months.
The element was the first to be discovered through radiochemical analysis in 1898. Researchers isolated the strongly radioactive substance from pitchblende, a uranium ore, after observing that the ore was more radioactive than the uranium it contained. The element was named polonium in honor of Marie Curie’s home country, Poland.
Natural Occurrence and Industrial Production
Polonium-210 exists naturally as a product in the long radioactive decay chain of Uranium-238. Its natural presence is minute, with only trace amounts found in the Earth’s crust, air, and soil. For instance, a metric ton of uranium ore contains less than 0.1 milligram of \(\text{Po-210}\), making extraction from natural sources impractical for commercial use.
To obtain commercially viable quantities, polonium is produced artificially in nuclear reactors. This synthesis involves bombarding the stable isotope Bismuth-209 (\(\text{Bi-209}\)) with neutrons. This neutron capture creates the short-lived Bismuth-210 (\(\text{Bi-210}\)), which rapidly undergoes beta decay to form \(\text{Po-210}\). Only about 100 grams of the element are estimated to be produced globally each year using this method.
The Danger Profile: Extreme Radioactivity and Toxicity
Polonium-210 is a pure alpha emitter, meaning its primary form of decay is the release of alpha particles. Alpha particles are composed of two protons and two neutrons, identical to a helium nucleus, and carry a high amount of energy. Polonium-210 is considered one of the most toxic substances known, being hundreds of billions of times more toxic than hydrogen cyanide by mass.
A distinguishing characteristic of alpha radiation is its low penetrating power; alpha particles cannot penetrate a sheet of paper or the dead outer layer of human skin. Therefore, polonium poses virtually no health risk when it remains outside the body. However, the danger profile changes drastically if the polonium is ingested, inhaled, or absorbed through a wound.
Once internalized, the high-energy alpha particles are released directly into sensitive living tissue. The short range of the alpha particles means all their destructive energy is concentrated within a very small area, causing localized, severe damage to cells and DNA. This internal irradiation can lead to acute radiation syndrome, with damage concentrating in organs like the bone marrow, spleen, kidneys, and liver. The amount required to be lethal is exceptionally small, with estimates suggesting that less than a microgram of \(\text{Po-210}\) can be a fatal dose.
Practical Applications
Despite its extreme toxicity, polonium’s unique physical properties have led to a few specialized applications. One gram of Polonium-210 generates approximately 140 watts of heat due to its rapid alpha decay, making it a powerful and lightweight heat source. This heat-generating capability has been used in Radioisotope Thermoelectric Generators (RTGs) for space exploration, where it provides thermal energy to power spacecraft systems.
The element is also used in anti-static devices because of its ability to ionize the surrounding air. When the alpha particles interact with the air molecules, they create charged ions that can neutralize static electricity. This function is utilized in specialized equipment, such as brushes used to eliminate static charge on photographic film or camera lenses, and in industrial machinery that produces sheet plastics or paper.