242 | The History and Use of Our Earth’s Chemical Elements Properties
242 | The History and Use of Our Earth’s Chemical Elements Properties
Most of the known chemistry of polonium is based on the naturally occurring radioactive isotope polonium-210, which is a natural radioactive decay by-product of the uranium decay series. Its melting point is 254°C, its boiling point is 962°C, and its density is 9.32g/cm 3 . Po-210 is a strong emitter of alpha particles. One gram of Po-210 will produce about 140 watts of energy, making it ideal as a lightweight thermoelectric power source for space vehicles. It has a half-life of 138.39 days.
Po-209, the most stable isotope of polonium, decays into lead-205 by alpha decay. It costs about $3,000 per microcurie, which is a very small amount of polonium.
Characteristics Polonium is more metallic in its properties than the elements above it in group 16. It is
the only element in group 16 that is naturally radioactive. It is in a position on the periodic table of elements where it can be a metal, metalloid, or nonmetal. It is more often considered
a metal because of its electrical conductivity decreases with an increase in temperature. As an oxide, it is similar to the reddish color of tellurium oxide. Chemically, it behaves similar to tellurium, lead, and bismuth.
AbundanceandSource Polonium is found only in trace amounts in the Earth’s crust. In nature it is found in
pitchblende (uranium ore) as a decay product of uranium. Because it is so scarce, it is usually artificially produced by bombarding bismuth-209 with neutrons in a nuclear (atomic) reactor, resulting in bismuth-210, which has a half-life of five days. Bi-210 subsequently decays into Po-210 through beta decay. The reaction for this process is 209 Bi( n,γ ) 210 Bi → 210 Po + β-. Only small commercial milligram amounts are produced by this procedure.
History Marie Sklodowska Curie (1867–1934) and Pierre Curie (1859–1906) are credited with
discovering polonium as they sought the source of radiation in pitchblende after they removed the uranium from its ore. Their discovery in 1898 led to the modern concepts of the nucleus of the atom, its structure, and how it reacts.
They knew there must be another radioactive element in the pitchblende after the uranium was removed. Marie Curie painstakingly processed a ton of pitchblende to recover only a small amount of uranium. Even so, there was still something radioactive in all that processed pitchblende. As it turned out, there were two radioactive elements that she was able to isolate. One was radium, and the other polonium. They were identified by using piezoelectricity, discovered by her husband Pierre Curie, which could measure the strength of radiation given off by the radioactive elements with which Marie Curie was working.
Marie Curie named polonium after her native country of Poland. She is also given credit for coining the world “radioactivity.” She is one of only two chemists to receive two Nobel Prizes. In 1903 both the Curies and Antoine-Henri Becquerel (1852–1908) shared the Nobel Prize for Physics for their work on radioactivity; in 1911 Madame Curie received the prize for chemistry for the discovery of radium and plonium. (The other scientist who received two Nobel Prizes was Linus Pauling [1901–1994], one for chemistry in 1954, and a Nobel Peace
243 Prize in 1962.) Madame Curie died from radiation poisoning that resulted from her work
Guide to the Elements |
with radioactive elements. CommonUses
There are not many uses for polonium. Probably the most important is as a source of alpha particles (nuclei of helium atoms) and high-energy neutrons for research and radiation studies. It is also used to calibrate radiation-detection devices.
Polonium is used to eliminate static electricity in industrial processes, such as rolling out paper, wire, or sheet metal in mills. Polonium is also sometimes used in “brushes” to remove dust from photographic film and in the manufacturing of spark plugs that make ignition sys- tems in automobiles more efficient, particularly in extremely cold temperatures. It can also be used as a portable, low-level power source and, since polonium is fissionable, used in nuclear weapons and nuclear electric power plants.
ExamplesofCompounds Following are two examples of polonium’s +2 and +4 oxidation states, in which the element
\mostly forms compounds with nonmetals. Polonium (II) oxide (PoO): Po 2+ +O 2- → PoO.
Polonium (IV) tetrachloride (PoCl 4 ): Po 4+ + 4Cl 1- → PoCl 4 .
Both the compounds SPoO 3 and SePoO 3 are similar to tellurium compounds in that they are bright red in color.
Hazards Even though polonium is a rare element, it is a very dangerous radiation source and should
be avoided. Cigarette smoke contains a minute amount of polonium, along with many other carcino- genic chemicals, many of which can cause lung cancer. Over one hundred trace elements and compounds have been identified in cigarette smoke besides polonium. Some examples are nicotine, cresol, carbon monoxide, pyridine, and the carcinogenic compound benzopyrene.
The Halogen Group (Nonmetal Oxidizers): Periods 2 to 6, Group 17 (VIIA)