radium

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francium ← radium → actinium Ba ↑ Ra ↓ Ubn 88Ra Periodic table
Appearance
silvery white metallic
General properties
Name, symbol, number	radium, Ra, 88
Pronunciation	/ˈreɪdiəm/ ray-dee-əm
Element category	alkaline earth metal
Group, period, block	2, 7, s
Standard atomic weight	(226)
Electron configuration	[Rn] 7s2
Electrons per shell	2, 8, 18, 32, 18, 8, 2 (Image)
Physical properties
Phase	solid
Density (near r.t.)	5.5 g·cm−3
Melting point	973 K, 700 °C, 1292 °F
Boiling point	2010 K, 1737 °C, 3159 °F
Heat of fusion	8.5 kJ·mol−1
Heat of vaporization	113 kJ·mol−1
Vapor pressure
P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 819 906 1037 1209 1446 1799
Atomic properties
Oxidation states	2 (strongly basic oxide)
Electronegativity	0.9 (Pauling scale)
Ionization energies	1st: 509.3 kJ·mol−1
	2nd: 979.0 kJ·mol−1
Covalent radius	221±2 pm
Van der Waals radius	283 pm
Miscellanea
Crystal structure	body-centered cubic
Magnetic ordering	nonmagnetic
Electrical resistivity	(20 °C) 1 µΩ·m
Thermal conductivity	18.6 W·m−1·K−1
CAS registry number	7440-14-4
Most stable isotopes
Main article: Isotopes of radium
iso NA half-life DM DE (MeV) DP 223Ra trace 11.43 d α 5.99 219Rn 224Ra trace 3.6319 d α 5.789 220Rn 226Ra ~100% 1601 y α 4.871 222Rn 228Ra trace 5.75 y β− 0.046 228Ac
v d e · r

Radium ( /ˈreɪdiəm/ ray-dee-əm) is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226, which has a half-life of 1601 years and decays into radon gas. Because of such instability, radium is luminescent, glowing a faint blue.

Radium, in the form of radium chloride, was discovered by Marie Skłodowska-Curie and Pierre Curie in 1898. They extracted the radium compound from uraninite and published the discovery at the French Academy of Sciences five days later. Radium was isolated in its metallic state by Marie Curie and André-Louis Debierne through the electrolysis of radium chloride in 1910. Since its discovery, it has given names like radium A and radium C₂ to several isotopes of other elements that are decay products of radium-226.

In nature, radium is found in uranium ores in trace amounts as small as a seventh of a gram per ton of uraninite. Radium is not necessary for living organisms, and adverse health effects are likely when it is incorporated into biochemical processes because of its radioactivity and chemical reactivity.

Characteristics

Physical characteristics

Although radium is not as well studied as its stable lighter homologue barium, the two elements have very similar properties. Their first two ionization energies are very similar: 509.3 and 979.0 kJ·mol⁻¹ for radium and 502.9 and 965.2 kJ·mol⁻¹ for barium. Such low figures yield both elements' high reactivity and the formation of the very stable Ra²⁺ ion and similar Ba²⁺.

Pure radium is a white, silvery, solid metal, melting at 700 °C (1292 °F) and boiling at 1737 °C (3159 °F), similar to barium. Radium has density of 5.5 g•cm³; the radium-barium density ratio is comparable to the radium-barium atomic mass ratio, as these elements have very similar body-centered cubic structures.

Chemical characteristics and compounds

Radium is the heaviest known alkaline earth metal; its chemical properties mostly resemble those of barium. When exposed to air, radium reacts violently with it, forming radium nitride,^[1] which causes blackening of this white metal. It exhibits only the +2 oxidation state in solution. Radium ions do not form complexes easily, due to highly basic character of the ions. Most radium compounds coprecipitate with all barium, most strontium, and most lead compounds, and are ionic salts. The radium ion is colorless, making radium salts white when freshly prepared, turning yellow and ultimately dark with age owing to self-decomposition from the alpha radiation. Compounds of radium flame red-purple and give a characteristic spectrum. Like other alkaline earth metals, radium reacts violently with water and oil to form radium hydroxide and is slightly more volatile than barium, which leads to lesser solubility of radium compounds compared to those of corresponding barium ones. Because of its geologically short half-life and intense radioactivity, radium compounds are quite rare, occurring almost exclusively in uranium ores.

Radium chloride, radium bromide, radium hydroxide and radium nitrate are soluble in water, with solubilities slightly lower than those of barium analogs for bromide and chloride, and higher for nitrate. Radium hydroxide is more soluble than hydroxides of other alkaline earth metals, actinium, and thorium, and more basic than barium hydroxide. It can be separated from these elements by their precipitation with ammonia.^[1] Out of insoluble radium compounds, radium sulfate, radium chromate, radium iodate, radium carbonate, and radium tetrafluoroberyllate are characterized.^[1] Radium oxide, however, remains uncharacterized, despite the fact that other alkaline-earth metals' oxides are common compounds for the corresponding metals.

Isotopes

Radium has 25 different known isotopes, four of which are found in nature, with ²²⁶Ra being the most common. ²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra are all generated naturally in the decay of either uranium (U) or thorium (Th). ²²⁶Ra is a product of ²³⁸U decay, and is the longest-lived isotope of radium with a half-life of 1601 years; next longest is ²²⁸Ra, a product of ²³²Th breakdown, with a half-life of 5.75 years.^[2]

Radium has no stable isotopes; however, four isotopes of radium are present in decay chains, having atomic masses of 223, 224, 226 and 228, all of which are present in trace amounts. The most abundant and the longest-living one is radium-226, with a half-life of 1601 years. To date, 33 isotopes of radium have been synthesized, ranging in mass number from 202 to 234.

To date, at least 12 nuclear isomers have been reported; the most stable of them is radium-205m, with a half-life of between 130 and 230 milliseconds. All ground states of isotopes from radium-205 to radium-214, and from radium-221 to radium-234, have longer ones.

Three other natural radioisotopes had received historical names in the early twentieth century: radium-223 was known as actinium X, radium-224 as thorium X and radium-228 as mesothorium I. Radium-226 has given historical names to its decay products after the whole element, such as radium A for polonium-218.

Radioactivity

Radium is over one million times as radioactive as the same mass of uranium. Its decay occurs in at least seven stages; the successive main products have been studied and were called radium emanation or exradio (now identified as radon), radium A (polonium), radium B (lead), radium C (bismuth), etc. Radon is a heavy gas, and the later products are solids. These products are themselves radioactive elements, each with an atomic weight a little lower than its predecessor.^[3][4]

Radium loses about 1% of its activity in 25 years, being transformed into elements of lower atomic weight, with lead being the final product of disintegration.^[5]

The SI unit of radioactivity is the becquerel (Bq), equal to one disintegration per second. The curie is a non-SI unit defined as that amount of radioactive material that has the same disintegration rate as 1 gram of radium-226 (3.7×10¹⁰ disintegrations per second, or 37 GBq).^[6]

Radium metal maintains itself at a higher temperature than its surroundings because of the radiation it emits – alpha particles, beta particles, and gamma rays. More specifically, the alpha particles are produced by the radium decay, whereas the beta particles and gamma rays are produced by relatively short-half-life elements further down the decay chain.^[7]

Occurrence

Radium is a decay product of uranium and is therefore found in all uranium-bearing ores. (One ton of pitchblende typically yields about one seventh of a gram of radium).^[8] Radium was originally acquired from pitchblende ore from Joachimsthal, Bohemia, now located in the Czech Republic. Carnotite sands in Colorado provide some of the element, but richer ores are found in the Congo and the area of the Great Bear Lake and the Great Slave Lake of northwestern Canada.^[9] Radium can also be extracted from the waste from nuclear reactors. Large radium-containing uranium deposits are located in Russia, Canada (the Northwest Territories), the United States (New Mexico, Utah and Colorado, for example) and Australia.

Production

All radium occurring today is produced by the decay of heavier elements, being present in decay chains. Owing to such short half-lives of its isotopes, radium is not primordial but trace. It cannot occur in large quantities due both to the fact that isotopes of radium have short half-lives and that parent nuclides have very long ones. Radium is found in tiny quantities in the uranium ore uraninite and various other uranium minerals, and in even tinier quantities in thorium minerals.

The amounts produced were aways relatively small; for example, in 1918 13.6 g of radium were produced in the United states.^[10] As of 1954, the total worldwide supply of purified radium amounted to about 5 pounds (2.3 kg).^[11]

History

Radium (Latin radius, ray) was discovered by Marie Skłodowska-Curie and her husband Pierre on December 21, 1898 in a uraninite sample. While studying the mineral, the Curies removed uranium from it and found that the remaining material was still radioactive. They then separated out a radioactive mixture consisting mostly of compounds of barium which gave a brilliant green flame color and crimson carmine spectral lines that had never been documented before. The Curies announced their discovery to the French Academy of Sciences on 26 December 1898.^[12] The naming of radium dates to circa 1899, from French radium, formed in Modern Latin from radius (ray), called for its power of emitting energy in the form of rays.^[13] In 1910, radium was isolated as a pure metal by Curie and André-Louis Debierne through the electrolysis of a pure radium chloride solution by using a mercury cathode and distilling in an atmosphere of hydrogen gas.^[14] The Curies' new element was first industrially produced in the beginning of the 20th century by Biraco, a subsidiary company of Union Minière du Haut Katanga (UMHK) in its Olen plant in Belgium. UMHK offered to Marie Curie her first gram of radium. It gave historical names to the decay products of radium, such as radium A, B, C, etc., now known to be isotopes of other elements.

On 4 February 1936, radium E (bismuth-210) became the first radioactive element to be made synthetically in the United States. Dr. John Jacob Livingood, at the radiation lab at University of California, Berkeley, was bombarding several elements with 5-MEV deuterons. He noted that irradiated bismuth emits fast electrons with a 5-day half-life, which matched the behavior of radium E.^{[15][16][17][18]}

The common historical unit for radioactivity, the curie, is based on the radioactivity of ²²⁶Ra.^[19]

Applications

Some of the few practical uses of radium are derived from its radioactive properties. More recently discovered radioisotopes, such as 60
Co and 137
Cs, are replacing radium in even these limited uses because several of these isotopes are more powerful emitters, safer to handle, and available in more concentrated form.^[20][21]

When mixed with beryllium, it is a neutron source for physics experiments.^[22]

Historical uses

Self-luminous white paint which contains radium on the face and hand of an old clock.

Radium hands in darkness

Radium was formerly used in self-luminous paints for watches, nuclear panels, aircraft switches, clocks, and instrument dials. A typical self-luminous watch that uses radium paint contains around 1 microgram of radium.^[11] In the mid-1920s, a lawsuit was filed by five dying "Radium Girl" dial painters who had painted radium-based luminous paint on the dials of watches and clocks. The dial painters' exposure to radium caused serious health effects which included sores, anemia, and bone cancer. This is because radium is treated as calcium by the body, and deposited in the bones, where radioactivity degrades marrow and can mutate bone cells.

During the litigation, it was determined that company scientists and management had taken considerable precautions to protect themselves from the effects of radiation, yet had not seen fit to protect their employees. Worse, for several years the companies had attempted to cover up the effects and avoid liability by insisting that the Radium Girls were instead suffering from syphilis. This complete disregard for employee welfare had a significant impact on the formulation of occupational disease labor law.^[23]

As a result of the lawsuit, the adverse effects of radioactivity became widely known, and radium-dial painters were instructed in proper safety precautions and provided with protective gear. In particular, dial painters no longer shaped paint brushes by lip (which led to accidental ingestion of the radium salts). Radium was still used in dials as late as the 1960s, but there were no further injuries to dial painters. This further highlighted that the plight of the Radium Girls was completely preventable.

After the 1960s, radium paint was first replaced with promethium paint, and later by tritium bottles which continue to be used today. Although the beta radiation from tritium is potentially dangerous if tritium is ingested, tritium has replaced radium in these applications.

Radium was once an additive in products such as toothpaste, hair creams, and even food items due to its supposed curative powers.^[24] Such products soon fell out of vogue and were prohibited by authorities in many countries after it was discovered they could have serious adverse health effects. (See, for instance, Radithor or Revigator types of "Radium water" or "Standard Radium Solution for Drinking".) Spas featuring radium-rich water are still occasionally touted as beneficial, such as those in Misasa, Tottori, Japan. In the U.S., nasal radium irradiation was also administered to children to prevent middle-ear problems or enlarged tonsils from the late 1940s through the early 1970s.^[25]

In 1909, the famous Rutherford experiment used radium as an alpha source to probe the atomic structure of gold. This experiment led to the Rutherford model of the atom and revolutionized the field of nuclear physics.

Radium (usually in the form of radium chloride) was used in medicine to produce radon gas which in turn was used as a cancer treatment; for example, several of these radon sources were used in Canada in the 1920s and 1930s.^[26] The isotope 223
Ra is currently under investigation for use in medicine as a cancer treatment of bone metastasis.

Precautions

Radium is highly radioactive and its decay product, radon gas, is also radioactive. Since radium is chemically similar to calcium, it has the potential to cause great harm by replacing calcium in bones. Exposure to radium can cause cancer and other disorders, because radium and its decay product radon emit alpha particles upon their decay, which kill and mutate cells. The dangers of radium were apparent from the start. The first case of so-called "radium-dermatitis" was reported in 1900, only 2 years after the element's discovery. The French physicist Antoine Becquerel carried a small ampoule of radium around in his waistcoat pocket for 6 hours and reported that his skin became ulcerated. Marie Curie also had a similar incident in which she experimented with a tiny sample that she kept in contact with her skin for 10 hours and noted how an ulcer appeared, although not for several days.^[27] Handling of radium has also been blamed for Curie's death due to aplastic anemia. Stored radium should be ventilated to prevent accumulation of radon. Emitted energy from the decay of radium also ionizes gases, affects photographic plates, and produces many other detrimental effects – to the extent that at the time of the Manhattan Project in 1944, the "tolerance dose" for workers was set at 0.1 microgram of ingested radium.^[28][29]

See also

• Decay chains
• Radium Girls

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References

Further reading

• Albert Stwertka (1998). Guide to the Elements – Revised Edition. Oxford University Press. ISBN 0-19-508083-1. 
• Denise Grady (October 6, 1998). "A Glow in the Dark, and a Lesson in Scientific Peril". The New York Times. http://www.nytimes.com/library/national/science/100698sci-radium.html. Retrieved 2007-12-25. 
• Nanny Fröman (1 December 1996). "Marie and Pierre Curie and the Discovery of Polonium and Radium". Nobel Foundation. http://nobelprize.org/nobel_prizes/physics/articles/curie/index.html. Retrieved 2007-12-25. 
• Macklis, R. M. (1993). "The great radium scandal". Scientific American 269 (2): 94–99. doi:10.1038/scientificamerican0893-94. PMID 8351514. 
• Clark, Claudia (1987). Radium Girls: Women and Industrial Health Reform, 1910–1935. University of North Carolina Press. ISBN 0-8078-4640-6. 

External links

• WebElements.com – Radium (also used as a reference)
• Lateral Science – Radium Discovery
• Photos of Radium Water Bath in Oklahoma
• NLM Hazardous Substances Databank – Radium, Radioactive
• Reproduction of a 1942 comic book ad selling a "Radiumscope" to children
• Annotated bibliography for radium from the Alsos Digital Library for Nuclear Issues
• The Poisoner Next Door - Japan Today, 10/20/2001