Radium

Radium is the heaviest known alkaline earth metal and is the only radioactive member of its group. Its physical and chemical properties most closely resemble its lighter congener, barium.[4]^(p 112)

Pure radium is a volatile silvery-white metal, although its lighter congeners calcium, strontium, and barium have a slight yellow tint.[4]^(p 112) This tint rapidly vanishes on exposure to air, yielding a black layer of what is probably radium nitride (Ra₃N₂).[5]^(p 4) Its melting point is either 700 °C (1,292 °F) or 960 °C (1,760 °F)[lower-alpha 1] and its boiling point is 1,737 °C (3,159 °F); however, this is not well established.[6] Both of these values are slightly lower than those of barium, confirming periodic trends down the group 2 elements.[7] Like barium and the alkali metals, radium crystallizes in the body-centered cubic structure at standard temperature and pressure: the radium–radium bond distance is 514.8 picometers.[8] Radium has a density of 5.5 g/cm³, higher than that of barium, again confirming periodic trends; the radium-barium density ratio is comparable to the radium-barium atomic mass ratio,[9] due to the two elements' similar crystal structures.[9][10]

Radium has 33 known isotopes, with mass numbers from 202 to 234: all of them are radioactive.[2] Four of these – ²²³Ra (half-life 11.4 days), ²²⁴Ra (3.64 days), ²²⁶Ra (1600 years), and ²²⁸Ra (5.75 years) – occur naturally in the decay chains of primordial thorium-232, uranium-235, and uranium-238 (²²³Ra from uranium-235, ²²⁶Ra from uranium-238, and the other two from thorium-232). These isotopes nevertheless still have half-lives too short to be primordial radionuclides and only exist in nature from these decay chains.[5]^(p 3) Together with the mostly artificial ²²⁵Ra (15 d), which occurs in nature only as a decay product of minute traces of neptunium-237,[11] these are the five most stable isotopes of radium.[2] All other 27 known radium isotopes have half-lives under two hours, and the majority have half-lives under a minute.[2] Of these, ²²¹Ra (half-life 28 s) also occurs as a ²³⁷Np daughter, and ²²⁰Ra and ²²²Ra would be produced by the still-unobserved double beta decay of natural radon isotopes.[12] At least 12 nuclear isomers have been reported; the most stable of them is radium-205m, with a half-life between 130~230 milliseconds; this is still shorter than twenty-four ground-state radium isotopes.[2]

In the early history of the study of radioactivity, the different natural isotopes of radium were given different names. In this scheme, ²²³Ra was named actinium X (AcX), ²²⁴Ra thorium X (ThX), ²²⁶Ra radium (Ra), and ²²⁸Ra mesothorium 1 (MsTh₁).[5]^(p 3) When it was realized that all of these are isotopes of the same element, many of these names fell out of use, and "radium" came to refer to all isotopes, not just ²²⁶Ra. Some of radium-226's decay products received historical names including "radium", ranging from radium A to radium G, with the letter indicating approximately how far they were down the chain from their parent ²²⁶Ra: Radium emanation = ²²²Rn, Ra A = ²¹⁸Po, Ra B = ²¹⁴Pb, Ra C = ²¹⁴Bi, Ra C₁ = ²¹⁴Po, Ra C₂ = ²¹⁰Tl, Ra D = ²¹⁰Pb, Ra E = ²¹⁰Bi, Ra F = ²¹⁰Po, and Ra G = ²⁰⁶Pb .[13]

²²⁶Ra is the most stable isotope of radium and is the last isotope in the (4n + 2) decay chain of uranium-238 with a half-life of over a millennium: it makes up almost all of natural radium. Its immediate decay product is the dense radioactive noble gas radon (specifically the isotope ²²²Rn), which is responsible for much of the danger of environmental radium.[14] It is 2.7 million times more radioactive than the same molar amount of natural uranium (mostly uranium-238), due to its proportionally shorter half-life.[15][16]

A sample of 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, natural radium (which is mostly ²²⁶Ra) emits mostly alpha particles, but other steps in its decay chain (the uranium or radium series) emit alpha or beta particles, and almost all particle emissions are accompanied by gamma rays.[17]

Experimental nuclear physics studies have shown that nuclei of several radium isotopes, such as ²²²Ra, ²²⁴Ra and ²²⁶Ra, have reflection-asymmetric ("pear-like") shapes. [18] In particular, this experimental information on radium-224 has been obtained at ISOLDE,CERN using a technique called Coulomb excitation.[19][20]

Radium, like barium, is a highly reactive metal and always exhibits its group oxidation state of +2.[5]^(p 4) It forms the colorless Ra²⁺ cation in aqueous solution, which is highly basic and does not form complexes readily.[5]^(p 4) Most radium compounds are therefore simple ionic compounds,[5]^(p 4) though participation from the 6s and 6p electrons (in addition to the valence 7s electrons) is expected due to relativistic effects and would enhance the covalent character of radium compounds such as RaF₂ and RaAt₂.[21] For this reason, the standard electrode potential for the half-reaction Ra²⁺ (aq) + 2e⁻ → Ra (s) is −2.916 V, even slightly lower than the value −2.92 V for barium, whereas the values had previously smoothly increased down the group (Ca: −2.84 V; Sr: −2.89 V; Ba: −2.92 V).[4]^(p 111) The values for barium and radium are almost exactly the same as those of the heavier alkali metals potassium, rubidium, and caesium.[4]^(p 111)

All isotopes of radium have half-lives much shorter than the age of the Earth, so that any primordial radium would have decayed long ago. Radium nevertheless still occurs in the environment, as the isotopes ²²³Ra, ²²⁴Ra, ²²⁶Ra, and ²²⁸Ra are part of the decay chains of natural thorium and uranium isotopes; since thorium and uranium have very long half-lives, these daughters are continually being regenerated by their decay.[5]^(p 3) Of these four isotopes, the longest-lived is ²²⁶Ra (half-life 1600 years), a decay product of natural uranium. Because of its relative longevity, ²²⁶Ra is the most common isotope of the element, making up about one part per trillion of the Earth's crust; essentially all natural radium is ²²⁶Ra.[4]^{(pp109–110)} Thus, radium is found in tiny quantities in the uranium ore uraninite and various other uranium minerals, and in even tinier quantities in thorium minerals. One ton of pitchblende typically yields about one seventh of a gram of radium.[23] One kilogram of the Earth's crust contains about 900 picograms of radium, and one liter of sea water contains about 89 femtograms of radium.[24]

Radium was discovered by Marie Skłodowska-Curie and her husband Pierre Curie on 21 December 1898, in a uraninite (pitchblende) sample from Jáchymov.[25] While studying the mineral earlier, the Curies removed uranium from it and found that the remaining material was still radioactive. In July 1898, while studying pitchblende, they isolated an element similar to bismuth which turned out to be polonium. They then isolated a radioactive mixture consisting of two components: compounds of barium, which gave a brilliant green flame color, and unknown radioactive compounds which gave carmine spectral lines that had never been documented before. The Curies found the radioactive compounds to be very similar to the barium compounds, except they were less soluble. This discovery made it possible for the Curies to isolate the radioactive compounds and discover a new element in them. The Curies announced their discovery to the French Academy of Sciences on 26 December 1898.[26][27] The naming of radium dates to about 1899, from the French word radium, formed in Modern Latin from radius (ray): this was in recognition of radium's power of emitting energy in the form of rays.[28][29][30]

In September 1910, Marie Curie and André-Louis Debierne announced that they had isolated radium as a pure metal through the electrolysis of pure radium chloride (RaCl₂) solution using a mercury cathode, producing radium–mercury amalgam.[31] This amalgam was then heated in an atmosphere of hydrogen gas to remove the mercury, leaving pure radium metal.[32] Later that same year, E. Eoler isolated radium by thermal decomposition of its azide, Ra(N₃)₂.[5]^(p 3) Radium metal was first industrially produced at the beginning of the 20th century by Biraco, a subsidiary company of Union Minière du Haut Katanga (UMHK) in its Olen plant in Belgium.[33]

The general historical unit for radioactivity, the curie, is based on the radioactivity of ²²⁶Ra: it was originally defined as the radioactivity of one gram of radium-226,[34] but the definition was later slightly refined to be 3.7×10¹⁰ disintegrations per second.

Uranium had no large scale application in the late 19th century and therefore no large uranium mines existed. In the beginning the only large source for uranium ore was the silver mines in Jáchymov, Austria-Hungary (now Czech Republic).[25] The uranium ore was only a byproduct of the mining activities.[55]

In the first extraction of radium, Curie used the residues after extraction of uranium from pitchblende. The uranium had been extracted by dissolution in sulfuric acid leaving radium sulfate, which is similar to barium sulfate but even less soluble in the residues. The residues also contained rather substantial amounts of barium sulfate which thus acted as a carrier for the radium sulfate. The first steps of the radium extraction process involved boiling with sodium hydroxide, followed by hydrochloric acid treatment to minimize impurities of other compounds. The remaining residue was then treated with sodium carbonate to convert the barium sulfate into barium carbonate (carrying the radium), thus making it soluble in hydrochloric acid. After dissolution, the barium and radium were reprecipitated as sulfates; this was then repeated to further purify the mixed sulfate. Some impurities that form insoluble sulfides were removed by treating the chloride solution with hydrogen sulfide, followed by filtering. When the mixed sulfates were pure enough, they were once more converted to mixed chlorides; barium and radium thereafter were separated by fractional crystallisation while monitoring the progress using a spectroscope (radium gives characteristic red lines in contrast to the green barium lines), and the electroscope.[56]

After the isolation of radium by Marie and Pierre Curie from uranium ore from Jáchymov, several scientists started to isolate radium in small quantities. Later, small companies purchased mine tailings from Jáchymov mines and started isolating radium. In 1904, the Austrian government nationalised the mines and stopped exporting raw ore. Until 1912 when radium production increased, radium availability was low.[55]

The formation of an Austrian monopoly and the strong urge of other countries to have access to radium led to a worldwide search for uranium ores. The United States took over as leading producer in the early 1910s. The 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. Neither of the deposits is mined for radium but the uranium content makes mining profitable.[25][57]

The Curies' process was still used for industrial radium extraction in 1940, but mixed bromides were then used for the fractionation. If the barium content of the uranium ore is not high enough it is easy to add some to carry the radium. These processes were applied to high grade uranium ores but may not work well with low grade ores.[58]

Small amounts of radium were still extracted from uranium ore by this method of mixed precipitation and ion exchange as late as the 1990s,[4]^{(pp 109–110)} but as of 2011, they are extracted only from spent nuclear fuel.[44]^(p437) In 1954, the total worldwide supply of purified radium amounted to about 5 pounds (2.3 kg)[35] and it is still in this range in 2015, while the annual production of pure radium compounds is only about 100 g in total as of 1984.[4]^{(pp 109–110)} The chief radium-producing countries are Belgium, Canada, the Czech Republic, Slovakia, the United Kingdom, and Russia.[4]^{(pp 109–110)} The amounts of radium produced were and are always relatively small; for example, in 1918, 13.6 g of radium were produced in the United States.[59] The metal is isolated by reducing radium oxide with aluminium metal in a vacuum at 1,200 °C.[22]

Radium is seeing increasing use in the field of atomic, molecular, and optical physics. Symmetry breaking forces scale proportional to ${\displaystyle \ Z^{3}\ ,}$[60][61] which makes radium, the heaviest alkaline earth element, well suited for constraining new physics beyond the standard model. Some radium isotopes, such as radium-225, have octupole deformed parity doublets that enhance sensitivity to charge parity violating new physics by two to three orders of magnitude compared to ¹⁹⁹Hg.[62][63][64]

Radium is also a promising candidate for trapped ion optical clocks. The radium ion has two subhertz-linewidth transitions from the ${\displaystyle \ \mathrm {7s^{2}S_{\tfrac {1}{2}}} \ }$ ground state that could serve as the clock transition in an optical clock.[65] A ²²⁶Ra+ trapped ion atomic clock has been demonstrated on the ${\displaystyle \ \mathrm {7s^{2}S_{\tfrac {1}{2}}} \ }$ to ${\displaystyle \ \mathrm {6d^{2}D_{\tfrac {5}{2}}} \ }$ transition.[66] Additionally, radium is particularly well suited for a transportable optical clock as all transitions necessary for clock operation can be addressed with direct diode lasers at common wavelengths.

Some of the few practical uses of radium are derived from its radioactive properties. More recently discovered radioisotopes, such as cobalt-60 and caesium-137, 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.[67][68]

The isotope ²²³Ra (the chloride is under the trade name Xofigo)[69] was approved by the United States Food and Drug Administration in 2013 for use in medicine as a cancer treatment of bone metastasis.[70][71] The main indication of treatment with Xofigo is the therapy of bony metastases from castration-resistant prostate cancer due to the favourable characteristics of this alpha-emitter radiopharmaceutical.[72] ²²⁵Ra has also been used in experiments concerning therapeutic irradiation, as it is the only reasonably long-lived radium isotope which does not have radon as one of its daughters.[73]

Radium is still used in 2007 as a radiation source in some industrial radiography devices to check for flawed metallic parts, similarly to X-ray imaging.[14] When mixed with beryllium, radium acts as a neutron source.[44][74] As of 2004, radium-beryllium neutron sources are still sometimes used,[14][75] but other materials such as polonium are more common: about 1,500 polonium-beryllium neutron sources, with an individual activity of 1,850 Ci (68 TBq), have been used annually in Russia. These RaBeF₄-based (α, n) neutron sources have been deprecated despite the high number of neutrons they emit (1.84×10⁶ neutrons per second) in favour of ²⁴¹Am–Be sources.[22] As of 2011, the isotope ²²⁶Ra is mainly used to form ²²⁷Ac by neutron irradiation in a nuclear reactor.[22]

Radium is highly radioactive, as is its immediate decay product, radon gas. When ingested, 80% of the ingested radium leaves the body through the feces, while the other 20% goes into the bloodstream, mostly accumulating in the bones. This is because the body treats radium as calcium and deposits it in the bones, where radioactivity degrades marrow and can mutate bone cells. Exposure to radium, internal or external, can cause cancer and other disorders, because radium and radon emit alpha and gamma rays upon their decay, which kill and mutate cells.[14] At the time of the Manhattan Project in 1944, the "tolerance dose" for workers was set at 0.1 micrograms of ingested radium.[76][77]

Some of the biological effects of radium include the first case of "radium-dermatitis", reported in 1900, two years after the element's discovery. The French physicist Antoine Becquerel carried a small ampoule of radium in his waistcoat pocket for six hours and reported that his skin became ulcerated. Pierre and Marie Curie were so intrigued by radiation that they sacrificed their own health to learn more about it. Pierre Curie attached a tube filled with radium to his arm for ten hours, which resulted in the appearance of a skin lesion, suggesting the use of radium to attack cancerous tissue as it had attacked healthy tissue.[78] Handling of radium has been blamed for Marie Curie's death, due to aplastic anemia. A significant amount of radium's danger comes from its daughter radon: Being a gas, it can enter the body far more readily than can its parent radium.[14]

As of 2015, ²²⁶Ra is considered to be the most toxic of the quantity radioelements, and it must be handled in tight glove boxes with significant airstream circulation that is then treated to avoid escape of its daughter ²²²Rn to the environment. Old ampoules containing radium solutions must be opened with care because radiolytic decomposition of water can produce an overpressure of hydrogen and oxygen gas.[22] The world's largest concentration of ²²⁶Ra is stored within the Interim Waste Containment Structure, approximately 9.6 mi (15.4 km) north of Niagara Falls, New York.[79] The Maximum Contaminant Level (MCL) for radium is 5pCi/L for drinking water, however, the OSHA does not set an exposure limit, as there is a radiation limit already set up.[80]

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.

• "The discovery of radium". Lateral Science. UK. 8 July 2012. Archived from the original on 9 March 2016. Retrieved 13 May 2017.
• Radium water bath in Oklahoma. markwshead.com (photographic images).
• "Radium, radioactive". NLM Hazardous Substances Databank. U.S. National Institutes of Health.
• "Annotated bibliography for radium". Alsos Digital Library for Nuclear Issues. Lexington, VA: Washington and Lee University. Archived from the original on 25 June 2019.
• "Radium". The Periodic Table of Videos. University of Nottingham.