Radium, in the form ofradium chloride, wasdiscovered byMarie andPierre Curie in 1898 from ore mined atJáchymov. They extracted the radium compound fromuraninite and published the discovery at theFrench Academy of Sciences five days later. Radium was isolated in itsmetallic state by Marie Curie andAndré-Louis Debierne through theelectrolysis of radium chloride in 1910, and soon afterwards the metal started being produced on larger scales inAustria, theUnited States, andBelgium. However, the amount of radium produced globally has always been small in comparison to other elements, and by the 2010s, annual production of radium, mainly via extraction fromspent nuclear fuel, was less than 100 grams.
In nature, radium is found inuranium ores in quantities as small as a seventh of a gram per ton of uraninite, and inthorium ores in trace amounts. Radium is not necessary forliving organisms, and its radioactivity and chemical reactivity make adverse health effects likely when it is incorporated into biochemical processes because of its chemical mimicry ofcalcium, due to them both beinggroup 2 elements. As of 2018, other than innuclear medicine, radium has no commercial applications. Formerly, from the 1910s to the 1970s, it was used as a radioactive source forradioluminescent devices and also inradioactive quackery for its supposed curative power. In nearly all of its applications, radium has been replaced with less dangerousradioisotopes, with one of its few remaining non-medical uses being the production ofactinium innuclear reactors.
Pure radium is avolatile,lustrous silvery-white metal, even though its lighter congenerscalcium,strontium, and barium have a slight yellow tint.[5] Radium's lustrous surface rapidly becomes black upon exposure to air, likely due to the formation ofradium nitride (Ra3N2).[6] Itsmelting point is either 700 °C (1,292 °F) or 960 °C (1,760 °F)[a] and itsboiling point is 1,737 °C (3,159 °F); however, this is not well established.[7] Both of these values are slightly lower than those of barium, confirmingperiodic trends down the group 2 elements.[8]Like barium and thealkali metals, radium crystallizes in thebody-centered cubic structure atstandard temperature and pressure: the radium–radium bond distance is 514.8 picometers.[9]Radium has a density of 5.5 g/cm3, higher than that of barium, and the two elements have similarcrystal structures (bcc at standard temperature and pressure).[10][11]
Radium has 33 known isotopes withmass numbers from 202 to 234, all of which areradioactive.[3] Four of these –223Ra (half-life 11.4 days),224Ra (3.64 days),226Ra (1600 years), and228Ra (5.75 years) – occur naturally in thedecay chains of primordialthorium-232,uranium-235, anduranium-238 (223Ra from uranium-235,226Ra from uranium-238, and the other two from thorium-232). These isotopes nevertheless still havehalf-lives too short to beprimordial radionuclides, and only exist in nature from these decay chains.[12]Together with the mostlyartificial225Ra (15 d), which occurs in nature only as a decay product of minute traces ofneptunium-237,[13]these are the five most stable isotopes of radium.[3] All other 27 known radium isotopes have half-lives under two hours, and the majority have half-lives under a minute.[3] Of these,221Ra (half-life 28 s) also occurs as a237Np daughter, and220Ra and222Ra would be produced by the still-unobserveddouble beta decay of natural radon isotopes.[14] At least 12 nuclear isomers have been reported, the most stable of which is radium-205m with a half-life between 130~230 milliseconds; this is still shorter than twenty-fourground-state radium isotopes.[3]
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 radioactivenoble gasradon (specifically the isotope222Rn), which is responsible for much of the danger of environmental radium.[15][b] It is 2.7 million times more radioactive than the samemolar amount of naturaluranium (mostly uranium-238), due to its proportionally shorter half-life.[16][17]
A sample of radium metal maintains itself at a highertemperature than its surroundings because of the radiation it emits. Natural radium (which is mostly226Ra) emits mostlyalpha particles, but other steps in its decay chain (theuranium or radium series) emit alpha orbeta particles, and almost all particle emissions are accompanied bygamma rays.[18]
Experimental nuclear physics studies have shown that nuclei of several radium isotopes, such as222Ra,224Ra and226Ra, have reflection-asymmetric ("pear-like") shapes.[19] In particular, this experimental information on radium-224has been obtained atISOLDE using a technique calledCoulomb excitation.[20][21]
Chemistry
Radium only exhibits the oxidation state of +2 in solution.[6] It forms the colorless Ra2+cation inaqueous solution, which is highlybasic and does not formcomplexes readily.[6] Most radium compounds are therefore simpleionic compounds,[6] though participation from the6s and 6p electrons (in addition to the valence 7s electrons) is expected due torelativistic effects and would enhance thecovalent character of radium compounds such asRaF2 and RaAt2.[22] For this reason, thestandard electrode potential for thehalf-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).[23] The values for barium and radium are almost exactly the same as those of the heavier alkali metalspotassium,rubidium, andcaesium.[23]
Compounds
Solid radium compounds are white as radium ions provide no specific coloring, but they gradually turn yellow and then dark over time due to self-radiolysis from radium'salpha decay.[6] Insoluble radium compoundscoprecipitate with all barium, moststrontium, and mostlead compounds.[24]
Radium oxide (RaO) is poorly characterized, as the reaction of radium with air results in the formation ofradium nitride.[25]Radium hydroxide (Ra(OH)2) is formed via the reaction of radium metal with water, and is the most readily soluble among the alkaline earth hydroxides and a stronger base than its barium congener,barium hydroxide.[26] It is also more soluble thanactinium hydroxide andthorium hydroxide: these three adjacent hydroxides may be separated by precipitating them withammonia.[26]
Radium chloride (RaCl2) is a colorless,luminescent compound. It becomes yellow after some time due to self-damage by thealpha radiation given off by radium when it decays. Small amounts of barium impurities give the compound arose color.[26] It is soluble in water, though less so thanbarium chloride, and its solubility decreases with increasing concentration ofhydrochloric acid. Crystallization from aqueous solution gives the dihydrate RaCl2·2H2O,isomorphous with its barium analog.[26]
Radium bromide (RaBr2) is also a colorless, luminous compound.[26] In water, it is more soluble than radium chloride. Like radium chloride, crystallization from aqueous solution gives the dihydrate RaBr2·2H2O, isomorphous with its barium analog. The ionizing radiation emitted by radium bromide excitesnitrogen molecules in the air, making it glow. Thealpha particles emitted by radium quickly gain two electrons to become neutralhelium, which builds up inside and weakens radium bromide crystals. This effect sometimes causes the crystals to break or even explode.[26]
Radium nitrate (Ra(NO3)2) is a white compound that can be made by dissolvingradium carbonate innitric acid. As the concentration of nitric acid increases, the solubility of radium nitrate decreases, an important property for the chemical purification of radium.[26]
Radium forms much the same insoluble salts as its lighter congener barium: it forms the insolublesulfate (RaSO4, the most insoluble known sulfate),chromate (RaCrO4),carbonate (RaCO3),iodate (Ra(IO3)2),tetrafluoroberyllate (RaBeF4), and nitrate (Ra(NO3)2). With the exception of the carbonate, all of these are less soluble in water than the corresponding barium salts, but they are allisostructural to their barium counterparts. Additionally,radium phosphate,oxalate, andsulfite are probably also insoluble, as theycoprecipitate with the corresponding insoluble barium salts.[27] The great insolubility of radium sulfate (at 20 °C, only 2.1 mg will dissolve in 1 kg of water) means that it is one of the less biologically dangerous radium compounds.[28] The large ionic radius of Ra2+ (148 pm) results in weak ability to formcoordination complexes and poor extraction of radium from aqueous solutions when not at high pH.[29]
Occurrence
All isotopes of radium have half-lives much shorter than theage of the Earth, so that any primordial radium would have decayed long ago. Radium nevertheless still occursin the environment, as the isotopes223Ra,224Ra,226Ra, and228Ra are part of the decay chains of natural thorium and uranium isotopes; since thorium and uranium have very long half-lives,[3] thesedaughters are continually being regenerated by their decay.[12] Of these four isotopes, the longest-lived is226Ra (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 onepart per trillion of the Earth's crust; essentially all natural radium is226Ra.[30] Thus, radium is found in tiny quantities in the uranium oreuraninite and various other uraniumminerals, and in even tinier quantities in thorium minerals. Oneton ofpitchblende typically yields about one seventh of agram of radium.[31] One kilogram of theEarth's crust contains about 900 picograms of radium, and oneliter ofsea water contains about 89 femtograms of radium.[32]
Marie and Pierre Curie experimenting with radium, a drawing byAndré CastaigneGlass tube of radium chloride kept by the US Bureau of Standards that served as the primary standard of radioactivity for the United States in 1927.
Radium wasdiscovered byMarie Skłodowska-Curie and her husbandPierre Curie on 21 December 1898 in auraninite (pitchblende) sample fromJáchymov.[33] 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 tobismuth which turned out to bepolonium. They then isolated a radioactive mixture consisting of two components: compounds ofbarium, which gave a brilliant green flame color, and unknown radioactive compounds which gavecarminespectral 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 theFrench Academy of Sciences on 26 December 1898.[34] The naming of radium dates to about 1899, from the French wordradium, formed in Modern Latin fromradius (ray): this was in recognition of radium's emission of energy in the form of rays.[35] The gaseous emissions of radium, radon, were recognized and studied extensively byFriedrich Ernst Dorn in the early 1900s, though at the time they were characterized as "radium emanations".[36]
In September 1910, Marie Curie andAndré-Louis Debierne announced that they had isolated radium as a puremetal through theelectrolysis of pure radiumchloride (RaCl2) solution using amercurycathode, producing radium–mercuryamalgam.[37] This amalgam was then heated in an atmosphere ofhydrogen gas to remove the mercury, leaving pure radium metal.[38]Later that same year, E. Ebler isolated radium metal bythermal decomposition of itsazide, Ra(N3)2.[39][40] Radium metal was first industrially produced at the beginning of the 20th century byBiraco, a subsidiary company ofUnion Minière du Haut Katanga (UMHK) in itsOlen plant in Belgium.[41] The metal became an important export of Belgium from 1922 up until World War II.[42]
The general historical unit for radioactivity, thecurie, is based on the radioactivity of226Ra. it was originally defined as the radioactivity of one gram of radium-226,[43] but the definition was later refined to be3.7×1010 disintegrations per second.[44]
Historical applications
Luminescent paint
Watch hands coated with radium paint under ultraviolet light
Radium was formerly used inself-luminous paints for watches, aircraft switches, clocks, and instrument dials and panels. A typical self-luminous watch that uses radium paint contains around 1 microgram of radium.[45] In the mid-1920s, a lawsuit was filed against theUnited States Radium Corporation by five dying "Radium Girls" – dial painters who had painted radium-basedluminous paint on the components of watches and clocks.[46] The dial painters were instructed to lick their brushes to give them a fine point, thereby ingesting radium.[47] Their exposure to radium caused serious health effects which included sores,anemia, andbone cancer.[15]
During the litigation, it was determined that the company's scientists and management had taken considerable precautions to protect themselves from the effects of radiation, but it did not seem to protect their employees. Additionally, for several years the companies had attempted to cover up the effects and avoid liability by insisting that the Radium Girls were instead suffering fromsyphilis.[48]
As a result of the lawsuit, and an extensive study by the U.S. Public Health Service, the adverse effects of radioactivity became widely known, and radium-dial painters were instructed in proper safety precautions and provided with protective gear. Radium continued to be used in dials, especially in manufacturing duringWorld War II, but from 1925 onward there were no further injuries to dial painters.[46]
From the 1960s the use of radium paint was discontinued. In many cases luminous dials were implemented with non-radioactive fluorescent materials excited by light; such devices glow in the dark after exposure to light, but the glow fades.[15] Where long-lasting self-luminosity in darkness was required, safer radioactivepromethium-147 (half-life 2.6 years) ortritium (half-life 12 years) paint was used; both continue to be used as of 2018.[49] These had the added advantage of not degrading the phosphor over time, unlike radium.[50] Tritium as it is used in these applications is considered safer than radium,[51] as it emits very low-energybeta radiation[52] (even lower-energy than the beta radiation emitted by promethium)[53] which cannot penetrate the skin,[54] unlike the gamma radiation emitted by radium isotopes.[51]
A zeppelinaltimeter fromWorld War I. The dial, previously painted with a luminescent radium paint, has turned yellow due to the degradation of the fluorescentzinc sulfide medium.
Clocks, watches, and instruments dating from the first half of the 20th century, often in military applications, may have been painted with radioactive luminous paint. They are usually no longer luminous; this is not due to radioactive decay of the radium (which has a half-life of 1600 years) but to the fluorescence of the zinc sulfide fluorescent medium being worn out by the radiation from the radium.[55] Originally appearing as white, most radium paint from before the 1960s has tarnished to yellow over time. The radiation dose from an intact device is usually only a hazard when many devices are grouped together or if the device is disassembled or tampered with.[56]
Use in electron tubes
Radium has been used inelectron tubes, such as the Western Electric 346B tube. These devices contain a small amount of radium (in the form ofradium bromide)[57] to ionize the fill gas, typically a noble gas likeneon orargon. This ionization ensures reliable and consistent operation by providing a steady current when a high voltage is applied, enhancing the device's performance and stability. The radium is sealed within a glass envelope with two electrodes, one of which is coated with the radioactive material to create an ion path between the electrodes.[58]
1918 ad for Radior, one of several cosmetic products claiming to contain radium for its purported curative properties[59]
Radium was once an additive in products such as cosmetics, soap, razor blades, and even beverages due to its supposed curative powers. Many contemporary products were falsely advertised as being radioactive.[60] 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 orRevigator types of "radium water" or "Standard Radium Solution for Drinking".)[55]Spas featuring radium-rich water are still occasionally touted as beneficial, such as those inMisasa, Tottori, Japan,[61] though the sources of radioactivity in these spas vary and may be attributed toradon and other radioisotopes.[62]
Medical and research uses
Radium (usually in the form ofradium chloride or radium bromide) was used inmedicine to produce radon gas, which in turn was used as acancer treatment.[7] Several of these radon sources were used in Canada in the 1920s and 1930s.[63] However, many treatments that were used in the early 1900s are not used anymore because of the harmful effects radium bromide exposure caused. Some examples of these effects areanaemia, cancer, andgenetic mutations.[64] As of 2011, safer gamma emitters such as60Co, which is less costly and available in larger quantities, are usually used to replace the historical use of radium in this application,[29] but factors including increasing costs of cobalt and risks of keeping radioactive sources on site have led to an increase in the use oflinear particle accelerators for the same applications.[65]
In the U.S., from 1940 through the 1960s, radium was used innasopharyngeal radium irradiation, a treatment that was administered to children to treathearing loss and chronicotitis. The procedure was also administered toairmen andsubmarine crew to treatbarotrauma.[66][67]
Early in the 1900s, biologists used radium to induce mutations and studygenetics. As early as 1904, Daniel MacDougal used radium in an attempt to determine whether it could provoke sudden large mutations and cause major evolutionary shifts.Thomas Hunt Morgan used radium to induce changes resulting in white-eyed fruit flies. Nobel-winning biologistHermann Muller briefly studied the effects of radium on fruit fly mutations before turning to more affordable x-ray experiments.[68]
Uranium had no large scale application in the late 19th century and therefore no large uranium mines existed. In the beginning, thesilver mines inJáchymov,Austria-Hungary (nowCzech Republic) were the only large sources for uranium ore.[33] The uranium ore was only abyproduct of the mining activities.[69]
In the first extraction of radium, Curie used the residues after extraction of uranium from pitchblende. The uranium had been extracted by dissolution insulfuric acid leaving radium sulfate, which is similar tobarium 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 byhydrochloric acid treatment to minimize impurities of other compounds. The remaining residue was then treated withsodium 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 withhydrogen 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 byfractional crystallisation while monitoring the progress using aspectroscope (radium gives characteristic red lines in contrast to the green barium lines), and theelectroscope.[70]
After the isolation of radium by Marie and Pierre Curie from uranium ore fromJáchymov, 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 governmentnationalised the mines and stopped exporting raw ore. Until 1912, when radium production increased, radium availability was low.[69]
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,[33] producing 70 g total from 1913 to 1920 inPittsburgh alone.[71]
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, additional barium can be added to carry the radium. These processes were applied to high grade uranium ores but may not have worked well with low grade ores.[72] Small amounts of radium were still extracted from uranium ore by this method of mixed precipitation and ion exchange as late as the 1990s,[30] but as of 2011, it is extracted only from spent nuclear fuel.[73] Pure radium metal is isolated by reducing radium oxide with aluminium metal in a vacuum at 1,200 °C.[29]
In 1954, the total worldwide supply of purified radium amounted to about 5 pounds (2.3 kg).[45]Zaire and Canada were briefly the largest producers of radium in the late 1970s.[71] As of 1997 the chief radium-producing countries were Belgium, Canada, the Czech Republic, Slovakia, the United Kingdom, and Russia.[30] The annual production of radium compounds was only about 100 g in total as of 1984;[30] annual production of radium had reduced to less than 100 g by 2018.[74]
Modern applications
Radium is seeing increasing use in the field ofatomic, molecular, and optical physics.[75][21]Symmetry breaking forces scale proportional to[76] which makes radium, the heaviest alkaline earth element, well suited for constraining new physics beyond thestandard model. Some radium isotopes, such as radium-225, haveoctupole deformed parity doublets that enhance sensitivity tocharge parity violating new physics by two to three orders of magnitude compared to199Hg.[77]
Radium is also a promising candidate for trapped ionoptical clocks. The radium ion has two subhertz-linewidth transitions from the ground state that could serve as the clock transition in an optical clock.[78] A226Ra+ trapped ion atomic clock has been demonstrated on the to transition, which has been considered for the creation of a transportable optical clock as all transitions necessary for clock operation can be addressed with direct diode lasers at common wavelengths.[79]
Some of the few practical uses of radium are derived from its radioactive properties. More recently discoveredradioisotopes, such ascobalt-60 andcaesium-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.[80]
The isotope223Ra was approved by the United StatesFood and Drug Administration in 2013 for use inmedicine as acancer treatment of bonemetastasis in the form of a solution[81] including radium-223 chloride.[82] The main indication of treatment is the therapy ofbony metastases from castration-resistant prostate cancer.[83]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.[84]
Radium was still used in 2007 as a radiation source in someindustrial radiography devices to check for flawed metallic parts, similarly toX-ray imaging.[15] When mixed withberyllium, radium acts as aneutron source.[55][85] Up until at least 2004, radium-beryllium neutron sources were still sometimes used,[15][86]but other materials such aspolonium andamericium have become more common for use in neutron sources. 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 of241Am–Be sources.[87] As of 2011[update], the isotope226Ra is mainly used to form227Ac byneutron irradiation in a nuclear reactor.[29]
Hazards
Radium is highly radioactive, as is its immediate decay product,radon gas. When ingested, 80% of the ingested radium leaves the body through thefeces, while the other 20% goes into thebloodstream, mostly accumulating in the bones. This is because the body treats radium ascalcium anddeposits it in the bones, where radioactivity degradesmarrow and can mutatebone cells. Exposure to radium, internal or external, can cause cancer and other disorders, because radium and radon emit alpha andgamma rays upon their decay, which kill and mutate cells.[15] Radium is generally considered the most toxic of the radioactive elements.[87]
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 physicistAntoine Becquerel carried a small ampoule of radium in his waistcoat pocket for six hours and reported that his skin becameulcerated. 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.[88]Handling of radium has been blamed for Marie Curie's death, due toaplastic anemia,[89] though analysis of her levels of radium exposure done after her death find them within accepted safe levels and attribute her illness and death to her use ofradiography.[90] A significant amount of radium's danger comes from its daughter radon, which as a gas can enter the body far more readily than can its parent radium.[15]
The first published recommendations for protection against radium and radiation in general were made by the British X-ray and Radium Protection Committee and were adopted internationally in 1928 at the first meeting of theInternational Commission on Radiological Protection (ICRP), following preliminary guidance written by theRöntgen Society.[91] This meeting led to further developments of radiation protection programs[92] coordinated across all countries represented by the commission.[93]
Exposure to radium is still regulated internationally by the ICRP, alongside theWorld Health Organization.[94] TheInternational Atomic Energy Agency (IAEA) publishes safety standards and provides recommendations for the handling of and exposure to radium in its works onnaturally occurring radioactive materials and the broader International Basic Safety Standards,[95] which are not enforced by the IAEA but are available for adoption by members of the organization.[96] In addition, in efforts to reduce the quantity of oldradiotherapy devices that contain radium, the IAEA has worked since 2022[97] to manage and recycle disused226Ra sources.[98][99]
In several countries, further regulations exist and are applied beyond those recommended by the IAEA and ICRP. For example, in the United States, theEnvironmental Protection Agency-defined Maximum Contaminant Level for radium is 5 pCi/L for drinking water;[100] at the time of theManhattan Project in the 1940s, the "tolerance level" for workers was set at 0.1 micrograms of ingested radium.[101] TheOccupational Safety and Health Administration does not specifically set exposure limits for radium, and instead limits ionizing radiation exposure in units ofroentgen equivalent man based on the exposed area of the body. Radium sources themselves, rather than worker exposures, are regulated more closely by theNuclear Regulatory Commission,[102] which requires licensing for anyone possessing226Ra with activity of more than 0.01 μCi.[103] The particular governing bodies that regulate radioactive materials and nuclear energy are documented by the Nuclear Energy Agency for member countries[104] – for instance, in theRepublic of Korea, the nation's radiation safety standards are managed by the Korea Radioisotope Institute, established in 1985, and the Korea Institute of Nuclear Safety, established in 1990[105] – and the IAEA leads efforts in establishing governing bodies in locations that do not have government regulations on radioactive materials.[106][107]
Notes
^Both values are encountered in sources and there is no agreement among scientists as to the true value of the melting point of radium.[6]
^Arblaster, John W. (2018).Selected Values of the Crystallographic Properties of Elements. Materials Park, Ohio: ASM International.ISBN978-1-62708-155-9.
^Weigel, F.; Trinkl, A. (1968). "Zur Kristallchemie des Radiums" [On radium's chemical chrystalography].Radiochim. Acta (in German).10 (1–2): 78.doi:10.1524/ract.1968.10.12.78.S2CID100313675.
^Young, David A. (1991)."Radium".Phase Diagrams of the Elements. University of California Press. p. 85.ISBN978-0-520-91148-2.
^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. Vol. 10. Dordrecht: Springer. p. 81.doi:10.1007/978-1-4020-9975-5_2.ISBN978-1-4020-9974-8.
^Section 14, Geophysics, Astronomy, and Acoustics; Abundance of Elements in the Earth's Crust and in the Sea, in Lide, David R. (ed.),CRC Handbook of Chemistry and Physics, 85th Edition. CRC Press. Boca Raton, Florida (2005).
Carvalho, Fernando P. (2011). "Marie Curie and the Discovery of Radium".The New Uranium Mining Boom. Springer Geology. Berlin, Heidelberg: Springer. pp. 3–13.doi:10.1007/978-3-642-22122-4_1.ISBN978-3-642-22121-7.
Weeks, Mary Elvira (1933). "The discovery of the elements. XIX. The radioactive elements".Journal of Chemical Education.10 (2): 79.Bibcode:1933JChEd..10...79W.doi:10.1021/ed010p79.
^Stwertka, Albert (1998).A Guide to the Elements (revised ed.). Oxford University Press. p. 194.ISBN978-0-19-508083-4.
^Curie, Marie & Debierne, André (1910)."Sur le radium métallique" [On metallic radium].Comptes Rendus (in French).151:523–525.Archived from the original on 20 July 2011. Retrieved1 August 2009.
^Lavrukhina, Avgusta Konstantinovna; Pozdnyakov, Aleksandr Aleksandrovich (1966).Аналитическая химия технеция, прометия, астатина и франция [Analytical Chemistry of Technetium, Promethium, Astatine, and Francium] (in Russian).Nauka. p. 118.
^Hydrogen-3(PDF) (Report). Nuclide safety data sheet. Environmental Health & Safety Office,Emory University. Archived fromthe original(PDF) on 20 May 2013 – via ehso.emory.edu.
^Hon. R. J. Strutt (1904).The Becquerel rays and the properties of radium. London: Edward Arnold. – via"Lateral Science"Archived 2 April 2015 at theWayback Machine.lateralscience.blogspot.se. November 2012
^ab"Production, Import, Use and Disposal".Toxicological Profile for Radium. Atlanta (GA): Agency for Toxic Substances and Disease Registry (US). 4 December 1990.
Radiation Source Use and Replacement: Abbreviated version (Report). Committee on Radiation Source Use and Replacement / Nuclear and Radiation Studies Board. Washington, DC: U.S. National Research Council / National Academies Press. January 2008. p. 24.ISBN978-0-309-11014-3.Archived from the original on 5 September 2015. Retrieved27 June 2015 – via Google Books.
^"7. Regulations and Advisories".Toxicological Profile for Radium. Atlanta (GA): Agency for Toxic Substances and Disease Registry (US). 7 December 1990.
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