|Alkaline earth metals|
The alkaline earth metals are six chemical elements in column (group) 2 of the Periodic table. They are beryllium (Be), magnesium(Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). They have very similar properties: they are all shiny, silvery-white, somewhat reactive metals at standard temperature and pressure.
Structurally, they have in common an outer s- electron shell which is full;. that is, this orbital contains its full complement of two electrons, which these elements readily lose to form cations with charge +2, and an oxidation state (oxidation number) of +2.
All the discovered alkaline earth metals occur in nature. Experiments have been conducted to attempt the synthesis of element 120, the next potential member of the group, but they have all met with failure.
As with other groups, the members of this family show patterns in their electronic configuration, especially the outermost shells, resulting in trends in chemical behavior:
|Z||Element||No. of electrons/shell||Electron configuration[n 1]|
|4||beryllium||2, 2||[He] 2s2|
|12||magnesium||2, 8, 2||[Ne] 3s2|
|20||calcium||2, 8, 8, 2||[Ar] 4s2|
|38||strontium||2, 8, 18, 8, 2||[Kr] 5s2|
|56||barium||2, 8, 18, 18, 8, 2||[Xe] 6s2|
|88||radium||2, 8, 18, 32, 18, 8, 2||[Rn] 7s2|
Most of the chemistry has been observed only for the first five members of the group. The chemistry of radium is not well-established due to its radioactivity; thus, the presentation of its properties here is limited.
The alkaline earth metals are all silver-colored and soft, and have relatively low densities, melting points, and boiling points. In chemical terms, all of the alkaline metals react with the halogens to form the alkaline earth metal halides, all of which being ionic crystalline compounds (except for beryllium chloride, which is covalent). All the alkaline earth metals except beryllium also react with water to form strongly alkaline hydroxides and, thus, should be handled with great care. The heavier alkaline earth metals react more vigorously than the lighter ones. The alkaline metals have the second-lowest first ionization energies in their respective periods of the periodic tablebecause of their somewhat low effective nuclear charges and the ability to attain a full outer shell configuration by losing just twoelectrons. The second ionization energy of all of the alkaline metals is also somewhat low.
Beryllium is an exception: It does not react with water or steam, and its halides are covalent. If beryllium did form compounds with an ionization state of +2, it would polarize electron clouds that are near it very strongly and would cause extensive orbital overlap, since beryllium has a high charge density. All compounds that include beryllium have a covalent bond. Even the compound beryllium fluoride, which is the most ionic beryllium compound, has a low melting point and a low electrical conductivity when melted.
All the alkaline earth metals have two electrons in their valence shell, so the energetically preferred state of achieving a filled electron shell is to lose two electrons to form doubly charged positive ions.
Compounds and reactions
The alkaline earth metals all react with the halogens to form ionic halides, such as calcium chloride (CaCl
2), as well as reacting withoxygen to form oxides such as strontium oxide (SrO). Calcium, strontium, and barium react with water to produce hydrogen gas and their respective hydroxides, and also undergo transmetalation reactions to exchange ligands.
Alkaline earth metals fluorides solubility-related constants[n 2] Metal M2+
Be 2,455 458 3,371 3,526 soluble Mg 1,922 458 2,838 2,978 0.0012 Ca 1,577 458 2,493 2,651 0.0002 Sr 1,415 458 2,331 2,513 0.0008 Ba 1,361 458 2,277 2,373 0.006
Physical and atomic
The table below is a summary of the key physical and atomic properties of the alkaline earth metals.
|Alkaline earth metal||Standard atomic weight
|First ionization energy
|Flame test color|
|Radium||[n 4]||973||700||2010||1737||5.5||0.9||509.3||221||Crimson red[n 5]|
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All of the alkaline earth metals except magnesium and strontium have at least one naturally occurring radioisotope: beryllium-7, beryllium-10, and calcium-41 are trace radioisotopes, calcium-48 and barium-130 have very long half-lives and, thus, occur naturally, and all isotopes of radium are radioactive. Calcium-48 is the lightest nuclide to undergo double beta decay.
The natural radioisotope of calcium, calcium-48, makes up about 0.1874% of natural calcium, and, thus, natural calcium is weakly radioactive. Barium-130 makes up approximately 0.1062% of natural barium, and, thus, barium is weakly radioactive, as well.
The alkaline earth metals are named after their oxides, the alkaline earths, whose old-fashioned names were beryllia, magnesia, lime, strontia, and baryta. These oxides are basic (alkaline) when combined with water. “Earth” is an old term applied by early chemists to nonmetallic substances that are insoluble in water and resistant to heating—properties shared by these oxides. The realization that these earths were not elements but compounds is attributed to the chemist Antoine Lavoisier. In his Traité Élémentaire de Chimie (Elements of Chemistry) of 1789 he called them salt-forming earth elements. Later, he suggested that the alkaline earths might be metal oxides, but admitted that this was mere conjecture. In 1808, acting on Lavoisier’s idea, Humphry Davy became the first to obtain samples of the metals by electrolysis of their molten earths, thus supporting Lavoisier’s hypothesis and causing the group to be named the alkaline earth metals.
The calcium compounds calcite and lime have been known and used since prehistoric times. The same is true for the beryllium compounds beryl and emerald. The other compounds of the alkaline earth metals were discovered starting in the early 15th century. The magnesium compound magnesium sulfate was first discovered in 1618 by a farmer at Epsom in England. Strontium carbonate was discovered in minerals in the Scottish village of Strontian in 1790. The last element is the least abundant: radioactiveradium, which was extracted from uraninite in 1898.
All elements except beryllium were isolated by electrolysis of molten compounds. Magnesium, calcium, and strontium were first produced by Humphry Davy in 1808, whereas beryllium was independently isolated by Friedrich Wöhler and Antoine Bussy in 1828 by reacting beryllium compounds with potassium. In 1910, radium was isolated as a pure metal by Curie and André-Louis Debierne also by electrolysis.
Beryl, a mineral that contains beryllium, has been known since the time of the Ptolemaic dynasty in Egypt. Although it was originally thought that beryl was an aluminium silicate, beryl was later found to contain a then-unknown element when, in 1797, Louis-Nicolas Vauquelin dissolved aluminium hydroxide from beryl in an alkali. In 1828, Friedrich Wöhler and Antoine Bussy independently isolated this new element, beryllium, by the same method, which involved a reaction of beryllium chloride with metallic potassium; this reaction was not able to produce large ingots of beryllium. It was not until 1898, when Paul Lebeau performed an electrolysis of a mixture of beryllium fluoride and sodium fluoride, that large pure samples of beryllium were produced.
Magnesium was first produced by Sir Humphry Davy in England in 1808 using electrolysis of a mixture of magnesia and mercuric oxide.Antoine Bussy prepared it in coherent form in 1831. Davy’s first suggestion for a name was magnium, but the name magnesium is now used.
Lime has been used as a material for building since 7000 to 14,000 BCE, and kilns used for lime have been dated to 2,500 BCE in Khafaja, Mesopotamia. Calcium as a material has been known since at least the first century, as the ancient Romans were known to have used calcium oxide by preparing it from lime. Calcium sulfate has been known to be able to set broken bones since the tenth century. Calcium itself, however, was not isolated until 1808, when Humphry Davy, in England, used electrolysis on a mixture of lime and mercuric oxide, after hearing that Jöns Jakob Berzelius had prepared a calcium amalgam from the electrolysis of lime in mercury.
In 1790, physician Adair Crawford, who had been working with barium, realized that Strontian ores showed different properties than other supposed ores of barium. Therefore, he concluded that these ores contained new minerals, which were named strontites in 1793 by Thomas Charles Hope, a chemistry professor at the University of Glasgow, who confirmed Crawford’s discovery. Strontium was eventually isolated in 1808 by Sir Humphry Davy by electrolysis of a mixture of strontium chloride and mercuric oxide. The discovery was announced by Davy on 30 June 1808 at a lecture to the Royal Society.
Barite, a mineral containing barium, was first recognized as containing a new element in 1774 by Carl Scheele, although he was able to isolate onlybarium oxide. Barium oxide was isolated again two years later by Johan Gottlieb Gahn. Later in the 18th century, William Withering noticed a heavy mineral in the Cumberland lead mines, which are now known to contain barium. Barium itself was finally isolated in 1808 when Sir Humphry Davyused electrolysis with molten salts, and Davy named the element barium, after baryta. Later, Robert Bunsen and Augustus Matthiessen isolated pure barium by electrolysis of a mixture of barium chloride and ammonium chloride.
While studying uraninite, on 21 December 1898, Marie and Pierre Curie discovered that, even after uranium had decayed, the material created was still radioactive. The material behaved somewhat similarly to barium compounds, although some properties, such as the color of the flame test and spectral lines, were much different. They announced the discovery of a new element on 26 December 1898 to the French Academy of Sciences.Radium was named in 1899 from the word radius, meaning ray, as radium emitted power in the form of rays.
Beryllium occurs in the earth’s crust at a concentration of two to six parts per million (ppm), much of which is in soils, where it has a concentration of six ppm. Beryllium is one of the rarest elements in seawater, even rarer than elements such as scandium, with a concentration of 0.2 parts per trillion. However, in freshwater, beryllium is somewhat more common, with a concentration of 0.1 parts per billion.
Magnesium and calcium are very common in the earth’s crust, with calcium the fifth-most-abundant element, and magnesium the eighth. None of the alkaline earth metals are found in their elemental state, but magnesium and calcium are found in many rocks and minerals: magnesium in carnellite, magnesite, and dolomite; and calcium in chalk, limestone, gypsum, and anhydrite.
Strontium is the fifteenth-most-abundant element in the Earth’s crust. Most strontium is found in the minerals celestite and strontianite.Barium is slightly less common, much of it in the mineral barite.
Radium, being a decay product of uranium, is found in all uranium-bearing ores. Due to its relatively short half-life, radium from the Earth’s early history has decayed, and present-day samples have all come from the much slower decay of uranium.
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Most beryllium is extracted from beryllium hydroxide. One production method is sintering, done by mixing beryl, sodium fluorosilicate, and soda at high temperatures to form sodium fluoroberyllate, aluminium oxide, and silicon dioxide. A solution of sodium fluoroberyllate and sodium hydroxide in water is then used to form beryllium hydroxide by precipitation. Alternatively, in the melt method, powdered beryl is heated to high temperature, cooled with water, then heated again slightly in sulfuric acid, eventually yielding beryllium hydroxide. The beryllium hydroxide from either method then produces beryllium fluoride and beryllium chloride through a somewhat long process. Electrolysis or heating of these compounds can then produce beryllium.
To produce barium, barite ore is separated from quartz, sometimes by froth flotation methods, resulting in relatively pure barite. Carbon is then used to reduce the baryte into barium sulfide, which is dissolved with other elements to form other compounds, such as barium nitrate. These in turn are thermally decompressed into barium oxide, which eventually yields pure barium after a reaction with aluminium. The most important supplier of barium is China, which produces more than 50% of world supply.
Beryllium is used mostly for military applications, but there are other uses of beryllium, as well. In electronics, beryllium is used as a p-type dopant in some semiconductors,and beryllium oxide is used as a high-strength electrical insulator and heat conductor. Due to its light weight and other properties, beryllium is also used in mechanics when stiffness, light weight, and dimensional stability are required at wide temperature ranges.
Magnesium has many different uses. One of its most common uses was in industry, where it has many structural advantages over other materials such as aluminium, although this usage has fallen out of favor recently due to magnesium’s flammability. Magnesium is also often alloyed with aluminium or zinc to form materials with more desirable properties than any pure metal. Magnesium has many other uses in industrial applications, such as having a role in the production of iron and steel, and the production oftitanium.
Calcium also has many uses. One of its uses is as a reducing agent in the separation of other metals from ore, such as uranium. It is also used in the production of the alloys of many metals, such as aluminium and copper alloys, and is also used to deoxidize alloys as well. Calcium also has a role in the making of cheese, mortars, and cement.
Strontium and barium do not have as many applications as the lighter alkaline earth metals, but still have uses. Strontium carbonate is often used in the manufacturing of redfireworks, and pure strontium is used in the study of neurotransmitter release in neurons. Barium has some use in vacuum tubes to remove gases, and barium sulfatehas many uses in the petroleum industry, as well as other industries.
Due to its radioactivity, radium no longer has many applications, but it used to have many. Radium used to be used often in luminous paints, although this use was stopped after workers got sick. As people used to think that radioactivity was a good thing, radium used to be added to drinking water, toothpaste, and many other products, although they are also not used anymore due to their health effects. Radium is no longer even used for its radioactive properties, as there are more powerful and safer emitters than radium.
Biological role and precautions
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Magnesium and calcium are ubiquitous and essential to all known living organisms. They are involved in more than one role, with, for example, magnesium or calcium ion pumpsplaying a role in some cellular processes, magnesium functioning as the active center in some enzymes, and calcium salts taking a structural role, most notably in bones.
Strontium plays an important role in marine aquatic life, especially hard corals, which use strontium to build their exoskeletons. It and barium have some uses in medicine, for example “barium meals” in radiographic imaging, whilst strontium compounds are employed in some toothpastes. Excessive amounts of strontium-90 are toxic due to its radioactivity and strontium-90 mimics calcium and then can kill.
Beryllium and radium, however, are toxic. Beryllium’s low aqueous solubility means it is rarely available to biological systems; it has no known role in living organisms and, when encountered by them, is usually highly toxic. Radium has a low availability and is highly radioactive, making it toxic to life.
The next alkaline earth metal after radium is thought to be element 120, although this may not be true due to relativistic effects. The synthesis of element 120 was first attempted in March 2007, when a team at the Flerov Laboratory of Nuclear Reactions in Dubna bombarded plutonium-244 with iron-58 ions; however, no atoms were produced, leading to a limit of 400 fb for the cross-section at the energy studied. In April 2007, a team at the GSI attempted to create element 120 by bombarding uranium-238 withnickel-64, although no atoms were detected, leading to a limit of 1.6 pb for the reaction. Synthesis was again attempted at higher sensitivities, although no atoms were detected. Other reactions have been tried, although all have been met with failure.
The chemistry of element 120 is predicted to be closer to that of calcium or strontium instead of barium or radium. This is unusual as periodic trends would predict element 120 to be more reactive than barium and radium. This lowered reactivity is due to the expected energies of element 120’s valence electrons, increasing element 120’s ionization energy and decreasing the metallic and ionic radii.
The Radium Girls were female factory workers who contracted radiation poisoning from painting watch dials withself-luminous paint at the United States Radium factory in Orange, New Jersey, around 1917. The women, who had been told the paint was harmless, ingested deadly amounts of radium by licking their paintbrushes to give them a fine point; some also painted their fingernails and teeth with the glowing substance.
Five of the women challenged their employer in a case that established the right of individual workers who contractoccupational diseases to sue their employers.
United States Radium Corporation
From 1917 to 1926, U.S. Radium Corporation, originally called the Radium Luminous Material Corporation, was engaged in the extraction and purification of radium from carnotite ore to produce luminous paints, which were marketed under the brand name “Undark“. As a defense contractor, U.S. Radium was a major supplier of radioluminescent watches to the military. Their plant in Ottawa, Illinois, employed more than a hundred workers, mainly women, to paint radium-lit watch faces and instruments, misleading them that it was safe.
The U.S. Radium Corporation hired approximately 70 women to perform various tasks including the handling of radium, while the owners and the scientists familiar with the effects of radium carefully avoided any exposure to it themselves; chemists at the plant used lead screens, masks and tongs. U.S. Radium had even distributed literature to the medical community describing the “injurious effects” of radium.
An estimated 4,000 workers were hired by corporations in the U.S. and Canada to paint watch faces with radium. They mixed glue, water and radium powder, and then used camel hair brushes to apply the glowing paint onto dials. The then-current rate of pay, for painting 250 dials a day, was about a penny and a half per dial (equivalent to $0.276 in 2015). The brushes would lose shape after a few strokes, so the U.S. Radium supervisors encouraged their workers to point the brushes with their lips, or use their tongues to keep them sharp. For fun, the Radium Girls painted their nails, teeth and faces with the deadly paint produced at the factory. Many of the workers became sick. It is unknown how many died from exposure to radiation.
Many of the women later began to suffer from anemia, bone fractures and necrosis of the jaw, a condition now known as radium jaw. It is thought that the X-ray machines used by the medical investigators may have contributed to some of the sickened workers’ ill-health by subjecting them to additional radiation. It turned out at least one of the examinations was a ruse, part of a campaign of disinformation started by the defense contractor. U.S. Radium and other watch-dial companies rejected claims that the afflicted workers were suffering from exposure to radium. For some time, doctors, dentists, and researchers complied with requests from the companies not to release their data. At the urging of the companies, worker deaths were attributed by medical professionals to other causes; syphilis, a notorious sexually transmitted disease at the time, was often cited in attempts to smear the reputations of the women.
The story of the abuse perpetrated against the workers is distinguished from most such cases by the fact that the ensuing litigation was covered widely by the media. Plant worker Grace Fryer decided to sue, but it took two years for her to find a lawyer willing to take on U.S. Radium. Even after the women found a lawyer, the slow-moving courts held out for months. At their first appearance in court on January 1928, two women were bedridden and none of them could raise their arms to take oath. A total of five factory workers – Grace Fryer, Edna Hussman, Katherine Schaub, and sisters Quinta McDonald and Albina Larice – dubbed the Radium Girls, joined the suit. The litigation and media sensation surrounding the case established legal precedents and triggered the enactment of regulations governing labor safety standards, including a baseline of “provable suffering“.
The Radium Girls saga holds an important place in the history of both the field of health physics and the labor rights movement. The right of individual workers to sue for damages from corporations due to labor abuse was established as a result of the Radium Girls case. In the wake of the case, industrial safety standards were demonstrably enhanced for many decades.
The case was settled in the autumn of 1928, before the trial was deliberated by the jury, and the settlement for each of the Radium Girls was $10,000 (equivalent to $137,000 in 2015) and a $600 per year annuity (equivalent to $8,200 in 2015) while they lived, and all medical and legal expenses incurred would also be paid by the company.
The lawsuit and resulting publicity was a factor in the establishment of occupational disease labor law. Radium dial painters were instructed in proper safety precautions and provided with protective gear; in particular, they no longer shaped paint brushes by lip and avoided ingesting or breathing the paint. Radium paint was still used in dials as late as the 1960s.
Robley D. Evans made the first measurements of exhaled radon and radium excretion from a former dial painter in 1933. At MIT he gathered dependable body content measurements from 27 dial painters. This information was used in 1941 by the National Bureau of Standards to establish the tolerance level for radium of 0.1 μCi (3.7 kBq).
The Center for Human Radiobiology was established at Argonne National Laboratory in 1968. The primary purpose of the Center was providing medical examinations for living dial painters. The project also focused on collection of information and, in some cases, tissue samples from the radium dial painters. When the project ended in 1993, detailed information of 2,403 cases had been collected. No symptoms were observed in those dial painter cases with less than 1,000 times the natural 226Ra levels found in unexposed individuals, suggesting a threshold for radium-induced malignancies.
Literature and film
- The story is told in Eleanor Swanson’s poem Radium Girls, collected in A Thousand Bonds: Marie Curie and the Discovery of Radium (2003, ISBN 0-9671810-7-0)
- D. W. Gregory told the story of Grace Fryer in the play Radium Girls, which premiered in 2000 at the Playwrights Theatre in Madison, New Jersey.
- There is an elaborate reference to the story in the Kurt Vonnegut novel Jailbird (1979, ISBN 0-385-33390-0)
- Poet Lavinia Greenlaw has written on the subject in The Innocence of Radium (Night Photograph, 1994)
- Historian Claudia Clark wrote an account of the case and its wider historical implications: Radium Girls: Women and Industrial Health Reform, 1910–1935 (published 1997).
- Ross Mullner’s book Deadly Glow: The Radium Dial Worker Tragedy describes many of the events (1999, ISBN 0-87553-245-4)
- The story is told by Jo Lawrence in her short animated film “Glow” (2007)
- The story is referenced in the 2006 film Pu-239
- The Michael A. Martone short story It’s Time is told from the perspective of an unnamed Radium Girl
- A fictionalized version of the story was featured in the Spike TV show 1000 Ways to Die (#196) and Science Channel‘s Dark Matters: Twisted But True
- Radium Halos: A Novel about the Radium Dial Painters a 2009 novel by Shelley Stout is historical fiction narrated by a sixty-five-year-old mental patient who worked at the factory when she was sixteen (ISBN 978-1448696222).
- Author Deborah Blum referenced the story in her 2010 book The Poisoner’s Handbook: Murder and the Birth of Forensic Medicine in Jazz Age New York.
- Author Robert R. Johnson features a story on the radium girls in his book Romancing the Atom. (ISBN 978-0313392795) 
- The Case of the Living Dead Women, a website displaying scans of 180 pages of newspaper clippings about a similar incident, the Ottawa, Illinois Radium Dial Companylitigation
- The Radium Dial Company workers’ story is dramatized in Melanie Marnich‘s stage play These Shining Lives.
- A fictionalized version of the story was featured in the 1937 short story “Letter to the Editor” by James H. Street, adapted into a 1937 film Nothing Sacred and a 1953 Broadway musical Hazel Flagg.
- The story is told in the American Experience episode The Poisoner’s Handbook, based on the non-fiction book with the same name by Deborah Blum.