Alkaline earth metal

History

Etymology

The majority of the alkaline earth metals are named after their oxides, the alkaline earths, whose old-fashioned names were beryllia, magnesia, lime, strontia, and baria. These oxides are basic (alkaline) when combined with water. "Earth" was a 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.

Radium comes from the French word radium, originating from the Modern Latin radius, meaning ray, in recognition of radium's emission of energy in the form of rays.

Discovery

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: radioactive radium, 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.

Beryllium

Beryl, a mineral that contains beryllium, has been known since the time of the Ptolemaic Kingdom in Egypt. Although it was originally thought that beryl was an aluminum silicate, beryl was later found to contain a then-unknown element when, in 1797, Louis-Nicolas Vauquelin dissolved aluminum 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

Magnesium was first produced by 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.

Calcium

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.

Strontium

In 1790, physician Adair Crawford discovered ores with distinctive properties, 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 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.

Barium

Barite, a mineral containing barium, was first recognized as containing a new element in 1774 by Carl Scheele, although he was able to isolate only barium 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 Humphry Davy used 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.

Radium

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.

Occurrence

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, being respectively the fifth and eighth most abundant elements. None of the alkaline earth metals are found in their elemental state. Common magnesium-containing minerals are carnallite, magnesite, and dolomite. Common calcium-containing minerals are chalk, limestone, gypsum, and anhydrite.

Strontium is the 15th most abundant element in the Earth's crust. The principal minerals are 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.

Production

Beryllium

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, aluminum 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.

Strontium

In general, strontium carbonate is extracted from the mineral celestite through two methods: by leaching the celestite with sodium carbonate, or in a more complicated way involving coal.

Barium

To produce barium, barite (impure barium sulfate) is converted to barium sulfide by carbothermic reduction (such as with coke). The sulfide is water-soluble and easily reacted to form pure barium sulfate, used for commercial pigments, or other compounds, such as barium nitrate. These in turn are calcined into barium oxide, which eventually yields pure barium after reduction with aluminum. The most important supplier of barium is China, which produces more than 50% of world supply.

Magnesium

Magnesium is usually produced from magnesite ore, as well as dolomite. When dolomite is crushed, roasted and mixed with seawater in large tanks, magnesium hydroxide settles to the bottom. Heating, mixing in coke, and reacting with chlorine, then produces molten magnesium chloride. This can be electrolyzed, releasing magnesium, which floats to the surface.

Calcium

Calcium is the fifth most abundant element in the Earth's crust, existing mostly as calcium carbonate. Other important minerals of calcium include gypsum, anhydrite, fluorite, and apatite. The largest producers of calcium in the world are China and Russia, where Davy's method of electrolysis is used on calcium chloride to produce the metal. Other large producers of calcium are the U.S. and Canada, where lime is reduced with aluminium at high temperatures.

Radium

Marie Skłodowska-Curie developed the first methods of radium extraction using residues of pitchblende, following her extraction of uranium. These residues contained mostly barium sulfate. A lengthy purification process involving boiling with sodium hydroxide, treating with hydrochloric acid, then adding sodium carbonate afforded a mixture of barium and radium carbonates, which could be converted to chlorides, and from which the radium salt could be separated via fractional crystallization. Curie's method was used until 1940, after which mixed bromides became more popular. As of 2011, radium is extracted only from spent reactor fuel as oxides. The pure metal can be isolated by reduction with aluminium in a vacuum at 1,200 °C.

Applications

Beryllium is used mainly in military applications, but non-military uses exist. 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. Beryllium alloys are used for mechanical parts when stiffness, light weight, and dimensional stability are required over a wide temperature range. Beryllium-9 is used in small-scale neutron sources that use the reaction ⁹Be + ⁴He (α) → ¹²C + ¹n, the reaction used by James Chadwick when he discovered the neutron. Its low atomic weight and low neutron absorption cross-section would make beryllium suitable as a neutron moderator, but its high price and the readily available alternatives such as water, heavy water and nuclear graphite have limited this to niche applications. In the FLiBe eutectic used in molten salt reactors, beryllium's role as a moderator is more incidental than the desired property leading to its use.

Magnesium has many uses. It offers advantages over other structural materials such as aluminum, but magnesium's usage is hindered by its flammability. Magnesium is often alloyed with aluminum, zinc and manganese to increase its strength and corrosion resistance. Magnesium has many other industrial applications, such as its role in the production of iron and steel, and in the Kroll process for production of titanium.

Calcium is used as a reducing agent in the separation of other metals such as uranium from ore. It is a major component of many alloys, especially aluminum and copper alloys, and is also used to deoxidize alloys. Calcium has roles in the making of cheese, mortars, and cement.

Strontium and barium have fewer applications than the lighter alkaline earth metals. Strontium carbonate is used in the manufacturing of red fireworks. Pure strontium is used in the study of neurotransmitter release in neurons. Radioactive strontium-90 finds some use in RTGs, which utilize its decay heat. Barium is used in vacuum tubes as a getter to remove gases. Barium sulfate has many uses in the petroleum industry, and other industries.

Radium has many former applications based on its radioactivity, but its use is no longer common because of the adverse health effects and long half-life. Radium was frequently used in luminous paints, although this use was stopped after it sickened workers. The nuclear quackery that alleged health benefits of radium formerly led to its addition to drinking water, toothpaste, and many other products. Radium is no longer used even when its radioactive properties are desired because its long half-life makes safe disposal challenging. For example, in brachytherapy, shorter-lived alternatives such as iridium-192 are usually used instead.

Representative reactions of alkaline earth metals

Reactions with halogens

Ca + Cl₂ → CaCl₂

Anhydrous calcium chloride is a hygroscopic substance that is used as a desiccant. Exposed to air, it will absorb water vapour from the air, forming a solution. This property is known as deliquescence.

Reactions with oxygen

2 Ca + O₂ → 2 CaO

2 Mg + O₂ → 2 MgO

Reactions with sulfur

8 Ca + S₈ → 8 CaS

Reactions with carbon

With carbon, they form acetylides directly. Beryllium forms carbide.

2 Be + C → Be₂C

CaO + 3 C → CaC₂ + CO (at 2500 °C in furnace)

CaC₂ + 2 H₂O → Ca(OH)₂ + C₂H₂

Mg₂C₃ + 4 H₂O → 2 Mg(OH)₂ + C₃H₄

Reactions with nitrogen

Only Be and Mg form nitrides directly.

3 Be + N₂ → Be₃N₂

3 Mg + N₂ → Mg₃N₂

Reactions with hydrogen

Alkaline earth metals react with hydrogen to generate saline hydride that are unstable in water.

Ca + H₂ → CaH₂

Reactions with water

Ca, Sr, and Ba readily react with water to form hydroxide and hydrogen gas. Be and Mg are passivated by an impervious layer of oxide. However, amalgamated magnesium will react with water vapor.

Mg + H₂O → MgO + H₂

Reactions with acidic oxides

Alkaline earth metals reduce the nonmetal from its oxide.

2 Mg + SiO₂ → 2MgO + Si

2 Mg + CO₂ → 2 MgO + C (in solid carbon dioxide)

Reactions with acids

Mg + 2 HCl → MgCl₂ + H₂

Be + 2 HCl → BeCl₂ + H₂

Reactions with bases

Be exhibits amphoteric properties. It dissolves in concentrated sodium hydroxide.

Be + NaOH + 2 H₂O → Na[Be(OH)₃] + H₂

Reactions with alkyl halides

Magnesium reacts with alkyl halides via an insertion reaction to generate Grignard reagents.

RX + Mg → RMgX (in anhydrous ether)

Identification of alkaline earth cations

The flame test

The table below presents the colors observed when the flame of a Bunsen burner is exposed to salts of alkaline earth metals. Be and Mg do not impart colour to the flame due to their small size.

In solution

Mg²⁺

Disodium phosphate is a very selective reagent for magnesium ions and, in the presence of ammonium salts and ammonia, forms a white precipitate of ammonium magnesium phosphate.

Mg²⁺ + NH₃ + Na₂HPO₄ → (NH₄)MgPO₄ + 2 Na⁺

Ca²⁺

Ca²⁺ forms a white precipitate with ammonium oxalate. Calcium oxalate is insoluble in water, but is soluble in mineral acids.

Ca²⁺ + (COO)₂(NH₄)₂ → (COO)₂Ca + NH₄⁺

Sr²⁺

Strontium ions precipitate with soluble sulfate salts.

Sr²⁺ + Na₂SO₄ → SrSO₄ + 2 Na⁺

All ions of alkaline earth metals form white precipitate with ammonium carbonate in the presence of ammonium chloride and ammonia.

Compounds of alkaline earth metals

Oxides

The alkaline earth metal oxides are formed from the thermal decomposition of the corresponding carbonates.

CaCO<₃ → CaO + CO₂ (at approx. 900 °C)

In laboratory, they are obtained from hydroxides:

Mg(OH)₂ → MgO + H₂O

or nitrates:

2 Ca(NO₃)₂ → 2 CaO + 4 NO₂ + O₂

The oxides exhibit basic character: they turn phenolphthalein red and litmus, blue. They react with water to form hydroxides in an exothermic reaction.

CaO + H₂O → Ca(OH)₂ + Q

Calcium oxide reacts with carbon to form acetylide.

CaO + 3 C → CaC₂ + CO (at 2500 °C)

CaC₂ + N₂ → CaCN₂ + C

CaCN₂ + H₂SO₄ → CaSO₄ + H₂N��CN

H₂N��CN + H₂O → (H₂N)₂CO (urea)

CaCN₂ + 2 H₂O → CaCO₃ + NH₃

Hydroxides

They are generated from the corresponding oxides on reaction with water. They exhibit basic character: they turn phenolphthalein pink and litmus, blue. Beryllium hydroxide is an exception as it exhibits amphoteric character.

Be(OH)₂ + 2 HCl → BeCl₂ + 2 H₂O

Be(OH)₂ + NaOH → Na[Be(OH)₃]

Salts

Ca and Mg are found in nature in many compounds such as dolomite, aragonite, magnesite (carbonate rocks). Calcium and magnesium ions are found in hard water. Hard water represents a multifold issue. It is of great interest to remove these ions, thus softening the water. This procedure can be done using reagents such as calcium hydroxide, sodium carbonate or sodium phosphate. A more common method is to use ion-exchange aluminosilicates or ion-exchange resins that trap Ca²⁺ and Mg²⁺ and liberate Na⁺ instead:

Na₂O·Al₂O₃·₆SiO₂ + Ca²⁺ → CaO·Al₂O₃·₆SiO₂ + 2 Na⁺

Biological role and precautions

Magnesium and calcium is essential to all known living organisms. Magnesium is in plants, where it occurs in chlorophyll. Calcium ions play a large role in cell signaling, and calcium salts also take a structural role, most notably bones for vertebrates, and some shells in invertebrates.

Strontium plays an important role in marine aquatic life, especially hard corals, which use strontium to build their exoskeletons. In medicine, strontium compounds are employed in some toothpastes to reduce dental hypersensitivity. While strontium typically has a low toxicity, radioactive isotopes of strontium (like strontium-90) can be toxic. Since strontium-90 mimics calcium (i.e. Behaves as a "bone seeker"), it bio-accumulates with a significant biological half life, so long-term strontium-90 exposure at low concentrations can still damage the cells.

Barium has very little biological role, as soluble barium (Ba²⁺ ions) are toxic. However, in the Loxodes genus, an organelle known as a Müller vesicle contains barium salts, allowing the cell to orient themselves spatially using gravity. Barium, especially its sulfate compound (BaSO₄), is also used in medicine. Since barium sulfate has a low toxicity and relatively high density of 4.5g/cm³, it is used as a radiocontrast agent in X-ray imaging ("barium meals" and "barium enemas")

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. Acute exposure to beryllium, typically by inhalation, causes inflammation in the airways. Long-term exposure can cause berylliosis or Chronic beryllium disease (CBD). Radium has a low availability and is highly radioactive, making it toxic to life. Exposure to large amounts of radium may cause anemia, cataracts, teeth fracture and death. Furthermore, since radium emits gamma radiation, simply being near areas with high levels of radium may be hazardous.

Extensions

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 with nickel-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 noticeably contrasts with periodic trends, which 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 next alkaline earth metal after element 120 has not been definitely predicted. Although a simple extrapolation using the Aufbau principle would suggest that element 170 is a congener of 120, relativistic effects may render such an extrapolation invalid. The next element with properties similar to the alkaline earth metals has been predicted to be element 166, though due to overlapping orbitals and lower energy gap below the 9s subshell, element 166 may instead be placed in group 12, below copernicium.