Strontium

Strontium is a chemical element; it has symbol Sr and atomic number 38. An alkaline earth metal, it is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is exposed to air. Strontium has physical and chemical properties similar to those of its two vertical neighbors in the periodic table, calcium and barium. It occurs naturally mainly in the minerals celestine and strontianite, and is mostly mined from these.
Both strontium and strontianite are named after Strontian, a village in Scotland near which the mineral was discovered in 1790 by Adair Crawford and William Cruickshank; it was identified as a new element the next year from its crimson-red flame test color. Strontium was first isolated as a metal in 1808 by Humphry Davy using the then newly discovered process of electrolysis. During the 19th century, strontium was mostly used in the production of sugar from sugar beets. At the peak of production of television cathode-ray tubes, as much as 75% of strontium consumption in the United States was used for the faceplate glass. With the replacement of cathode-ray tubes with other display methods, consumption of strontium has dramatically declined.
While natural strontium is stable, the synthetic strontium-90 is radioactive and is one of the most dangerous components of nuclear fallout, as strontium is absorbed by the body in a similar manner to calcium. Natural stable strontium is not hazardous to health at low levels.

Characteristics

Strontium is a divalent silvery metal with a pale yellow tint whose properties are mostly intermediate between and similar to those of its group neighbors calcium and barium. It is softer than calcium and harder than barium. Its melting and boiling points are lower than those of calcium ; barium continues this downward trend in the melting point, but not in the boiling point. The density of strontium is similarly intermediate between those of calcium and barium. Three allotropes of metallic strontium exist, with transition points at 235 and 540 °C.
The standard electrode potential for the Sr²⁺/Sr couple is −2.89 V, approximately midway between those of the Ca²⁺/Ca and Ba²⁺/Ba couples, and close to those of the neighboring alkali metals. Strontium is intermediate between calcium and barium in its reactivity toward water, with which it reacts on contact to produce strontium hydroxide and hydrogen gas. Strontium metal burns in air to produce both strontium oxide and strontium nitride, but since it does not react with nitrogen below 380 °C, at room temperature it forms only the oxide spontaneously. Besides the simple oxide SrO, the peroxide can be made by direct oxidation of strontium metal under a high pressure of oxygen, and there is some evidence for a yellow superoxide. Strontium hydroxide,, is a strong base, though it is not as strong as the hydroxides of barium or the alkali metals. All four dihalides of strontium are known.
Due to the large size of the heavy s-block elements, including strontium, a vast range of coordination numbers is known, from 2, 3, or 4 all the way to 22 or 24 in and. The Sr²⁺ ion is quite large, so that high coordination numbers are the rule. The large size of strontium and barium plays a significant part in stabilising strontium complexes with polydentate macrocyclic ligands such as crown ethers: for example, while 18-crown-6 forms relatively weak complexes with calcium and the alkali metals, its strontium and barium complexes are much stronger.
Organostrontium compounds contain one or more strontium–carbon bonds. They have been reported as intermediates in Barbier-type reactions. Although strontium is in the same group as magnesium, and organomagnesium compounds are very commonly used throughout chemistry, organostrontium compounds are not similarly widespread because they are more difficult to make and more reactive. Organostrontium compounds tend to be more similar to organoeuropium or organosamarium compounds due to the similar ionic radii of these elements. Most of these compounds can only be prepared at low temperatures; bulky ligands tend to favor stability. For example, strontium dicyclopentadienyl,, must be made by directly reacting strontium metal with mercurocene or cyclopentadiene itself; replacing the ligand with the bulkier ligand on the other hand increases the compound's solubility, volatility, and kinetic stability.
Because of its extreme reactivity with oxygen and water, strontium occurs naturally only in compounds with other elements, such as in the minerals strontianite and celestine. It is kept under a liquid hydrocarbon such as mineral oil or kerosene to prevent oxidation; freshly exposed strontium metal rapidly turns a yellowish color with the formation of the oxide. Finely powdered strontium metal is pyrophoric, meaning that it will ignite spontaneously in air at room temperature. Volatile strontium salts impart a bright red color to flames, and these salts are used in pyrotechnics and in the production of flares. Like calcium and barium, as well as the alkali metals and the divalent lanthanides europium and ytterbium, strontium metal dissolves directly in liquid ammonia to give a dark blue solution of solvated electrons.

Isotopes

Natural strontium is a mixture of four stable isotopes: ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr. Of these isotopes, ⁸⁸Sr is the most abundant, making up about 82.6% of all natural strontium, though the abundance varies somewhat due to the production of radiogenic ⁸⁷Sr as the daughter of long-lived beta-decaying ⁸⁷Rb. This is the basis of rubidium–strontium dating.
Of the unstable isotopes, the primary decay mode of the isotopes lighter than ⁸⁶Sr is electron capture or positron emission to isotopes of rubidium, and that of the isotopes heavier than ⁸⁸Sr is electron emission to isotopes of yttrium. Of special note are ⁸⁹Sr and ⁹⁰Sr. The former has a half-life of 50.56 days and is used to treat bone cancer due to strontium's chemical similarity and hence ability to replace calcium. While ⁹⁰Sr has been used similarly, it is more an isotope of concern in fallout from nuclear weapons and nuclear accidents due to its longer life. Its presence in bones, where all strontium accumulates, can cause bone cancer, cancer of nearby tissues, and leukemia. The 1986 Chernobyl nuclear accident contaminated about 30,000 km² with greater than 10 kBq/m² of ⁹⁰Sr, or an estimated 5% of the reactor's total content of ⁹⁰Sr.

History

Strontium is named after the Scottish village of Strontian, where it was discovered in the ores of the lead mines.
In 1790, Adair Crawford, a physician engaged in the preparation of barium, and his colleague William Cruickshank, recognised that the Strontian ores exhibited properties that differed from those in other "heavy spars" sources. This allowed Crawford to conclude on page 355 "... it is probable indeed, that the scotch mineral is a new species of earth which has not hitherto been sufficiently examined." The physician and mineral collector Friedrich Gabriel Sulzer analysed together with Johann Friedrich Blumenbach the mineral from Strontian and named it strontianite. He also came to the conclusion that it was distinct from the witherite and contained a new earth. In 1793 Thomas Charles Hope, a professor of chemistry at the University of Glasgow studied the mineral and proposed the name strontites. He confirmed the earlier work of Crawford and recounted: "... Considering it a peculiar earth I thought it necessary to give it an name. I have called it Strontites, from the place it was found; a mode of derivation in my opinion, fully as proper as any quality it may possess, which is the present fashion." The element was eventually isolated by Sir Humphry Davy in 1808 by the electrolysis of a mixture containing strontium chloride and mercuric oxide, and announced by him in a lecture to the Royal Society on 30 June 1808. In keeping with the naming of the other alkaline earths, he changed the name to strontium.
The first large-scale application of strontium was in the production of sugar from sugar beet. Although a crystallisation process using strontium hydroxide was patented by Augustin-Pierre Dubrunfaut in 1849 the large scale introduction came with the improvement of the process in the early 1870s. The German sugar industry used the process well into the 20th century. Before World War I the beet sugar industry used 100,000 to 150,000 tons of strontium hydroxide for this process per year. The strontium hydroxide was recycled in the process, but the demand to substitute losses during production was high enough to create a significant demand initiating mining of strontianite in the Münsterland. The mining of strontianite in Germany ended when mining of the celestine deposits in Gloucestershire started. These mines supplied most of the world strontium supply from 1884 to 1941. Although the celestine deposits in the Granada basin were known for some time the large scale mining did not start before the 1950s.
During atmospheric nuclear weapons testing, it was observed that strontium-90 is one of the nuclear fission products with a relatively high yield. The similarity to calcium and the chance that the strontium-90 might become enriched in bones made research on the metabolism of strontium an important topic.

Occurrence

Strontium commonly occurs in nature, being the 15th most abundant element on Earth, estimated to average approximately 360 parts per million in the Earth's crust and is found chiefly as the sulfate mineral celestine and the carbonate strontianite. Of the two, celestine occurs much more frequently in deposits of sufficient size for mining. Because strontium is used most often in the carbonate form, strontianite would be the more useful of the two common minerals, but few deposits have been discovered that are suitable for development. Because of the way it reacts with air and water, strontium only exists in nature when combined to form minerals. Naturally occurring strontium is stable, but its synthetic isotope Sr-90 is only produced by nuclear fallout.
In groundwater strontium behaves chemically much like calcium. At intermediate to acidic pH Sr²⁺ is the dominant strontium species. In the presence of calcium ions, strontium commonly forms coprecipitates with calcium minerals such as calcite and anhydrite at an increased pH. At intermediate to acidic pH, dissolved strontium is bound to soil particles by cation exchange.
The mean strontium content of ocean water is 8 mg/L. At a concentration between 82 and 90 μmol/L of strontium, the concentration is considerably lower than the calcium concentration, which is normally between 9.6 and 11.6 mmol/L. It is nevertheless much higher than that of barium, 13 μg/L.