strontium (Sr), chemical element, one of the alkaline-earth metals of Group 2 (IIa) of the periodic table. It is used as an ingredient in red signal flares and phosphors and is the principal health hazard in radioactive fallout.

Element Properties
atomic number38
atomic weight87.62
melting point769 °C (1,416 °F)
boiling point1,384 °C (2,523 °F)
specific gravity2.63
oxidation state+2
electron configuration[Kr]5s2

Occurrence, properties, and uses

Strontium is a soft metal like lead and, when freshly cut, has a silvery lustre. It rapidly reacts in air to take on a yellowish colour; therefore, it must be protected from oxygen for storage. It does not occur free in nature. Although it is widely distributed with calcium, there are only two principal ores of strontium alone, celestine (SrSO4) and strontianite (SrCO3).

A mineral from a lead mine near the village of Strontian, in Argyll, Scotland, was originally misidentified as a type of barium carbonate, but Adair Crawford and William Cruickshank in 1789 noted that it was likely a different substance. The chemist Thomas Charles Hope named the new mineral strontites, after the village, and the corresponding “earth” (strontium oxide, SrO) was accordingly referred to as strontia. The metal was isolated (1808) by Sir Humphry Davy, who electrolyzed a mixture of the moist hydroxide or chloride with mercuric oxide, using a mercury cathode, and then evaporated the mercury from the resultant amalgam. He used the stem of the word strontia to form the name of the element.

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Its cosmic abundance is estimated as 18.9 atoms (on a scale where the abundance of silicon = 106 atoms). It composes about 0.04 percent of Earth’s crust. The most important commercial source of strontium is celestine; more than two-thirds of the world’s supply comes from China, with Spain and Mexico supplying much of the remainder. Strontium may be obtained in the form of sticks by the contact cathode method of electrolysis, in which a cooled iron rod, acting as a cathode, just touches the surface of a fused mixture of potassium and strontium chlorides and is raised as the strontium solidifies on it. Metallic strontium may be also obtained by reduction of the oxide with aluminum. The metal is malleable and ductile and a good conductor of electricity, but there are relatively few uses for elemental strontium. One of them is as an alloying agent for aluminum or magnesium in cast engine blocks and wheels; the strontium improves the machinability and creep resistance of the metal.

Naturally occurring strontium is a mixture of four stable isotopes: strontium-88 (82.6 percent), strontium-86 (9.9 percent), strontium-87 (7.0 percent), and strontium-84 (0.56 percent). Depending on the location, it is possible for the ratios of strontium-87 to strontium-86 to differ by more than a factor of 5. This variation is used in dating geological samples and in identifying the provenance of skeletons and clay artifacts. About 16 synthetic radioactive isotopes have been produced by nuclear reactions, of which the longest-lived is strontium-90 (28.9-year half-life). This isotope, formed by nuclear explosions, is considered the most dangerous constituent of fallout. Because of its chemical resemblance to calcium, it is assimilated in bones and teeth, where it continues ejecting electrons that cause radiation injury by damaging bone marrow, impairing the process of forming new blood cells, and possibly inducing cancer. Under controlled conditions, however, it has been used for treatment of some superficial cancers and bone cancer. It is also used as a source in thickness gauges and has been used in radioisotope thermoelectric generators, where the heat of its radioactive decay is converted to electricity for long-lived, lightweight power sources in navigation buoys, remote weather stations, and space vehicles. Strontium-89 is employed in the treatment of bone cancer, as it targets bone tissues, delivers its beta radiation, and then decays in a few months’ time (half-life 51 days).

Strontium is not an essential element for higher life-forms, and its salts are generally nontoxic. The same “bone-seeking” property that makes strontium-90 dangerous is beneficially employed in strontium supplements to increase bone density and growth.

Compounds

In general, the chemistry of strontium is quite similar to that of calcium. In its compounds strontium has an exclusive oxidation state of +2, as the Sr2+ ion. The metal is an active reducing agent and readily reacts with halogens, oxygen, and sulfur to yield halides, oxide, and sulfide.

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Strontium compounds have rather limited commercial value because the corresponding calcium and barium compounds generally serve the same purpose yet are cheaper. A few, however, have found application in industry and elsewhere. There is currently no substitute for the brilliant crimson colour produced by strontium salts such as strontium nitrate, Sr(NO3)2, and strontium chlorate, Sr(ClO3)2, in fireworks, flares, and tracer ammunition. About 5–10 percent of all strontium production is consumed in pyrotechnics. Strontium hydroxide, Sr(OH)2, is sometimes used to extract sugar from molasses because it forms a soluble saccharide from which the sugar can be easily regenerated by the action of carbon dioxide. Strontium monosulfide, SrS, is employed as a depilatory and as an ingredient in phosphors for electroluminescent devices and luminous paints.

Strontium ferrites comprise a family of compounds of general formula SrFexOy, formed from the high-temperature (1,000–1,300 °C, or 1,800–2,400 °F) reaction between SrCO3 and Fe2O3. Permanent ceramic magnets are made from strontium ferrites and find use in applications as diverse as loudspeakers, motors for automobile windshield wipers, and children’s toys.

Timothy P. Hanusa

radioactive isotope

chemistry
Also known as: radioactive nuclide, radioisotope, radionuclide
Also called:
radioisotope, radionuclide, or radioactive nuclide
Top Questions

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radioactive isotope, any of several species of the same chemical element with different masses whose nuclei are unstable and dissipate excess energy by spontaneously emitting radiation in the form of alpha, beta, and gamma rays.

A brief treatment of radioactive isotopes follows. For full treatment, see isotope: Radioactive isotopes.

Every chemical element has one or more radioactive isotopes. For example, hydrogen, the lightest element, has three isotopes with mass numbers 1, 2, and 3. Only hydrogen-3 (tritium), however, is a radioactive isotope, the other two being stable. More than 1,000 radioactive isotopes of the various elements are known. Approximately 50 of these are found in nature; the rest are produced artificially as the direct products of nuclear reactions or indirectly as the radioactive descendants of these products.

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isotope: Radioactive isotopes

Radioactive isotopes have many useful applications. In medicine, for example, cobalt-60 is extensively employed as a radiation source to arrest the development of cancer. Other radioactive isotopes are used as tracers for diagnostic purposes as well as in research on metabolic processes. When a radioactive isotope is added in small amounts to comparatively large quantities of the stable element, it behaves exactly the same as the ordinary isotope chemically; it can, however, be traced with a Geiger counter or other detection device. Iodine-131 has proved effective in treating hyperthyroidism. Another medically important radioactive isotope is carbon-14, which is used in a breath test to detect the ulcer-causing bacteria Heliobacter pylori.

In industry, radioactive isotopes of various kinds are used for measuring the thickness of metal or plastic sheets; their precise thickness is indicated by the strength of the radiations that penetrate the material being inspected. They also may be employed in place of large X-ray machines to examine manufactured metal parts for structural defects. Other significant applications include the use of radioactive isotopes as compact sources of electrical power—e.g., plutonium-238 in spacecraft. In such cases, the heat produced in the decay of the radioactive isotope is converted into electricity by means of thermoelectric junction circuits or related devices.

The table lists some naturally occurring radioactive isotopes.

Some significant naturally occurring radioactive isotopes
isotope half-life (years, unless noted)
Source: National Nuclear Data Center, Brookhaven National Laboratory, NuDat 2.6 (2016).
3H 12.32
14C 5,700
50V >2.1 × 1017
87Rb 4.81 × 1010
90Sr 28.9
115In 4.41 × 1014
123Te >9.2 × 1016
130Te >3.0 × 1024
131I 8.0252 days
137Cs 30.08
138La 1.02 × 1011
144Nd 2.29 × 1015
147Sm 1.06 × 1011
148Sm 7 × 1015
176Lu 3.76 × 1010
187Re 4.33 × 1010
186Os 2 × 1015
222Rn 3.8235 days
226Ra 1,600
230Th 75,400
232Th 1.4 × 1010
232U 68.9
234U 245,500
235U 7.04 × 108
236U 2.342 × 107
237U 6.75 days
238U 4.468 × 109
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