barium

barium (Ba), chemical element, one of the alkaline-earth metals of Group 2 (IIa) of the periodic table. The element is used in metallurgy, and its compounds are used in pyrotechnics, petroleum production, and radiology.

Occurrence, properties, and uses

Barium, which is slightly harder than lead, has a silvery white luster when freshly cut. It readily oxidizes when exposed to air and must be protected from oxygen during storage. In nature it is always found combined with other elements. The Swedish chemist Carl Wilhelm Scheele discovered (1774) a new base (baryta, or barium oxide, BaO) as a minor constituent in pyrolusite, and from that base he prepared some crystals of barium sulfate, which he sent to Johan Gottlieb Gahn, the discoverer of manganese. A month later Gahn found that the mineral barite is also composed of barium sulfate, BaSO₄. A particular crystalline form of barite found near Bologna, Italy, in the early 17th century, after being heated strongly with charcoal, glowed for a time after exposure to bright light. The phosphorescence of “Bologna stones” was so unusual that it attracted the attention of many scientists of the day, including Galileo. Only after the electric battery became available could Sir Humphry Davy finally isolate (1808) the element itself by electrolysis.

Barium minerals are dense (e.g., BaSO₄, 4.5 grams per cubic centimetre; BaO, 5.7 grams per cubic centimeter), a property that was the source of many of their names and of the name of the element itself (from the Greek barys, “heavy”). Ironically, metallic barium is comparatively light, only 30 percent denser than aluminum. Its cosmic abundance is estimated as 3.7 atoms (on a scale where the abundance of silicon = 10⁶ atoms). Barium constitutes about 0.03 percent of Earth’s crust, chiefly as the minerals barite (also called barytes or heavy spar) and witherite. Between six and eight million tons of barite are mined every year, more than half of it in China. Lesser amounts are mined in India, the United States, and Morocco. Commercial production of barium depends upon the electrolysis of fused barium chloride, but the most effective method is the reduction of the oxide by heating with aluminum or silicon in a high vacuum. A mixture of barium monoxide and peroxide can also be used in the reduction. Only a few tons of barium are produced each year.

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The metal is used as a getter in electron tubes to perfect the vacuum by combining with final traces of gases, as a deoxidizer in copper refining, and as a constituent in certain alloys. The alloy with nickel readily emits electrons when heated and is used for this reason in electron tubes and in spark plug electrodes. The detection of barium (atomic number 56) after uranium (atomic number 92) had been bombarded by neutrons was the clue that led to the recognition of nuclear fission in 1939.

Naturally occurring barium is a mixture of six stable isotopes: barium-138 (71.7 percent), barium-137 (11.2 percent), barium-136 (7.8 percent), barium-135 (6.6 percent), barium-134 (2.4 percent), and barium-132 (0.10 percent). Barium-130 (0.11 percent) is also naturally occurring but undergoes decay by double electron capture with an extremely long half-life (more than 4 × 10²¹ years). More than 30 radioactive isotopes of barium are known, with mass numbers ranging from 114 to 153. The isotope with the longest half-life (barium-133, 10.5 years) is used as a gamma-ray reference source.

Compounds

In its compounds, barium has an oxidation state of +2. The Ba²⁺ ion may be precipitated from solution by the addition of carbonate (CO₃²⁻), sulfate (SO₄²⁻), chromate (CrO₄²⁻), or phosphate (PO₄³⁻) anions. All soluble barium compounds are toxic to mammals, probably by interfering with the functioning of potassium ion channels.

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Barium sulfate (BaSO₄) is a white, heavy insoluble powder that occurs in nature as the mineral barite. Almost 80 percent of world consumption of barium sulfate is in drilling muds for oil. It is also used as a pigment in paints, where it is known as blanc fixe (i.e., “permanent white”) or as lithopone when mixed with zinc sulfide. The sulfate is widely used as a filler in paper and rubber and finds an important application as an opaque medium in the X-ray examination of the gastrointestinal tract.

Most barium compounds are produced from the sulfate via reduction to the sulfide, which is then used to prepare other barium derivatives. About 75 percent of all barium carbonate (BaCO₃) goes into the manufacture of specialty glass, either to increase its refractive index or to provide radiation shielding in cathode-ray and television tubes. The carbonate also is used to make other barium chemicals, as a flux in ceramics, in the manufacture of ceramic permanent magnets for loudspeakers, and in the removal of sulfate from salt brines before they are fed into electrolytic cells (for the production of chlorine and alkali). On heating, the carbonate forms barium oxide, BaO, which is employed in the preparation of cuprate-based high-temperature superconductors such as YBa₂Cu₃O_7−x. Another complex oxide, barium titanate (BaTiO₃), is used in capacitors, as a piezoelectric material, and in nonlinear optical applications.

Barium chloride (BaCl₂·2H₂O), consisting of colorless crystals that are soluble in water, is used in heat-treating baths and in laboratories as a chemical reagent to precipitate soluble sulfates. Although brittle, crystalline barium fluoride (BaF₂) is transparent to a broad region of the electromagnetic spectrum and is used to make optical lenses and windows for infrared spectroscopy. The oxygen compound barium peroxide (BaO₂) was used in the 19th century for oxygen production (the Brin process) and as a source of hydrogen peroxide. Volatile barium compounds impart a yellowish green color to a flame, the emitted light being of mostly two characteristic wavelengths. Barium nitrate, formed with the nitrogen-oxygen group NO₃⁻, and chlorate, formed with the chlorine-oxygen group ClO₃⁻, are used for this effect in green signal flares and fireworks.