Barium (Ba)

• Element Symbol: Ba
• Atomic Number: 56
• Atomic Mass: 137.327
• Group # in Periodic Table: 2
• Group Name: Alkaline earth metals
• Period in Periodic Table: 6
• Block of Periodic Table: s-block
• Discovered by: Sir Humphry Davy (1808)

Barium is a silvery-white alkaline earth metal. The alkaline earth metals, which are the elements in group 2 of the periodic table, also include beryllium, magnesium, calcium, strontium, and radium. These metals are highly reactive, and they are therefore not found in their pure form in nature. However, compounds containing alkaline earth metals can be very common on Earth. Minerals that contain barium include barite (barium sulfate), witherite (barium carbonate) and pyrolusite (barium oxide).

The discovery of barium is credited to British chemist Sir Humphry Davy. Davy was able to isolate the element from barium hydroxide in 1808 using a new technique called electrolysis. Although the element was not observed in its pure form until this point, minerals containing barium compounds had been familiar for hundreds of years prior to 1808. In the early 1600s, Italian shoemaker Vincenzo Casciarolo discovered pebbles of barite in Bologna, Italy, that produced blue phosphorescence when they were heated and then exposed to sunlight. The aptly named Bologna stones would glow into the night. In the 1770s, Swedish chemist Johan Gottlieb Gahn discovered that barite, the mineral in the stones, was made of barium sulfate. Another Swedish chemist, Carl Wilhelm Scheele, discovered a dense and basic substance in the mineral pyrolusite in 1774. This substance was the compound barium oxide. It was named "baryta," from the Greek word barys, meaning "heavy."

Physical Properties

Barium is silvery white and solid in its standard state at 298 kelvins (K). It is a relatively soft alkaline earth metal, with a Mohs hardness of 1.25. Although many of its compounds are dense, barium itself has a relatively low density of 3.51 grams per cubic centimeter (g/cm³) at standard state. Its melting point is 727 degrees Celsius (°C), and its boiling point is 1870 °C. The specific heat of barium is 205 joules per kilogram-kelvin (J/kg·K). Barium is a good conductor of both heat and electricity. Its thermal conductivity is 18 watts per meter-kelvin (W/m·K), its electrical conductivity is 2.9 × 10⁶ siemens per meter (S/m), and its resistivity is 3.5 × 10⁻⁷ meter-ohms (m·Ω). Barium is paramagnetic, meaning it has a weak attraction to magnetic fields.

Chemical Properties

Barium has a body-centered cubic crystal structure. Some compounds of barium, such as barium sulfate, produce phosphorescence. Barium shows strong reactivity with many nonmetals, including oxygen, carbon, nitrogen, and phosphorus. Its reactions may release intense heat energy. Barium reacts with air to produce a thin film of barium oxide on its surface. It reacts with water to produce heat energy, barium hydroxide, and hydrogen gas. Barium and its compounds are toxic to humans and other mammals. However, barium sulfate can be ingested, because it does not dissolve in water or stomach acid.

The electron affinity of barium is 13.95 kilojoules per mole (kJ/mol). Barium has two valence electrons. Its ionization energies are 502.9 kJ/mol, 965.2 kJ/mol, and 3600 kJ/mol. Like all other alkaline earth metals, it has an oxidation state of +2.

Barium has six naturally occurring stable isotopes. The most common of them is barium-138, which represents around 71.7 percent of the barium on Earth. Around 11.2 percent of natural barium is barium-137. Around 7.8 percent of natural barium is barium-136. The other three stable isotopes, which make up around 9.2 percent of natural barium, are barium-135, barium-134, and barium-132.

Barium has one naturally occurring radioactive isotope: barium-130. This isotope decays extremely slowly, with a half-life of 4 × 10²¹ years. There are more than thirty synthetically produced radioactive isotopes of barium. The mass numbers of these radioisotopes range from 114 to 153, and their half-lives range from just a few minutes to 10.5 years (barium-133). This element has an electron configuration of [Xe]6s².

Applications

Barium compounds are abundant in Earth’s crust within the minerals barite and witherite (and, to a lesser degree, pyrolusite). Major deposits of these minerals are located in China, India, Morocco, the United States, Turkey, Kazakhstan, and Iran. Barite is the main mineral mined for barium. Around five to seven tons of barite are mined each year.

Barium metal is most often produced from barite through the reduction of barium sulfate to barium sulfide. A series of chemical reactions yields barium oxide from barium sulfide. Barium oxide can then be reduced at high temperatures with aluminum to obtain the barium metal.

Barium metal can also be produced through a reaction between barium sulfate and hydrochloric acid. This reaction produces barium chloride, which can then be subjected to electrolysis to produce the pure metal.

Barium metal is useful as a so-called getter that removes oxygen. A small amount of pure barium is added to the inside of vacuum tubes, which were once in standard use in electronic equipment such as amplifiers. The barium reacts with any trace of oxygen or water in the tube to ensure a complete vacuum.

Barium is used in some metal alloys. A barium-nickel alloy is used in the electrodes found in vehicle spark plugs. When heated, the barium emits electrons from these electrodes, and this emission produces the spark that ignites the oxygen-fuel mixture in the vehicle. Ignition of fuel provides power to the vehicle.

Barium metal is also one of the components of YBCO superconductors. The YBCO stands for yttrium, barium, copper, and oxygen. These four elements can be combined at high temperatures (900 °C) to form a compound that acts as a superconductor at low temperatures (−180 °C). A superconductor transfers electricity without resistance. Superconducting materials are used as magnets in magnetic resonance imaging (MRI) and magnetic levitation (maglev) trains.

Barium compounds have various current and historic uses. In the petroleum industry, barium sulfate is a component of fluids that are used when gas or oil wells are drilled. The high density of this barium compound allows it to sink into the hole and force broken rocks up during drilling. Its density also helps prevent the escape of oil or gas from the hole. Barium sulfate is also used as a pigment in paints. In medicine, a barium sulfate preparation called a barium meal can be used to image the gastrointestinal system. Depending on which area of the system is to be imaged, the barium meal is either taken by mouth or inserted through an enema. The barium compound scatters x-rays, allowing the structure of the stomach or intestines to be viewed. This procedure is no longer widely used, as endoscopy or colonoscopy can obtain images from inside the gastrointestinal system.

Other barium compounds have been used to make lenses and specialty glass. Some are used to create the greens seen in fireworks displays.

Bibliography

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