Strontium

Strontium

Strontium is the fourth element in Group 2 of the periodic table, a group of elements known as the alkaline earth metals. It has an atomic number of 38, an atomic mass of 87.62, and a chemical symbol of Sr.

Properties

Strontium is a silvery white metal that combines with oxygen of the air to form a thin film of strontium oxide (SrO). The film gives the metal a yellowish hue and protects it from further oxidation. Strontium has a melting point of 1,395°F (757°C), a boiling point of 2,491°F (1,366°C), and a density of 2.6 grams per cubic centimeter.

Strontium is a very active element that is normally kept under kerosene, mineral oil, or some other organic liquid to keep it from reacting with oxygen in the air. In a finely divided form, the metal may catch fire spontaneously and burn vigorously. Strontium is active enough to combine even with hydrogen and nitrogen when heated, behaviors unusual for most metals. Strontium also reacts with cold water to produce hydrogen gas as one product: Sr 2H₂O Sr(OH)₂ + H₂

Occurrence and Extraction

Strontium is relatively abundant in the Earth's crust, ranking about 15th among the elements found there. Its most common minerals are celestine (primarily strontium sulfate; SrSO₄) and strontianite (primarily strontium carbonate; SrCO₃). The most important sources of strontium worldwide are Mexico, Spain, Turkey, and Iran. A small amount of the element is also obtained from mines in California and Texas.

Strontium metal is still extracted by a method originally developed by the English chemist and physicist Humphry Davy, the electrolysis of molten stronium chloride (SrCl₂): SrCl₂ --electric current Sr + Cl₂

Discovery and Naming

Credit for the discovery of strontium usually goes to the Irish physician Adair Crawford (1748-1795). Crawford was interested in the study of minerals and other chemicals and spent some time analyzing the mineral known as baryte, a primary source of the element barium. Crawford was surprised to find that a small portion of baryte was always left over after all other known elements had been identified. He decided that the remaining material must be a new element. He suggested naming the element strontia after a region in Scotland from which his sample of baryte had come. Nearly two decades later, Davy showed that strontia was a compound of a metal and oxygen, a metal to which the name strontium was then given.

Uses

Strontium and its compounds have relatively few commercial uses. Some compounds are added to glass and ceramics to give them a beautiful red color. Compounds of strontium are also used to provide the red colors seen in a fireworks display.

Health Issues

One radioactive isotope of strontium, strontium-90, is of some interest in the field of health. The isotope is produced in relatively large amounts during the explosion of nuclear weapons. When such an explosion takes place in the atmosphere, the isotope eventually falls back to earth and coats grass, leaves, and other plant material. Cattle, sheep and other domestic animals then eat that plant material and ingest the strontium-90. It becomes part of the milk that may be passed on to humans as a food.

This is significant because the human body treats strontium in much the same way it treats calcium, the element above it in the periodic table. The body uses the strontium to build bones and teeth. But strontium-90 is radioactive, giving off radiation that can cause harm to the body, most seriously, bone cancer.

The health risks posed by strontium-90 became an important issues during the 1950s and 1960s when the United States, the then-Soviet Union, China, and a few other nations tested nuclear weapons in the atmosphere. Concerns about the potential health risks of strontium-90 were one of the major factors that convinced those nations to discontinued atmospheric testing of nuclear weapons. Such tests are now almost entirely conducted underground where strontium-90 can not be released to the atmosphere.