Calcium (Ca) is a non-toxic alkaline earth metal. Alkaline earth metals are in the second group on the periodic table, with calcium being in period 4, atomic number 20. All of the elements in the group are very reactive and none are found free in nature. In air, calcium rapidly forms calcium nitride (Ca3N2), and it also reacts with water. Elemental calcium is a relatively soft, silvery-gray metal, found in minerals like limestone and gypsum, usually as the compound calcium carbonate (CaCO3) or calcium sulfate (CaSO4). Metal calcium burns with a yellowish-red flame, and limestone produces an intense light when burned, as well. Limestone was used to light theater stages in the 1800s before electricity was readily available. This is the origin of the phrase “in the limelight.” Calcium is the fifth most abundant element of the Earth’s crust, making up approximately 4.2 percent of the crust.
Calcium was first isolated in 1808 by Sir Humphrey Davy through the electrolysis of calcium oxide (CaO) and mercuric oxide (HgO). It was named from the Latin word “calx,” which is what the Romans called limestone. Calcium in various compounds was used for thousands of years. The Romans used calx as far back as the first century for setting broken bones with casts and as cement in building many of their famous structures. The pyramids of Egypt were also constructed of limestone blocks. Calcium was not available on a large scale until the beginning of the twentieth century. Today, metallic calcium is obtained commercially using aluminum in hot, low-pressure containers to displace the calcium atoms. There is very little demand for this elemental calcium, though, because of its reactivity.
Elemental calcium is used in some refining processes for thorium, uranium and zirconium. It can also be used to remove oxygen, sulfur and carbon from alloys, and is used as a getter to remove any gases from vacuum tubes. Calcium itself can be alloyed with aluminum, beryllium, copper, lead and magnesium. Calcium carbonate is a common compound appearing in more everyday uses, including chalk, some cleaning powders, toothpastes and antacids. Calcium carbonate forms quicklime, which is used to treat drinking water, as well as slaked lime, used throughout the chemical industry. Gypsum, containing calcium sulfate, is used for drywall as well as modern plaster of paris. Calcium nitrate (Ca(NO3)2) is a naturally occurring fertilizer. Calcium phosphate (Ca3(PO4)2) is found in bones and teeth, making calcium the most abundant metal in the human body. The shells of snails and many shellfish are also composed of calcium.
