What is calcium? |

Calcium is a chemical element. In its elemental form, it is a soft, gray, reactive metal. Due to its reactivity, however, calcium easily reacts with other atoms to form compounds and does not appear naturally in its elemental form. Calcium is the fifth most abundant element in Earth’s crust and the fifth most abundant element dissolved in seawater. Its ubiquity has sealed calcium’s significance in human history. Calcium’s compounds were known to humans long before the element itself was isolated. The numerous inorganic compounds of calcium used across a spectrum of industries are often called "limes," and the word calcium is derived from the Latin word for lime, calx. Calcium is also biologically significant: it plays roles in the biochemistry and physiology of cells, cell signaling, and musculoskeletal tissue.

Background

Calcium’s compounds were known and useful to humans thousands of years before the discovery of elemental calcium. For example, quicklime, the common name for calcium oxide (CaO), has been found in the flooring of an archeological site near the Mediterranean Sea dating to 7000–6000 BCE. Calcium oxide has a diversity of uses. When heated to high temperatures, it will glow intensely, and this distinct illumination was used in theatrical productions prior to the advent of electrical lighting and is the origin of the term "limelight." Calcium oxide is also used in the production of cement, paper, weaponry, and biodiesel.

Calcium oxide is a product of the thermal decomposition of another critical calcium compound, calcium carbonate (CaCO3), and much like calcium oxide, its uses are diverse. For example, it is used in art as a pigment and as a paint primer as well as a dietary supplement, in pharmaceuticals, food preservatives, ore purification, ceramics, adhesives, and in absorbent materials such as disposable diapers. Biologically, calcium carbonate is the main component of marine shells, eggshells, pearls, and snails. Calcium carbonate can be found in plants, and the dietary benefits of dark green vegetables such as kale and broccoli comes primarily from calcium in carbonate form. Geologically, calcium carbonate is found in marble, travertine, chalk, and limestone. Many of these materials have been historically important building materials. As such, calcium carbonate’s reactivity with acids is visible in historic limestone structures (such as the Great Pyramids of Giza) that have been noted to be damaged by exposure to acid rain. Limestone comprises 10 percent of the total sedimentary rock found on Earth, and most cave systems move through limestone bedrock. Other calcium compounds with familiar uses are calcium hypochlorite, which is used in household deodorants and in bleaching agents; calcium stearate, which is used to create cosmetics, some plastics, and wax crayons; calcium chloride, which is used for ice removal; and calcium tungstate, which is used in fluorescent lights. In addition to these uses of calcium compounds, the compound hydroxyapatite, Ca10(PO4)6(OH)2 can be found in bone, dental enamel, and dentin tissue.

Despite the ubiquity of calcium’s compounds in our bodies, homes, landscapes, and art, the elemental form of calcium was not discovered until 1808 by the British chemist, Sir Humphry Davy (1778–1829). Davy used electrolysis through a lime mixture to isolate calcium, similar to the technique he used to isolate chemically related elements such as potassium, sodium, boron, and magnesium.

Overview

Calcium is the twentieth element on the periodic table. It has an atomic number of 20, meaning it has twenty protons. Calcium atoms can have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes. Ninety-seven percent of naturally occurring calcium has an equal number of protons and neutrons, but calcium has stable isotopes with twenty-two, twenty-three, twenty-four, and twenty-six neutrons.

A neutral calcium atom would have twenty electrons, but calcium is not naturally found in a neutral state. Calcium so readily forms chemical compounds that elemental calcium was not isolated until the technology existed for using direct current to drive an otherwise nonspontaneous chemical reaction. Calcium was familiar to humans through its myriad compounds.

Calcium belongs to the group or family of elements known as the alkaline earth metals (also referred to as the group 2 elements), and like the other alkaline earth metals, is an s-block element because its outermost electrons are in the s orbital. The alkaline earth metals are the elements found in the second column from the left of the modern periodic table, and calcium’s chemical and physical properties share some similarities with its nearest neighbors on the periodic table.

In addition to their shared metallic properties, calcium has other chemical and physical properties that are common among alkaline earth metals such as relatively low melting points, boiling points, and densities. Like other alkaline earth metals, calcium is a reactive metal, with reactivity increasing as the atoms get heavier. Also, like other alkaline earth metals (with the exception of beryllium), calcium reacts strongly with water to produce hydrogen gas and calcium (or respective) hydroxide.

Understanding the atomic structure of calcium provides insight into its reactive nature and its importance in the biochemistry of life and the chemical compounds encountered in nature and used in industry

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