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See also Calcium, New York, United States.

Calcium is the chemical element in the periodic table that has the symbol Ca and atomic number 20. Calcium is a soft grey alkaline earth metal that is used as a reducing agent in the extraction of thorium, zirconium and uranium. Calcium is also the fifth most abundant element in the Earth's crust. It is essential for living organisms, particularly in cell physiology, and is the most common metal in many animals.

Notable characteristics

Calcium is a rather soft, grey, metallic element that is purified by electrolysis from calcium fluoride. It burns with a yellow-red flame and forms a white nitride coating when exposed to air. It reacts with water, displacing hydrogen and forming calcium hydroxide.

Calcium is essential in muscle contraction, building strong bones and teeth, blood clotting, nerve impulse transmission, regulating heartbeat, and fluid balance within cells. In the U.S., between about 50% and 75% of adults do not get sufficient calcium in their diet.^[1] Adults need between 1,000 and 1,300 mg of calcium in their daily diet.^[1]

The isotope calcium-40 has a nucleus of 20 protons and 20 neutrons. Calcium has 20 electrons distributed as follows: 2 in the K shell (principal quantum number 1), 8 in the L shell (principal quantum number 2), 8 in the M shell (principal quantum number 3), and 2 in the N shell (principal quantum number 4). The outer shell is the valence shell, with 2 electrons in the lone 4s orbital, the 3p orbitals being empty. See also electron configuration.

Occurrence

Calcium is not naturally found in its elemental state. Calcium is found mostly as limestone, gypsum and fluorite. Stalagmites and stalactites contain calcium carbonate.

See also Calcium minerals.

Applications

Calcium is an important component of a healthy diet. A deficit can affect bone and teeth formation, while overretention can cause kidney stones. Vitamin D is needed to absorb calcium. Dairy products, such as milk and cheese, are a well-known source of calcium. However, some individuals are allergic to dairy products and even more people, particularly those of non-European descent, are lactose-intolerant, leaving them unable to consume dairy products. Fortunately, many other good sources of calcium exist. These include: seaweeds such as kelp, wakame and hijiki; nuts and seeds (like almonds and sesame); beans; seafood such as oysters and shrimp; soft-boned fish; amaranth; whole wheat; collard greens; okra; rutabaga; broccoli; and fortified products such as orange juice and bread.

For more information about calcium in living nature, see calcium in biology and calcium metabolism.

Other uses include:

• as a reducing agent in the extraction of other metals, such as uranium, zirconium, and thorium.
• as a deoxidizer, desulfurizer, or decarburizer for various ferrous and nonferrous alloys.
• as an alloying agent used in the production of aluminium, beryllium, copper, lead, and magnesium alloys.
• in the making of cements and mortars to be used in construction.

History

Calcium (Latin calcis, meaning "lime") was known as early as the first century when the Ancient Romans prepared lime as calcium oxide. It was not actually isolated until 1808 in England when Sir Humphrey Davy electrolyzed a mixture of lime and mercuric oxide. Davy was trying to isolate calcium and when he heard that Berzelius and Pontin prepared calcium amalgam by electrolyzing lime in mercury, he tried it himself. He worked with electrolysis throughout his life and also discovered/isolated magnesium, strontium and barium.

Compounds

Calcium, combined with phosphate to form hydroxylapatite, is the mineral portion of human and animal bones and teeth. The mineral portion of some corals can also be transformed into hydroxylapatite.

Quicklime (CaO) is used in many chemical refinery processes and is made by heating and carefully adding water to limestone. When CaO is mixed with sand, it hardens into a mortar and is turned into plaster by carbon dioxide uptake. Mixed with other compounds, CaO forms an important part of Portland cement.

When water percolates through limestone or other soluble carbonate rocks, it partially dissolves part of the rock and causes cave formation and characteristic stalactites and stalagmites and also forms hard water. Other important calcium compounds are nitrate, sulfide, chloride, carbide, cyanamide, and hypochlorite.

Isotopes

Calcium has four stable isotopes (⁴⁰Ca and ⁴²Ca through ⁴⁴Ca), plus two more isotopes (⁴⁶Ca and ⁴⁸Ca) that have such long half-lives that for all practical purposes they can be considered stable. It also has a cosmogenic isotope, radioactive ⁴¹Ca, which has a half-life of 103,000 years. Unlike cosmogenic isotopes that are produced in the atmosphere, ⁴¹Ca is produced by neutron activation of ⁴⁰Ca. Most of its production is in the upper metre or so of the soil column where the cosmogenic neutron flux is still sufficiently strong. ⁴¹Ca has received much attention in stellar studies because it decays to ⁴¹K, a critical indicator of solar-system anomalies.

Naturally occurring calcium is 97% in the form of ⁴⁰Ca. ⁴⁰Ca is one of the daughter products of ⁴⁰K decay, along with ⁴⁰Ar. While K-Ar dating has been used extensively in the geological sciences, the prevalence of ⁴⁰Ca in nature has impeded its use in dating. Techniques using mass spectrometry and a double spike isotope dilution have been used for K-Ca age dating.

Dietary calcium supplements

There are conflicting recommendations about when to take calcium supplements. However, most experts agree that no more than 500 mg should be taken at a time – any excess will go to waste. It is recommended to spread doses throughout the day, with the last dose near bedtime. Recommended daily calcium intake varies from 1000 to 1500 milligrams, depending upon the stage of life.

• Calcium carbonate is the most common and least expensive calcium supplement. It can be difficult to digest and causes gas in some people. Taking magnesium with it can help to prevent constipation. Calcium carbonate is 40% elemental calcium. 1000 mg will provides 400 mg of calcium. Take this supplement with food to aid in absorption.

• Calcium citrate is more easily absorbed, easier to digest and less likely to cause constipation and gas than calcium carbonate. It also has a lower risk of contributing to the formation of kidney stones. Calcium citrate is 21% elemental calcium. 1000 mg will provide 210 mg of calcium. It is more expensive than calcium carbonate and more of it must be taken to get the same amount of calcium.

• Calcium phosphate costs more than calcium carbonate, but less than calcium citrate. It is easily absorbed and is less likely to cause constipation and gas than either.

• Calcium lactate and calcium aspartate are both more easily digested, but more expensive than calcium carbonate.

See also

• Calcium compounds
• Disorders of calcium metabolism

Notes

1. ↑ ^a b Dietary Supplement Fact Sheet: Calcium. URL accessed on 2006-03-23.

References

• Rebecca J. Donatelle. Health, The Basics. 6th ed. San Francisco: Pearson Education, Inc. 2005.

External links

• WebElements.com – Calcium
• USDA National Nutrient Database, Calcium content of selected foods

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