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Calcium (IPA: ) is the chemical element in the periodic table that has the symbol Ca and atomic number 20. It has an atomic mass of 40.078. Calcium is a soft grey alkaline earth metal, and is 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.
Calcium is a rather soft (though harder than lead), gray, metallic element that can be extracted by electrolysis from fused calcium chloride.[1] It burns with a yellow-red flame and forms a white nitride coating when exposed to air. It reacts with water, displacing a hydrogen atom from the structure, then forming calcium hydroxide.
The most abundant isotope, <sup>40</sup>Ca, has a nucleus of 20 protons and 20 neutrons. Its electron configuration is 2 electrons 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 3d orbitals being empty.
Calcium is not naturally found in its elemental state. Calcium occurs most commonly in sedimentary rocks in the minerals calcite, dolomite and gypsum. It occurs in igneous and metamorphic rocks chiefly in the silicate minerals: plagioclase, amphiboles, pyroxenes and garnets.
See also .
Some uses are:
In the visible portion of the spectrum of many stars, including the Sun, show strong absorption lines of singly-ionized Calcium. Prominent among these are the H-line at 3968.5 Å and the K line at 3933.7 Å of singly-ionized Calcium, or Ca II. For the Sun and stars with low temperatures, the prominence of the H and K lines can be an indication of strong magnetic activity in the chromosphere. Measurement of periodic variations of these active regions can also be used to deduce the rotation periods of these stars.[2]
Calcium (Latin calx, meaning "limestone") was known as early as the first century when the Ancient Romans prepared lime as calcium oxide. It was not isolated until 1808 in England when Sir Humphry Davy electrolyzed a mixture of lime and mercuric oxide. Davy was trying to isolate calcium; 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.
Other compounds include Calcium carbonate (CaCO<sub>3</sub>), one of the common compounds of calcium. It is heated to form quicklime (CaO) which is then added to water (H<sub>2</sub>O). This forms another material known as slaked lime (Ca(OH)<sub>2</sub>) which is an inexpensive base material used throughout the chemical industry. Chalk, marble, and limestone are all forms of calcium carbonate.
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.
Calcium oxide (lime) is used in many chemical refinery processes and is made by heating and carefully adding water to limestone. When lime is mixed with sand, it hardens into a mortar and is turned into plaster by carbon dioxide uptake. Mixed with other compounds, lime 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.
Calcium has four stable isotopes (<sup>40</sup>Ca and <sup>42</sup>Ca through <sup>44</sup>Ca), plus two more isotopes (<sup>46</sup>Ca 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 <sup>41</sup>Ca, which has a half-life of 103,000 years. Unlike cosmogenic isotopes that are produced in the atmosphere, <sup>41</sup>Ca is produced by neutron activation of <sup>40</sup>Ca. Most of its production is in the upper metre or so of the soil column where the cosmogenic neutron flux is still sufficiently strong. <sup>41</sup>Ca has received much attention in stellar studies because it decays to <sup>41</sup>K, a critical indicator of solar-system anomalies.
97% of naturally occurring calcium is in the form of <sup>40</sup>Ca. <sup>40</sup>Ca is one of the daughter products of <sup>40</sup>K decay, along with <sup>40</sup>Ar. While K-Ar dating has been used extensively in the geological sciences, the prevalence of <sup>40</sup>Ca 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.
Calcium is an important component of a healthy diet. A deficit can affect bone and tooth 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 Indo-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); blackstrap molasses; beans; oranges; amaranth; collard greens; okra; rutabaga; broccoli; dandelion leaves; kale; sardines; and fortified products such as orange juice and soy milk. The calcium content of most foods can be found in the USDA National Nutrient Database.[3]
Calcium is essential for the normal growth and maintenance of bones and teeth, and calcium requirements must be met throughout life. Long-term calcium deficiency can lead to osteoporosis, in which the bone deteriorates and there is an increased risk of fractures. Calcium has also been found to assist in the production of lymphatic fluids.
Recommended Adequate Intake by the IOM for Calcium:[4]
For more information about calcium in living nature, see calcium in biology and calcium metabolism.
Calcium supplements are used to prevent and to treat calcium deficiencies. 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 because the percent of calcium absorbed decreases as the amount of calcium in the supplement increases.[4] It is recommended to spread doses throughout the day, with the last dose near bedtime. Recommended daily calcium intake varies from 1000 to 1500 mg, depending upon the stage of life.
In July 2006, a report citing research from Fred Hutchinson Cancer Research Center in Seattle, Washington claimed that women in their 50s gained 5 pounds less in a period of 10 years by taking more than 500 mg of calcium supplements than those who did not. However, the doctor in charge of the study, Dr. Alejandro J. Gonzalez also noted it would be stretching it to suggest calcium supplements as a weight-limiting aid.[5]