|Name, Symbol, Number||calcium, Ca, 20|
|Chemical series||alkaline earth metals|
|Group, Period, Block||2, 4, s|
|Atomic mass||40.078(4) g/mol|
|Electron configuration||[Ar] 4s2|
|Electrons per shell||2, 8, 8, 2|
|Density (near r.t.)||1.55 g/cm³|
|Liquid density at m.p.||1.378 g/cm³|
|Melting point||1115 K
(842 °C, 1548 °F)
|Boiling point||1757 K
(1484 °C, 2703 °F)
|Heat of fusion||8.54 kJ/mol|
|Heat of vaporization||154.7 kJ/mol|
|Heat capacity||(25 °C) 25.929 J/(mol·K)|
|Crystal structure||cubic face centered|
(strongly basic oxide)
|Electronegativity||1.00 (Pauling scale)|
|1st: 589.8 kJ/mol|
|2nd: 1145.4 kJ/mol|
|3rd: 4912.4 kJ/mol|
|Atomic radius||180 pm|
|Atomic radius (calc.)||194 pm|
|Covalent radius||174 pm|
|Electrical resistivity||(20 °C) 33.6 nΩ·m|
|Thermal conductivity||(300 K) 201 W/(m·K)|
|Thermal expansion||(25 °C) 22.3 µm/(m·K)|
|Speed of sound (thin rod)||(20 °C) 3810 m/s|
|Speed of sound (thin rod)||(r.t.) 20 m/s|
|Shear modulus||7.4 GPa|
|Bulk modulus||17 GPa|
|Brinell hardness||167 MPa|
|CAS registry number||7440-70-2|
Calcium (chemical symbol Ca, atomic number 20) is the fifth most plentiful chemical element in the Earth's crust, occurring in various rocks, minerals, coral, and shells of marine animals. Soft gray in color, it is classified as an alkaline earth metal. It is the most abundant mineral in the human body and an important component of a healthy diet. It is essential for building strong bones and teeth, muscle contraction, oocyte activation, blood clotting, nerve impulse transmission, heartbeat regulation, and fluid balance within cells. It is used as a reducing agent for the extraction of several metals; an agent for generating metal alloys; and a deoxidizer, desulfurizer, or decarbonizer for various alloys. Calcium carbonate is used in construction materials; calcium oxide is used for treating water, sewage, and acidic soils; and calcium hydroxide is used for processing water for beverages and neutralizing acids in the tanning industry.
Occurrence and isolation
In nature, calcium (Latin calcis, meaning "lime") is not found in its elemental state. Rather, it occurs in the form of various compounds: calcium carbonate in limestone rocks, marble, coral, and the shells of mollusks; calcium magnesium carbonate in the mineral dolomite; calcium sulfate dihydrate in the mineral gypsum; calcium fluoride in the mineral fluorite; and calcium phosphate in the apatite group of minerals. In addition, pearls and eggshells are made of calcium carbonate.
The ancient Romans are known to have prepared lime (calcium oxide) as early as the first century. The element calcium, however, was not isolated until 1808 in England, when Sir Humphry Davy electrolyzed a mixture of lime and mercuric oxide. He first obtained an amalgam, which, when heated to distill out the mercury, gave him a residue of calcium. Subsequent methods of extraction have included electrolysis of calcium fluoride or calcium chloride.
As a member of the series of alkaline earth metals, calcium lies in group 2 (former group 2A) of the periodic table, between magnesium and strontium. In addition, it is placed in period 4, between potassium and scandium.
In terms of the electronic configuration of each neutral atom, the outermost shell (or valence shell) contains 2 electrons in the 4s orbital. In the formation of compounds, calcium readily loses these 2 electrons to produce Ca2+ cations.
Calcium has four stable isotopes: 40Ca, 42Ca, 43Ca, and 44Ca. Of these, 40Ca is the most abundant isotope, constituting 97 percent of naturally occurring calcium. It has a nucleus of 20 protons and 20 neutrons. Two more isotopes, 46Ca and 48Ca, have such long half-lives that for all practical purposes they can be considered stable.
In addition, calcium has a radioactive isotope, 41Ca, with a half-life of 103,000 years. It is described as a "cosmogenic" isotope, which means that it is produced by the action of high-energy cosmic rays on the nuclei of target atoms. In the case of 41Ca, it is produced by neutron activation of 40Ca. Most of its production is in the upper meter or so of the soil column.
Calcium is used for a number of purposes. Examples are as follows.
- It is a reducing agent in the extraction of other metals, such as uranium, zirconium, and thorium.
- It is a deoxidizer, desulfurizer, or decarbonizer for various alloys.
- It is an alloying agent for the production of aluminum, beryllium, copper, lead, and magnesium alloys.
- It is used in making cements and mortars as construction materials.
As mentioned above, calcium carbonate (CaCO3) is a common substance found in limestone rock, marble, and coral. It is the main component of seashells, eggshells, and the shells of snails. It is commonly used medicinally as a calcium supplement or as an antacid. Calcium carbonate is the active ingredient in agricultural lime.
Calcium carbonate exhibits the properties typical of other carbonates.
- It reacts with strong acids, releasing carbon dioxide and forming a salt of the acid:
- CaCO3 + 2HCl → CaCl2 + CO2 + H2O
- It releases carbon dioxide on heating (to above 825 °C), to form calcium oxide.
- CaCO3 → CaO + CO2
- It reacts with water that is saturated with carbon dioxide to form the soluble calcium bicarbonate.
- CaCO3 + CO2 + H2O → Ca(HCO3)2
The last of these reactions is important in the erosion of limestone and other carbonate rocks, forming caves, stalactites, and stalagmites. In addition, it leads to the formation of hard water in many regions.
Calcium carbonate is used mainly in the construction industry. For instance, it may be used in the form of limestone aggregate for roadbuilding, or in the form of marble, a building material in its own right. In addition, calcium carbonate is an ingredient of cement and the starting material for the preparation of builder's lime by burning in a kiln.
This compound is widely used as an extender in paints, a filler in plastics, and an ingredient in adhesives and sealants. It has been a major component of blackboard chalk. Known as "whiting" in ceramic artwork, it is a common ingredient for many glazes in its white powdered form. Recently, it has been used to replace kaolin in the production of glossy paper.
Medicinally, calcium carbonate is widely used as an inexpensive calcium supplement and antacid. It is also used in the pharmaceutical industry as a base material for tablets of other drugs. It is also an additive in some soy milk products as a source of dietary calcium.
Calcium oxide (CaO)—commonly known as lime, quicklime, or burnt lime—is a white, caustic, and alkaline crystalline solid. As a commercial product, lime often includes magnesium oxide, silicon oxide, and smaller amounts of aluminum oxide and iron oxide. Calcium oxide reacts vigorously with water to produce calcium hydroxide (Ca(OH)2).
Calcium oxide is usually made by the thermal decomposition of materials (such as limestone) that contain calcium carbonate. In this process, called calcination or lime-burning, the material is heated to around 900 °C, to remove carbon dioxide in a non-reversible chemical reaction (American Scientist). This appears to have been one of the first chemical reactions discovered by human civilization.
When lime is mixed with sand, it hardens into a mortar and is turned into plaster by carbon dioxide uptake. Mixed with other compounds, it forms an important part of Portland cement. Lime is also used in the industrial production of metals—including steel, magnesium, aluminum—as it helps remove impurities as slag. It is used in water and sewage treatment, to reduce acidity, soften water, and remove phosphates and other impurities. In paper making, it is used in dissolving lignin, bleaching, and working as a coagulant. In agriculture, it helps improve acidic soils. For pollution control, it is used in gas scrubbers to desulfurize waste gases and treat many liquid effluents. It is a refactory and a dehydrating agent and is used to purify citric acid, glucose, and dyes. It is a carbon dioxide absorber. It is also used in pottery, concrete, and paints. The food industry sometimes uses it (in conjunction with water) to heat items like "Meals Ready To Eat" (MREs) and coffee.
Calcium hydroxide (Ca(OH)2), known as slaked lime, is a colorless crystal or white powder. It is produced when calcium oxide is "slaked" (hydrated) with water. It can also be made by mixing aqueous solutions of calcium chloride and sodium hydroxide.
If heated, calcium hydroxide decomposes into calcium oxide and water. A suspension of fine calcium hydroxide particles in water, called lime water (or milk of lime), is a medium-strength chemical base that reacts violently with acids and attacks many metals.
Calcium hydroxide has varied uses. For instance, it is used as an ingredient in whitewash, mortar, and plaster. It is a reagent in the tanning industry, for the neutralization of extra acid; in the petroleum refining industry, for the manufacture of additives to oils; in the chemical industry, for the manufacture of calcium stearate; in the food industry, for processing water for beverages. In Native American and Latin American cooking, corn cooked with calcium hydroxide (called "cal") becomes nixtamal, which is considered tastier and easier to digest. Calcium hydroxide is also a filler used in the preparation of dry mixes for painting and decorating; manufacturing mixes for pesticides; and manufacturing brake pads.
Hydroxylapatite (Ca5(PO4)3(OH))—a compound with calcium, phosphate, and hydroxide ions—is the mineral portion of human and animal bones and teeth. The mineral portion of some corals can also be transformed into hydroxylapatite.
Other important calcium compounds are its nitrate, sulfide, chloride, carbide, cyanamide, and hypochlorite.
Health and nutrition
Calcium is the most abundant mineral in the human body and is an important component of a healthy diet. It is essential for the normal growth and maintenance of bones and teeth. In addition, it plays an important part in muscle contraction, oocyte activation, blood clotting, nerve impulse transmission, regulating heartbeat, and fluid balance within cells. Calcium requirements must be met throughout life, but requirements are greatest during periods of growth, such as childhood, during pregnancy and when breast-feeding. vitamin D is needed to absorb calcium.
Long-term calcium deficiency can lead to osteoporosis, in which the bone deteriorates and there is an increased risk of fractures. Overretention, on the other hand, can cause kidney stones.
Milk, yogurt, cheese and other dairy products are a prime source of calcium and are also fortified with vitamin D. Calcium needs can be met by consuming at least three or four servings of dairy products daily. It should be noted that some dairy products, such as hard cheese and whole milk, contain significant amounts of saturated fat, which can contribute to cardiovascular disease. Therefore, a diet consisting of low-fat dairy products should be considered. Recommended daily calcium intake varies from 1,000 to 1,500 milligrams (mg), depending upon the person's stage of life.
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, there are many other good sources of calcium. These include: seaweeds such as kelp, wakame, and hijiki; nuts and seeds, including almonds and sesame; beans; seafood, such as oysters and shrimp; soft-boned fish; amaranth; whole wheat; collard greens; okra; rutabaga; and broccoli. In addition, several products are fortified with calcium, including soya milk, rice milk, orange juice, and bread.
Dietary calcium supplements
- 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 percent elemental calcium, which means that 1,000 mg will provide 400 mg of calcium. Taking this supplement with food can aid in absorption.
- Calcium citrate is more easily absorbed (bioavailability is 2.5 times higher than calcium carbonate), 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 percent elemental calcium, which means that 1,000 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 more easily digested but more expensive than calcium carbonate.
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.
- Barefoot, Robert R., and Carl J. Reich, M.D. 2002. The Calcium Factor: The Scientific Secret of Health and Youth. 6th ed. Wickenburg, AZ: Deonna Enterprises Pub. ISBN 0963370324.
- Chang, Raymond. 2006. Chemistry. 9th ed. New York: McGraw-Hill Science/Engineering/Math. ISBN 0073221031.
- Donatelle, Rebecca J. 2005. Health, The Basics. 6th ed. San Francisco: Pearson Education. ISBN 0805328521.
- Mayer, Mark. 2006. Calcification: The Aging Factor. Lulu.com. ISBN 184728633X.
All links retrieved March 26, 2013.
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