Calcium
- For other uses, see Calcium (disambiguation).
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| Name, Symbol, Number | calcium, Ca, 20 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Chemical series | alkaline earth metals | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Group, Period, Block | 2, 4, s | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Appearance | silvery white 
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| Atomic mass | 40.078(4) g/mol | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Electron configuration | [Ar] 4s2 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Electrons per shell | 2, 8, 8, 2 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Physical properties | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Phase | solid | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 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) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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| Atomic properties | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Crystal structure | cubic face centered | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Oxidation states | 2 (strongly basic oxide) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Electronegativity | 1.00 (Pauling scale) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Ionization energies (more) |
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 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Miscellaneous | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Magnetic ordering | paramagnetic | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 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 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Poisson ratio | 0.31 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Mohs hardness | 1.75 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Brinell hardness | 167 MPa | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| CAS registry number | 7440-70-2 | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Notable isotopes | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Calcium (chemical symbol Ca, atomic number 20) is the fifth most abundant chemical element in the Earth's crust. Soft gray in color, it is classified as an alkaline earth metal. It is used as a reducing agent in the extraction of thorium, zirconium, and uranium. It is essential for living organisms, particularly in cell physiology, and is the most common metal in many animals.
- Calcium is essential in muscle contraction, oocyte activation, building strong bones and teeth, blood clotting, nerve impulse transmission, regulating heartbeat, and fluid balance within cells. In the United States, between about 50% and 75% of adults do not get sufficient calcium in their diet.[1] Adults need between 1,000 and 1,300 milligrams (mg) of calcium in their daily diet.[1]
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 an essential macromineral in the human diet, soil conservation practices often consider the sustainable equilibrium of calcium concentrations in the earth.
Notable characteristics
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.
Calcium 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, an alkaline substance.
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.
Isotopes
Calcium has four stable isotopes: 40Ca, 42Ca, 43Ca, and 44Ca. Of these, 40Ca is the most abundant isotope, constituting 97% 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.
Applications
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.
Compounds
Calcium carbonate
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
Calcium oxide (CaO)—commonly known as lime, quicklime, or burnt lime—is a white, caustic, and alkaline crystalline solid. It is widely used as a fertilizer and building material. As a commercial product, lime often includes magnesium oxide, silicon oxide, and smaller amounts of aluminum oxide and iron oxide.
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.
As hydrated or slaked lime, Ca(OH)2 (mineral name: portlandite), it was used in mortar and plaster to increase the rate of hardening. Hydrated lime is very simple to make as lime is a basic anhydride and reacts vigorously with water. Lime was also used in glass production and its ability to work with silicates is also used in modern metal production (steel, magnesium, aluminium and other non-ferrous metals) industries to remove impurities as slag.
It is also used in water and sewage treatment to reduce acidity, to soften, as a flocculant and to remove phosphates and other impurities; in paper making to dissolve lignin, as a coagulant and in bleaching; in agriculture to improve acidic soils; and in pollution control - in gas scrubbers to desulfurize waste gases and to treat many liquid effluents. It is a refactory and a dehydrating agent and is used to purify citric acid, glucose, dyes and as a CO2 absorber. It is also used in pottery, concrete, paints and the food industry, where it is sometimes used (in conjunction with water) to heat items like MREs (Meals Ready To Eat) and coffee.
Hydroxylapatite
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.
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.
Other important calcium compounds are its nitrate, sulfide, chloride, carbide, cyanamide, and hypochlorite.
Health and nutrition
Calcium is the most abundant mineral in the 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
In the United States, between about 50% and 75% of adults do not get sufficient calcium in their diet.[1] For this reason, individuals may consider taking 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% 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% 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.
See also
- Calcium compounds
- Disorders of calcium metabolism
ReferencesISBN links support NWE through referral fees
- Donatelle, Rebecca J. Health, The Basics (6th ed.) San Francisco: Pearson Education, Inc. 2005.
Footnotes
- ↑ 1.0 1.1 1.2 "Dietary Supplement Fact Sheet: Calcium" Accessed on March 23, 2006. Cite error: Invalid
<ref>tag; name "DietaryFactSheet" defined multiple times with different content
External links
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