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20 potassiumcalciumscandium


Name, Symbol, Number calcium, Ca, 20
Chemical series alkaline earth metals
Group, Period, Block 24, s
Appearance silvery white
Standard atomic weight 40.078(4) g·mol−1
Electron configuration [Ar] 4s2
Electrons per shell 2, 8, 8, 2
Physical properties
Phase solid
Density (near r.t.) 1.55 g·cm−3
Liquid density at m.p. 1.378 g·cm−3
Melting point 1115 K
(842 °C, 1548 °F)
Boiling point 1757 K
(1484 °C, 2703 °F)
Heat of fusion 8.54 kJ·mol−1
Heat of vaporization 154.7 kJ·mol−1
Heat capacity (25 °C) 25.929 J·mol−1·K−1
Vapor pressure
P/Pa 1 10 100 1 k 10 k 100 k
at T/K 864 956 1071 1227 1443 1755
Atomic properties
Crystal structure cubic face centered
Oxidation states 2
(strongly basic oxide)
Electronegativity 1.00 (Pauling scale)
Ionization energies
1st: 589.8 kJ·mol−1
2nd: 1145.4 kJ·mol−1
3rd: 4912.4 kJ·mol−1
Atomic radius 180 pm
Atomic radius (calc.) 194 pm
Covalent radius 174 pm
Magnetic ordering paramagnetic
Electrical resistivity (20 °C) 33.6 nΩ·m
Thermal conductivity (300 K) 201 W·m−1·K−1
Thermal expansion (25 °C) 22.3 µm·m−1·K−1
Speed of sound (thin rod) (20 °C) 3810 m/s
Young's modulus 20 GPa
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
Selected isotopes
Main article: Isotopes of calcium
iso NA half-life DM DE (MeV) DP
40Ca 96.941% Ca is stable with 20 neutrons
41Ca syn 1.03×105 y ε - 41K
42Ca 0.647% Ca is stable with 22 neutrons
43Ca 0.135% Ca is stable with 23 neutrons
44Ca 2.086% Ca is stable with 24 neutrons
45Ca syn 162.7 d β- 0.258 45Sc
46Ca 0.004% >2.8×1015 y β-β-  ? 46Ti
47Ca syn 4.536 d β- 0.694, 1.99 47Sc
γ 1.297 -
48Ca 0.187% >4×1019 y β-β-  ? 48Ti
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Calcium (IPA: /ˈkalsiəm/) 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.


[edit] Notable characteristics

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, 40Ca, 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.

[edit] Occurrence

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 Calcium minerals.

[edit] Applications

Some uses are:

[edit] Calcium compounds

[edit] H and K lines

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]

[edit] History

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 (CaCO3), one of the common compounds of calcium. It is heated to form quicklime (CaO) which is then added to water (H2O). This forms another material known as slaked lime (Ca(OH)2) which is an inexpensive base material used throughout the chemical industry. Chalk, marble, and limestone are all forms of calcium carbonate.

[edit] 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.

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.

[edit] Isotopes

Main article: Isotopes of calcium

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.

97% of naturally occurring calcium is 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.

[edit] Nutrition

Age Calcium (mg/day)
0–6 months 210
7–12 months 270
1–3 years 500
4–8 years 800
9–18 years 1300
19–50 years 1000
51+ years 1200

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. Others, such as vegans, avoid dairy products for ethical and health reasons. 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.

[edit] Dietary calcium supplements

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]

  • 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 provide 400 mg of calcium. It is recommended to take this supplement with food to aid in absorption. In some calcium supplements based on calcium carbonate, vitamin D is added to aid in absorption. Vitamin D is needed for the absorption of calcium from the stomach and for the functioning of calcium in the body.[6][7]
  • 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 about 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 difficult to digest and are more expensive than calcium carbonate
  • Calcium chelates have been chemically bonded with an agent that the body recognizes as food. This form is generally known to be better absorbed by the human body than all other forms of calcium due to the bond.

The National Nutritional Food Association – NNFA (Newport Beach, Calif.) defines a chelate very specifically, and several criteria must be met in order for chelation to actually occur. Some of the claimed "chelates" on the market are the various Krebs (Citric Acid) Cycle chelates, such as citrate, malate, and aspartate. Dicalcium malate (chelated with malic acid) is a newer form of a true calcium chelate. It contains a high amount of elemental calcium (30%).

[edit] Do calcium supplements prevent fractures due to osteoporosis?

Randomized controlled trials found both positive[8][9] and negative[10][11][12][13] benefit. The different results may be explained by doses of calcium and underlying rates of calcium supplementation in the control groups.[14]

[edit] Do calcium supplements prevent cancer?

A meta-analysis[9] by the international Cochrane Collaboration of two randomized controlled trials[15][16]found that calcium "might contribute to a moderate degree to the prevention of adenomatous colonic polyps".

More recent studies were conflicting:

[edit] See also

[edit] Notes

  1. ^ Pauling, Linus General Chemistry p. 627, 1970 ed. Dover Publications
  2. ^ Staff (1995). H-K Project. Mount Wilson Observatory. Retrieved on 2006-08-10.
  3. ^ USDA National Nutrient Database
  4. ^ a b Dietary Supplement Fact Sheet: Calcium
  5. ^ Calcium may help women keep weight in check. Retrieved on 2006-09-25.
  6. ^ article about Calcium with Vitamin D. Retrieved on 2006-08-23.
  7. ^ Caltro. Retrieved on 2006-08-23.
  8. ^ Dawson-Hughes B, Harris SS, Krall EA, Dallal GE (1997). "Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older". N. Engl. J. Med. 337 (10): 670-6. PMID 9278463. 
  9. ^ a b Weingarten MA, Zalmanovici A, Yaphe J (2005). "Dietary calcium supplementation for preventing colorectal cancer and adenomatous polyps". Cochrane database of systematic reviews (Online) (3): CD003548. DOI:10.1002/14651858.CD003548.pub3. PMID 16034903. 
  10. ^ Jackson RD, LaCroix AZ, Gass M, et al (2006). "Calcium plus vitamin D supplementation and the risk of fractures". N. Engl. J. Med. 354 (7): 669-83. DOI:10.1056/NEJMoa055218. PMID 16481635. 
  11. ^ Grant AM, Avenell A, Campbell MK, et al (2005). "Oral vitamin D3 and calcium for secondary prevention of low-trauma fractures in elderly people (Randomised Evaluation of Calcium Or vitamin D, RECORD): a randomised placebo-controlled trial". Lancet 365 (9471): 1621-8. DOI:10.1016/S0140-6736(05)63013-9. PMID 15885294. 
  12. ^ Porthouse J, Cockayne S, King C, et al (2005). "Randomised controlled trial of calcium and supplementation with cholecalciferol (vitamin D3) for prevention of fractures in primary care". BMJ 330 (7498): 1003. DOI:10.1136/bmj.330.7498.1003. PMID 15860827. 
  13. ^ Prince RL, Devine A, Dhaliwal SS, Dick IM (2006). "Effects of calcium supplementation on clinical fracture and bone structure: results of a 5-year, double-blind, placebo-controlled trial in elderly women". Arch. Intern. Med. 166 (8): 869-75. DOI:10.1001/archinte.166.8.869. PMID 16636212. 
  14. ^ Fletcher RH (2006). "Calcium plus vitamin D did not prevent hip fracture or colorectal cancer in postmenopausal women". ACP J. Club 145 (1): 4-5. PMID 16813354. 
  15. ^ Baron JA, Beach M, Mandel JS, et al (1999). "Calcium supplements for the prevention of colorectal adenomas. Calcium Polyp Prevention Study Group". N. Engl. J. Med. 340 (2): 101-7. PMID 9887161. 
  16. ^ Bonithon-Kopp C, Kronborg O, Giacosa A, Räth U, Faivre J (2000). "Calcium and fibre supplementation in prevention of colorectal adenoma recurrence: a randomised intervention trial. European Cancer Prevention Organisation Study Group". Lancet 356 (9238): 1300-6. PMID 11073017. 
  17. ^ Wactawski-Wende J, Kotchen JM, Anderson GL, et al (2006). "Calcium plus vitamin D supplementation and the risk of colorectal cancer". N. Engl. J. Med. 354 (7): 684-96. DOI:10.1056/NEJMoa055222. PMID 16481636. 
  18. ^ Lappe JM, Travers-Gustafson D, Davies KM, Recker RR, Heaney RP (2007). "Vitamin D and calcium supplementation reduces cancer risk: results of a randomized trial". Am. J. Clin. Nutr. 85 (6): 1586-91. PMID 17556697. 
  19. ^ Lin J, Manson JE, Lee IM, Cook NR, Buring JE, Zhang SM (2007). "Intakes of calcium and vitamin d and breast cancer risk in women". Arch. Intern. Med. 167 (10): 1050-9. DOI:10.1001/archinte.167.10.1050. PMID 17533208. 

[edit] References

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

[edit] External links

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