Calcium

This article is about the metallic element. For the place, see Calcium, New York.

Calcium,  ₂₀Ca

Spectral lines of calcium
General properties
Name, symbol	calcium, Ca
Pronunciation	/ˈkælsiəm/ KAL-see-əm
Appearance	dull gray, silver; with a pale yellow tint
Calcium in the periodic table
Mg ↑ Ca ↓ Sr potassium ← calcium → scandium
Atomic number (Z)	20
Group, block	group 2 (alkaline earth metals), s-block
Period	period 4
Element category	alkaline earth metal
Standard atomic weight (±) (Ar)	40.078(4)
Electron configuration	[Ar] 4s2
per shell	2, 8, 8, 2
Physical properties
Phase	solid
Melting point	1115 K ​(842 °C, ​1548 °F)
Boiling point	1757 K ​(1484 °C, ​2703 °F)
Density near r.t.	1.55 g/cm3
when liquid, at m.p.	1.378 g/cm3
Heat of fusion	8.54 kJ/mol
Heat of vaporization	154.7 kJ/mol
Molar heat capacity	25.929 J/(mol·K)
Vapor pressure P (Pa) 1 10 100 1 k 10 k 100 k at T (K) 864 956 1071 1227 1443 1755
Atomic properties
Oxidation states	+2, +1 ​(a strongly basic oxide)
Electronegativity	Pauling scale: 1.00
Ionization energies	1st: 589.8 kJ/mol 2nd: 1145.4 kJ/mol 3rd: 4912.4 kJ/mol (more)
Atomic radius	empirical: 197 pm
Covalent radius	176±10 pm
Van der Waals radius	231 pm
Miscellanea
Crystal structure	​face-centered cubic (fcc)
Speed of sound thin rod	3810 m/s (at 20 °C)
Thermal expansion	22.3 µm/(m·K) (at 25 °C)
Thermal conductivity	201 W/(m·K)
Electrical resistivity	33.6 nΩ·m (at 20 °C)
Magnetic ordering	diamagnetic
Magnetic susceptibility (χmol)	+40.0·10−6 cm3/mol
Young's modulus	20 GPa
Shear modulus	7.4 GPa
Bulk modulus	17 GPa
Poisson ratio	0.31
Mohs hardness	1.75
Brinell hardness	170–416 MPa
CAS Number	7440-70-2
History
Discovery and first isolation	Humphry Davy (1808)
Most stable isotopes of calcium
iso NA half-life DM DE (MeV) DP 40Ca 96.941% is stable with 20 neutrons 41Ca trace 1.03×105 y ε – 41K 42Ca 0.647% is stable with 22 neutrons 43Ca 0.135% is stable with 23 neutrons 44Ca 2.086% is stable with 24 neutrons 45Ca syn 162.7 d β− 0.258 45Sc 46Ca 0.004% is stable with 26 neutrons 47Ca syn 4.5 d β− 0.694, 1.99 47Sc γ 1.297 – 48Ca 0.187% 4.3×1019 y β−β− 4.274 48Ti

Calcium is a chemical element with symbol Ca and atomic number 20. Calcium is a soft gray Group 2 alkaline earth metal, fifth-most-abundant element by mass in the Earth's crust. The ion Ca²⁺ is also the fifth-most-abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate. Free calcium metal is too reactive to occur in nature. Calcium is produced in supernova nucleosynthesis.

Calcium is an essential trace element in living organisms. It is the most abundant metal by mass in many animals, and it is an important constituent of bone, teeth, and shells. In cell biology, the movement of the calcium ion into and out of the cytoplasm functions as a signal for many cellular processes. Calcium carbonate and calcium citrate are often taken as dietary supplements. Calcium is on the World Health Organization's List of Essential Medicines.

Uses

Calcium has a wide variety of applications, almost all of which are associated with calcium compounds and salts.

• Calcium metal is used as a deoxidizer, desulfurizer, and decarbonizer for production of some ferrous and nonferrous alloys. In steelmaking and production of iron, Ca reacts with oxygen and sulfur-containing impurities.
• Calcium carbonate (CaCO₃) is used in manufacturing cement and mortar, lime, limestone (usually used in the steel industry) and aids in production in the glass industry. It also has chemical and optical uses as mineral specimens in toothpastes, for example.
• Calcium hydroxide solution (Ca(OH)₂) (also known as limewater) is used to detect the presence of carbon dioxide in a gas sample bubbled through a solution. The solution turns cloudy where CO₂ is present.
• Calcium arsenate (Ca₃(AsO₄)₂) is used in insecticides.
• Calcium carbide (CaC₂) is used to make acetylene gas (for use in acetylene torches for welding) and various plastics.
• Calcium chloride (CaCl₂) is used in ice removal and dust control on dirt roads, as a conditioner for concrete, as an additive in canned tomatoes, and to provide body for automobile tires.
• Calcium citrate (Ca₃(C₆H₅O₇)₂) is used as a food preservative.
• Calcium cyclamate (Ca(C₆H₁₁NHSO₃)₂) is used as a sweetening agent in several countries. In the United States, it has been outlawed as a suspected carcinogen.
• Calcium gluconate (Ca(C₆H₁₁O₇)₂) is used as a food additive and in vitamin pills.
• Calcium hypochlorite (Ca(OCl)₂) is used as a swimming pool disinfectant, as a bleaching agent, as an ingredient in deodorant, and in algaecide and fungicide.
• Calcium permanganate (Ca(MnO₄)₂) is used in liquid rocket propellant, textile production, as a water sterilizing agent and in dental procedures.
• Calcium phosphate (Ca₃(PO₄)₂) is used as a supplement for animal feed, fertilizer, in commercial production for dough and yeast products, in the manufacture of glass, and in dental products.
• Calcium phosphide (Ca₃P₂) is used in fireworks, rodenticide, torpedoes, and flares.
• Calcium stearate (Ca(C₁₈H₃₅O₂)₂) is used in the manufacture of wax crayons, cements, certain kinds of plastics, and cosmetics, as a food additive, in the production of water resistant materials, and in the production of paints.
• Calcium sulfate (CaSO₄·2H₂O) (gypsum) is used as common blackboard chalk, as well as, in its hemihydrate form, Plaster of Paris.
• Calcium tungstate (CaWO₄) is used in luminous paints, fluorescent lights, and in X-ray studies.
• Hydroxylapatite (Ca₅(PO₄)₃(OH), but is usually written Ca₁₀(PO₄)₆(OH)₂) makes up seventy percent of bone. Carbonated-calcium deficient hydroxylapatite is the main mineral of which dental enamel and dentin are comprised.

Physiology

In solution, the calcium ion varies remarkably to the human taste, being reported as mildly salty, sour, "mineral-like", or even "soothing." It is apparent that many animals can taste, or develop a taste, for calcium, and use this sense to detect the mineral in salt licks or other sources. In human nutrition, soluble calcium salts may be added to tart juices without much effect to the average palate.

Recommended adequate intake by the IOM for calcium:

Age	Calcium (mg/day)
0–6 months	200
7–12 months	260
1–3 years	700
4–8 years	1000
9–18 years	1300
19–50 years	1000
51–70 years (male)	1000
51–70 years (female)	1200
71+ years	1200

Calcium is an important component of a healthy diet and a mineral necessary for life. The National Osteoporosis Foundation states, "Calcium plays an important role in building stronger, denser bones early in life and keeping bones strong and healthy later in life." Approximately 99 percent of the calcium in the human body is in the bones and teeth. Intracellular calcium overload can cause oxidative stress and apoptosis in some cells, sometimes leading to several diseases. In the electrical conduction system of the heart, calcium replaces sodium as the mineral that depolarizes the cell, proliferating the action potential. In cardiac muscle, sodium influx commences an action potential, but during potassium efflux, the cardiac myocyte experiences calcium influx, prolonging the action potential and creating a plateau phase of dynamic equilibrium. Long-term calcium deficiency can lead to rickets and poor blood clotting; in menopausal women, deficiency can lead to osteoporosis, a condition in which the bone deteriorates and fractures more readily. While a lifelong deficit of calcium can affect bone and tooth formation, over-retention can cause hypercalcemia (elevated levels of calcium in the blood), impaired kidney function, and decreased absorption of other minerals. Vitamin D is needed to absorb calcium.

Cell physiology

The release of calcium ions from the sarcoplasmic reticulum into the cytoplasm is an essential intracellular signal, important in many cellular functions and processes, including muscle contraction, neuronal transmission as in an excitatory synapse, cellular motility (including the movement of flagella and cilia), fertilisation, cell growth or proliferation, learning, memory (as with synaptic plasticity), and secretion of saliva. Calcium signalling can be studied by loading a cell's cytoplasm with a calcium-sensitive fluorescent dye such as Fura-2. Many of these dyes were developed by Roger Y. Tsien.

Dietary supplement

500 milligram calcium supplements made from calcium carbonate

Main article: Calcium supplements

Calcium supplements are used to prevent and to treat calcium deficiencies. However, it has been found that the taking of calcium supplements by people with a history of stroke or with white matter lesions greatly increased their chances of developing dementia. The Office of Dietary Supplements (National Institutes of Health) recommends that no more than 600 mg of supplement should be taken at a time because the percent of calcium absorbed decreases as the amount of calcium in the supplement increases. It is therefore recommended to spread doses throughout the day. Recommended daily calcium intake for adults ranges from 1000 to 1300 mg. Calcium supplements may have side effects such as bloating and constipation in some people. It is suggested that taking the supplements with food may aid in nullifying these side effects.

Toxicity

Compared with other metals, the calcium ion and most calcium compounds have low toxicity. This is not surprising, given the very high natural abundance of calcium compounds in the environment and in organisms.

Calcium poses few serious environmental problems.

Calcium metal is hazardous because of its sometimes-violent reactions with water and acids. Calcium metal is found in some drain cleaners, where it functions to generate heat and calcium hydroxide that saponifies the fats and liquefies the proteins (e.g., hair) that block drains. When swallowed, calcium metal has the same effect on the mouth, esophagus, and stomach, and can be fatal.

Notable characteristics

Flame test. Brick-red color originates from calcium.

Calcium can be extracted by electrolysis from a fused salt like calcium chloride. Calcium is relatively soft for a metal; although harder than lead, it can be cut with a knife with difficulty. Calcium is chemically reactive; when exposed to the air, it rapidly forms a gray-white coating of calcium oxide and calcium nitride. In bulk form (typically as chips or "turnings"), the metal is somewhat difficult to ignite, more difficult even than magnesium chips; but, when lit, the metal burns in air with a brilliant high-intensity orange-red light. Calcium metal reacts with water, producing hydrogen gas at a moderate rate without generating much heat, making it useful for generating hydrogen. In powdered form, however, the reaction with water is extremely rapid, as the increased surface area of the powder accelerates the reaction. Part of the reason for the slowness of the calcium–water reaction is a partial passivation (chemically protective coating) of insoluble white calcium hydroxide; in acidic solutions, where this compound is more soluble, calcium reacts vigorously.

With a density of 1.54 g/cm³, calcium is the lightest of the alkaline earth metals; magnesium (specific gravity 1.74) and beryllium (1.84) are denser though lighter in atomic mass. From strontium onward, the alkali earth metals become denser with increasing atomic mass. Calcium has two allotropes.

Calcium metal has a higher electrical resistivity than copper or aluminium, yet weight-for-weight, due to its much lower density, it is a better conductor than either. Its use as such in terrestrial applications is usually limited by its high reactivity with air; however, it has potential for use as wiring in off-world applications.

Calcium is the fifth-most-abundant element by mass in the human body, where it is a cellular ionic messenger with many functions. Calcium also serves as a structural element in bone. It is the relatively high atomic number of calcium that causes bone to be radio-opaque.

H and K lines

Visible spectra of many stars, including the Sun, exhibit strong emission 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. In the Sun or other stars with low temperatures, the prominence of the H and K lines in the visible spectra can be an indication of strong magnetic activity in the chromosphere.

Compounds

See also: Category:Calcium compounds.

Calcium chemistry is almost exclusively that of Ca²⁺ salts. Ca²⁺ is a "hard cation", that is, it characteristically favors oxide ligands. Hence the abundance of carbonates, nitrates, phosphates, and sulfates in the mineral kingdom. Many of these species crystallize with water. Because it is generally nontoxic and abundant, calcium is found in many foods and useful materials. Most calcium salts are colorless. As with magnesium salts and other alkaline earth metal salts, the halides are soluble in water.

Combined with phosphate, calcium forms hydroxylapatite (Ca₅(PO₄)₃(OH)), the mineral portion of animal bones, teeth, and some corals. Large-scale chemical processes are involved in the conversion of calcium phosphate minerals into fertilizer.

Calcium is the main problematic ion in hard water: it forms insoluble deposits of calcium carbonate that are problematic in plumbing. It also reacts with soap to form soap scum. Calcium carbonate occurs naturally as limestone and chalk. When water percolates through limestone or other calcium-containing rocks, it partially dissolves the rock. The slow re-precipitation of minerals derived from dissolved calcium leads to formation of stalactites and stalagmites.

When heated above 825 °C, calcium carbonate converts calcium oxide (CaO), also known as quicklime:

CaCO₃ → CaO + CO₂

When added to water, quicklime vigorously reacts (hence its name) to form calcium hydroxide:

CaO + H₂O → Ca(OH)₂

Also known as slaked lime, Ca(OH)₂ is an inexpensive base material used throughout the chemical industry. When 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.

Combined with sulfate, calcium forms the mineral gypsum. When heated to about 300 °F (150 °C), it undergoes partial dehydration:

CaSO₄·2H₂O → CaSO₄·0.5H₂O + 1.5 H₂O

The resulting powder, when mixed with water, forms a stiff but workable paste that hardens to give Plaster of Paris.

Organocalcium compounds, those containing Ca-C bonds are known, but generally of specialized interest in the research laboratory. One major exception is calcium carbide, which arises from heating calcium compounds with coal or other carbon-rich reducing agents.

CaO + 3 C → CaC₂ + CO

It was historically important precursor to acetylene.

CaC₂ + 2 H₂O → C₂H₂ + Ca(OH)₂

Other important calcium compounds are calcium nitrate used in fertilizers, calcium chloride used as for deicing roads, calcium cyanamide, and calcium hypochlorite, used for bleaching.

Structure of the polymeric [Ca(H₂O)₆]²⁺ center in hydrated calcium chloride, illustrating the high coordination number typical for calcium complexes.

Focusing on chemical structure, Ca²⁺ is a relatively large ion that tends to adopt a high coordination number. In CaF₂, the mineral fluorite, each Ca²⁺ ion is surrounded by eight F⁻ ligands.

Isotopes

Main article: Isotopes of calcium

Calcium has five stable isotopes (⁴⁰Ca, ⁴²Ca, ⁴³Ca, ⁴⁴Ca and ⁴⁶Ca), plus one more (⁴⁸Ca) that has such a long half-life, it can be considered stable for many purposes. The 20% range in relative mass among naturally occurring calcium isotopes is greater than for any element other than hydrogen and helium. Calcium also has a cosmogenic isotope, radioactive ⁴¹Ca, which has a half-life of 103,000 years. Unlike cosmogenic isotopes produced in the atmosphere, ⁴¹Ca is produced by neutron activation of ⁴⁰Ca, primarily in the top metre of the soil column, where the cosmogenic neutron flux is sufficiently strong. ⁴¹Ca has received much attention in stellar studies because it decays to ⁴¹K, a critical indicator of solar-system anomalies.

Ninety-seven percent of naturally occurring calcium is in the form of ⁴⁰Ca, which is a daughter product of ⁴⁰K and ⁴⁰Ar decay. While K–Ar dating is 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 are used for K-Ca age dating.

⁴⁰Ca has a nucleus of 20 protons and 20 neutrons and is the heaviest stable isotope of any element that has equal numbers of protons and neutrons. In supernova explosions, calcium is formed from the reaction of carbon with various numbers of alpha particles (helium nuclei), until the most common calcium isotope (containing 10 helium nuclei) has been synthesized.

Isotope fractionation

As with the isotopes of other elements, a variety of processes fractionate, or alter the relative abundance of, calcium isotopes. The best studied of these processes is the mass-dependent fractionation of calcium isotopes that accompanies the precipitation of calcium minerals, such as calcite, aragonite and apatite, from solution. Isotopically light calcium is preferentially incorporated into minerals, leaving the solution from which the mineral precipitated enriched in isotopically heavy calcium. At room temperature the magnitude of this fractionation is roughly 0.25‰ (0.025%) per atomic mass unit (AMU). Mass-dependent differences in calcium isotope composition conventionally are expressed by the ratio of two isotopes (usually ⁴⁴Ca/⁴⁰Ca) in a sample compared to the same ratio in a standard reference material. ⁴⁴Ca/⁴⁰Ca varies by about 1% among common earth materials.

Calcium isotope fractionation during mineral formation has led to several applications of calcium isotopes. In particular, the 1997 observation by Skulan and DePaolo that calcium minerals are isotopically lighter than the solutions from which the minerals precipitate is the basis of analogous applications in medicine and in paleooceanography. In animals with skeletons mineralized with calcium, the calcium isotopic composition of soft tissues reflects the relative rate of formation and dissolution of skeletal mineral. In humans, changes in the calcium isotopic composition of urine have been shown to be related to changes in bone mineral balance. When the rate of bone formation exceeds the rate of bone resorption, the ration ⁴⁴Ca/⁴⁰Ca in soft tissue rises. Soft tissue ⁴⁴Ca/⁴⁰Ca falls when bone resorption exceeds bone formation. Because of this relationship, calcium isotopic measurements of urine or blood may be useful in the early detection of metabolic bone diseases like osteoporosis.

A similar system exists in the ocean, where ⁴⁴Ca/⁴⁰Ca in seawater tends to rise when the rate of removal of Ca²⁺ from seawater by mineral precipitation exceeds the input of new calcium into the ocean, and fall when calcium input exceeds mineral precipitation. It follows that rising ⁴⁴Ca/⁴⁰Ca corresponds to falling seawater Ca²⁺ concentration, and falling ⁴⁴Ca/⁴⁰Ca corresponds to rising seawater Ca²⁺ concentration. In 1997 Skulan and DePaolo presented the first evidence of change in seawater ⁴⁴Ca/⁴⁰Ca over geologic time, along with a theoretical explanation of these changes. More recent papers have confirmed this observation, demonstrating that seawater Ca²⁺ concentration is not constant, and that the ocean probably never is in “steady state” with respect to its calcium input and output. This has important climatological implications, as the marine calcium cycle is closely tied to the carbon cycle (see below).

Sources

See also: Category:Calcium minerals.

Calcium is not naturally found in its elemental state. Calcium occurs most commonly in sedimentary rocks in the minerals calcite, dolomite, and gypsum. It also occurs in igneous and metamorphic rocks chiefly in the silicate minerals: plagioclases, amphiboles, pyroxenes, and garnets.

Food

Dairy products, such as milk and cheese, are a well-known sources of calcium.

Vegetable sources for calcium include:

• seaweeds such as kelp, wakame, and hijiki;
• Nuts and seeds such almonds, hazelnuts, sesame, and pistachio;
• Blackstrap molasses;
• beans (especially soy beans);
• figs, quinoa, okra; rutabaga; broccoli; dandelion leaves; and kale.
• Several foods and drinks, such as orange juice, soy milk, tofu, breakfast cereals, and breads are often fortified with calcium.

Numerous vegetables, notably spinach, chard, and rhubarb have a high calcium content, but they may also contain varying amounts of oxalic acid that binds calcium and reduces its absorption. The same problem may affect the absorption of calcium from amaranth, collard greens, and chicory greens. This process may also be related to the generation of calcium oxalate.

An overlooked source of calcium is eggshell, which can be ground into a powder and mixed into food or a glass of water.

The calcium content of most foods can be found in the USDA National Nutrient Database.

• Milk is an excellent source of dietary calcium for those whose bodies tolerate it because it has a high concentration of calcium and the calcium in milk is excellently absorbed.
• Soymilk and other vegetable milks are usually sold with calcium added so that their calcium concentration is as high as in milk.

Geochemical cycling

Calcium provides a link between tectonics, climate, and the carbon cycle. In the simplest terms, uplift of mountains exposes calcium-bearing rocks to chemical weathering and releases Ca²⁺ into surface water. This Ca²⁺ eventually is transported to the ocean where it reacts with dissolved CO₂ to form limestone. Some of this limestone settles to the sea floor where it is incorporated into new rocks. Dissolved CO₂, along with carbonate and bicarbonate ions, are termed "dissolved inorganic carbon" (DIC).

Travertine terraces Pamukkale, Turkey

The actual reaction is more complicated and involves the bicarbonate ion (HCO−
3) that forms when CO₂ reacts with water at seawater pH:

Ca2+
+ 2HCO−
3 → CaCO
3 (limestone) + CO
2 + H
2O

Note that at seawater pH, most of the CO₂ is immediately converted back into HCO−
3. The reaction results in a net transport of one molecule of CO₂ from the ocean/atmosphere into the lithosphere.

The result is that each Ca²⁺ ion released by chemical weathering ultimately removes one CO₂ molecule from the surficial system (atmosphere, ocean, soils and living organisms), storing it in carbonate rocks where it is likely to stay for hundreds of millions of years. The weathering of calcium from rocks thus scrubs CO₂ from the ocean and atmosphere, exerting a strong long-term effect on climate. Analogous cycles involving magnesium, and to a much smaller extent strontium and barium, have the same effect.

As the weathering of limestone (CaCO₃) liberates equimolar amounts of Ca²⁺ and CO₂, it has no net effect on the CO₂ content of the atmosphere and ocean. The weathering of silicate rocks like granite, on the other hand, is a net CO₂ sink because it produces abundant Ca²⁺ but very little CO₂.

History

'Ain Ghazal figure

Lime as building material was used since prehistoric times going as far back as 7000 to 14000 BC. Statues made from lime plaster have been dated into the 7 millennia BC. The first dated lime kiln dates back to 2500 BC and was found in Khafajah mesopotamia. Calcium (from Latin calx, genitive calcis, meaning "lime") was known as early as the first century when the Ancient Romans prepared lime as calcium oxide. Literature dating back to 975 AD notes that plaster of paris (calcium sulfate), is useful for setting broken bones. It was not isolated until 1808 in England when Sir Humphry Davy electrolyzed a mixture of lime and mercuric oxide. Calcium metal was not available in large scale until the beginning of the 20th century.

Further reading

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

External links

• WebElements.com – Calcium
• Calcium at The Periodic Table of Videos (University of Nottingham)
• UK Food Standards Agency: Calcium
• Nutrition fact sheet from the National Institutes of Health