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Calcium is a
chemical element 400px|The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nuclei. Unlike chemical compounds, chemical elements cannot be br ...

chemical element
with the
symbol A symbol is a mark, sign, or word that indicates, signifies, or is understood as representing an idea, object, or relationship. Symbols allow people to go beyond what is known or seen by creating linkages between otherwise very different concep ...

symbol
Ca and
atomic number 300px|The Rutherford–Bohr model of the [[hydrogen atom () or a hydrogen-like ion (). In this model it is an essential feature that the photon energy (or frequency) of the electromagnetic radiation emitted (shown) when an electron jumps from one ...

atomic number
20. As an
alkaline earth metal The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are all shi ...

alkaline earth metal
, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues
strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when i ...

strontium
and
barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free eleme ...

barium
. It is the fifth most abundant element in Earth's crust, and the third most abundant metal, after
iron Iron () is a chemical element with symbol Fe (from la|ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is by mass the most common element on Earth, right in front of o ...

iron
and
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has a ...

aluminium
. The most common calcium compound on Earth is
calcium carbonate Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is t ...

calcium carbonate
, found in
limestone Limestone is a common type of carbonate sedimentary rock. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (). Limestone forms when these minerals precipitate out of water con ...

limestone
and the fossilised remnants of early sea life;
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk chalk, and drywall. A ma ...

gypsum
,
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ort ...

anhydrite
,
fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs scale of ...

fluorite
, and
apatite Apatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most co ...

apatite
are also sources of calcium. The name derives from
Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally spoken in the area around Rome, known as Latium. Through the power of the Roman Republic, it became the dominant language ...

Latin
''calx'' "
lime
lime
", which was obtained from heating limestone. Some calcium compounds were known to the ancients, though their chemistry was unknown until the seventeenth century. Pure calcium was isolated in 1808 via
electrolysis In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from natur ...

electrolysis
of its oxide by
Humphry Davy Sir Humphry Davy, 1st Baronet (17 December 177829 May 1829) was a Cornish chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, a series of elements for the ...

Humphry Davy
, who named the element. Calcium compounds are widely used in many industries: in foods and pharmaceuticals for
calcium supplementation Calcium supplements are salts of calcium used in a number of conditions. Supplementation is generally only required when there is not enough calcium in the diet. By mouth they are used to treat and prevent low blood calcium, osteoporosis, and ric ...

calcium supplementation
, in the paper industry as bleaches, as components in cement and electrical insulators, and in the manufacture of soaps. On the other hand, the metal in pure form has few applications due to its high reactivity; still, in small quantities it is often used as an alloying component in steelmaking, and sometimes, as a calcium–lead alloy, in making automotive batteries. Calcium is the most abundant metal and the fifth-most abundant element in the
human body The human body is the structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organ systems. They ensure homeostasis and the viability of the human body. It comprises a head, ne ...

human body
. As
electrolyte An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. The dissolved electrolyte separates into cations and anions, which disperse uniformly through the solvent. Electricall ...

electrolyte
s, calcium ions play a vital role in the
physiological Physiology (; ) is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical and phys ...

physiological
and
biochemical Biochemistry or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology and ...

biochemical
processes of organisms and
cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Closed spaces * Monastic cell, a small room, hut, or cave in which a monk or religious recluse lives * Prison cell, a room used to hold peopl ...

cell
s: in
signal transduction Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events, most commonly protein phosphorylation catalyzed by protein kinases, which ultimately results in a cellular res ...

signal transduction
pathways where they act as a
second messenger Second messengers are intracellular signaling molecules released by the cell in response to exposure to extracellular signaling molecules—the first messengers. (Intracellular signals, a non-local form or cell signaling, encompassing both first mes ...

second messenger
; in
neurotransmitter Neurotransmitters are chemical messengers that transmit a signal from a neuron across the synapse to a target cell, which can be a different neuron, muscle cell, or gland cell. Neurotransmitters are chemical substances made by the neuron specifi ...

neurotransmitter
release from
neurons A neuron or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. It is the main component of nervous tissue in all animals except sponges and placozoa. Plants and fungi do ...

neurons
; in contraction of all
muscle Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce ...

muscle
cell types; as cofactors in many
enzyme Enzymes () are proteins that act as biological catalysts (biocatalysts). Catalysts accelerate chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules k ...

enzyme
s; and in
fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proces ...

fertilization
. Calcium ions outside cells are important for maintaining the
potential difference Voltage, electric potential difference, electromotive force emf, electric pressure or electric tension is the difference in electric potential between two points, which (in a static electric field) is defined as the work needed per unit of ch ...

potential difference
across excitable
cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates the interior of all cells from the outside ...

cell membrane
s,
protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

protein
synthesis, and bone formation.


Characteristics


Classification

Calcium is a very ductile silvery metal (sometimes described as pale yellow) whose properties are very similar to the heavier elements in its group,
strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when i ...

strontium
,
barium Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free eleme ...

barium
, and
radium Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitroge ...

radium
. A calcium atom has twenty electrons, arranged in the
electron configuration In atomic physics and quantum chemistry, the electron configuration is the distribution of electrons of an atom or molecule (or other physical structure) in atomic or molecular orbitals. For example, the electron configuration of the neon atom i ...

electron configuration
s2. Like the other elements placed in group 2 of the periodic table, calcium has two
valence electron In chemistry and physics, a valence electron is an outer shell electron that is associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed; in a single covalent bond, both atoms in the bon ...

valence electron
s in the outermost s-orbital, which are very easily lost in chemical reactions to form a dipositive ion with the stable electron configuration of a
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemic ...

noble gas
, in this case
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in the Earth's atmosphere, at 0.934% (9340 ppmv). It is more than t ...

argon
. Hence, calcium is almost always
divalent In chemistry, the valence or valency of an element is the measure of its combining capacity with other atoms when it forms chemical compounds or molecules. Description The combining capacity, or affinity of an atom of a given element is determin ...

divalent
in its compounds, which are usually . Hypothetical univalent salts of calcium would be stable with respect to their elements, but not to
disproportionation In chemistry, disproportionation, sometimes called dismutation, is a redox reaction in which one compound of intermediate oxidation state converts to two compounds, one of higher and one of lower oxidation states. More generally, the term can be ap ...

disproportionation
to the divalent salts and calcium metal, because the
enthalpy of formationThe standard enthalpy of formation or standard heat of formation of a compound is the change of enthalpy during the formation of 1 mole of the substance from its constituent elements, with all substances in their standard states. The standard pressur ...

enthalpy of formation
of MX2 is much higher than those of the hypothetical MX. This occurs because of the much greater
lattice energy The lattice energy is the energy required to dissociate one mole of an ionic compound to its constituent ions in the gaseous state. It is a measure of the cohesive forces that bind ions. Lattice energy is relevant to many practical properties inc ...

lattice energy
afforded by the more highly charged Ca2+ cation compared to the hypothetical Ca+ cation.Greenwood and Earnshaw, pp. 112–3 Calcium, strontium, barium, and radium are always considered to be
alkaline earth metal The alkaline earth metals are six chemical elements in group 2 of the periodic table. They are beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).. The elements have very similar properties: they are all shi ...

alkaline earth metal
s; the lighter
beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a relatively rare element in the universe, usually occurring as a product of the spallation of larger atomic nuclei that have collided with cosmic rays. Within the co ...

beryllium
and
magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray solid which bears a close physical resemblance to the other five elements in the second column (group 2, or alkaline earth metals) of the per ...

magnesium
, also in group 2 of the periodic table, are often included as well. Nevertheless, beryllium and magnesium differ significantly from the other members of the group in their physical and chemical behaviour: they behave more like
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has a ...

aluminium
and
zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a silvery-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodic t ...

zinc
respectively and have some of the weaker metallic character of the
post-transition metal The metallic elements in the periodic table located between the transition metals and the weakly nonmetallic metalloids have received many names in the literature, such as ''post-transition metals'', ''poor metals'', ''other metals'', ''p-block ...

post-transition metal
s, which is why the traditional definition of the term "alkaline earth metal" excludes them. This classification is mostly obsolete in English-language sources, but is still used in other countries such as Japan. As a result, comparisons with strontium and barium are more germane to calcium chemistry than comparisons with magnesium.


Physical

Calcium metal melts at 842 °C and boils at 1494 °C; these values are higher than those for magnesium and strontium, the neighbouring group 2 metals. It crystallises in the [[face-centered cubic arrangement like strontium; above 450 °C, it changes to an [[anisotropy|anisotropic [[hexagonal close-packed arrangement like magnesium. Its density of 1.55 g/cm3 is the lowest in its group. Calcium is harder than [[lead but can be cut with a knife with effort. While calcium is a poorer conductor of electricity than [[copper or
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has a ...

aluminium
by volume, it is a better conductor by mass than both due to its very low density. While calcium is infeasible as a conductor for most terrestrial applications as it reacts quickly with atmospheric oxygen, its use as such in space has been considered.Hluchan and Pomerantz, p. 484


Chemical

The chemistry of calcium is that of a typical heavy alkaline earth metal. For example, calcium spontaneously reacts with water more quickly than magnesium and less quickly than strontium to produce [[calcium hydroxide and hydrogen gas. It also reacts with the [[oxygen and [[nitrogen in the air to form a mixture of [[calcium oxide and [[calcium nitride.C. R. Hammond ''The elements'' (p. 4–35) in When finely divided, it spontaneously burns in air to produce the nitride. In bulk, calcium is less reactive: it quickly forms a hydration coating in moist air, but below 30% [[relative humidity it may be stored indefinitely at room temperature.Hluchan and Pomerantz, p. 483 Besides the simple oxide CaO, the [[peroxide CaO2 can be made by direct oxidation of calcium metal under a high pressure of oxygen, and there is some evidence for a yellow [[superoxide Ca(O2)2. Calcium hydroxide, Ca(OH)2, is a strong base, though it is not as strong as the hydroxides of strontium, barium or the alkali metals. All four dihalides of calcium are known. [[Calcium carbonate (CaCO3) and [[calcium sulfate (CaSO4) are particularly abundant minerals.Greenwood and Earnshaw, pp. 122–5 Like strontium and barium, as well as the alkali metals and the divalent [[lanthanides [[europium and [[ytterbium, calcium metal dissolves directly in liquid [[ammonia to give a dark blue solution. Due to the large size of the Ca2+ ion, high coordination numbers are common, up to 24 in some [[intermetallic compounds such as CaZn13. Calcium is readily complexed by oxygen [[chelates such as [[ethylenediaminetetraacetic acid|EDTA and [[polyphosphates, which are useful in [[analytic chemistry and removing calcium ions from [[hard water. In the absence of [[steric hindrance, smaller group 2 cations tend to form stronger complexes, but when large [[polydentate [[macrocycles are involved the trend is reversed. Although calcium is in the same group as magnesium and [[organomagnesium compounds are very commonly used throughout chemistry, organocalcium compounds are not similarly widespread because they are more difficult to make and more reactive, although they have recently been investigated as possible [[catalysts. Organocalcium compounds tend to be more similar to organoytterbium compounds due to the similar [[ionic radius|ionic radii of Yb2+ (102 pm) and Ca2+ (100 pm). Most of these compounds can only be prepared at low temperatures; bulky ligands tend to favor stability. For example, calcium di[[cyclopentadienyl, Ca(C5H5)2, must be made by directly reacting calcium metal with [[mercurocene or [[cyclopentadiene itself; replacing the C5H5 ligand with the bulkier C5(CH3)5 ligand on the other hand increases the compound's solubility, volatility, and kinetic stability.


Isotopes

Natural calcium is a mixture of five stable [[isotopes (40Ca, 42Ca, 43Ca, 44Ca, and 46Ca) and one isotope with a half-life so long that it can be considered stable for all practical purposes ([[calcium-48|48Ca, with a half-life of about 4.3 × 1019 years). Calcium is the first (lightest) element to have six naturally occurring isotopes. By far the most common isotope of calcium in nature is 40Ca, which makes up 96.941% of all natural calcium. It is produced in the [[silicon-burning process from fusion of [[alpha particles and is the heaviest stable nuclide with equal proton and neutron numbers; its occurrence is also supplemented slowly by the decay of [[primordial nuclide|primordial [[potassium-40|40K. Adding another alpha particle leads to unstable 44Ti, which quickly decays via two successive [[electron captures to stable 44Ca; this makes up 2.806% of all natural calcium and is the second-most common isotope. The other four natural isotopes, 42Ca, 43Ca, 46Ca, and 48Ca, are significantly rarer, each comprising less than 1% of all natural calcium. The four lighter isotopes are mainly products of the [[oxygen-burning process|oxygen-burning and silicon-burning processes, leaving the two heavier ones to be produced via [[neutron capture processes. 46Ca is mostly produced in a "hot" [[s-process, as its formation requires a rather high neutron flux to allow short-lived 45Ca to capture a neutron. 48Ca is produced by electron capture in the [[r-process in [[type Ia supernovae, where high neutron excess and low enough entropy ensures its survival. 46Ca and 48Ca are the first "classically stable" nuclides with a six-neutron or eight-neutron excess respectively. Although extremely neutron-rich for such a light element, 48Ca is very stable because it is a [[magic number (physics)|doubly magic nucleus, having 20 protons and 28 neutrons arranged in closed shells. Its [[beta decay to 48[[scandium|Sc is very hindered because of the gross mismatch of [[nuclear spin: 48Ca has zero nuclear spin, being [[even and odd atomic nuclei|even–even, while 48Sc has spin 6+, so the decay is [[forbidden mechanism|forbidden by the conservation of [[angular momentum. While two excited states of 48Sc are available for decay as well, they are also forbidden due to their high spins. As a result, when 48Ca does decay, it does so by [[double beta decay to 48[[titanium|Ti instead, being the lightest nuclide known to undergo double beta decay. The heavy isotope 46Ca can also theoretically undergo double beta decay to 46Ti as well, but this has never been observed; the lightest and most common isotope 40Ca is also doubly magic and could undergo [[double electron capture to 40[[argon|Ar, but this has likewise never been observed. Calcium is the only element to have two primordial doubly magic isotopes. The experimental lower limits for the half-lives of 40Ca and 46Ca are 5.9 × 1021 years and 2.8 × 1015 years respectively. Apart from the practically stable 48Ca, the longest lived [[radioisotope of calcium is 41Ca. It decays by electron capture to stable 41[[potassium|K with a half-life of about a hundred thousand years. Its existence in the early Solar System as an [[extinct radionuclide has been inferred from excesses of 41K: traces of 41Ca also still exist today, as it is a [[cosmogenic nuclide, continuously reformed through [[neutron activation of natural 40Ca. Many other calcium radioisotopes are known, ranging from 35Ca to 60Ca. They are all much shorter-lived than 41Ca, the most stable among them being 45Ca (half-life 163 days) and 47Ca (half-life 4.54 days). The isotopes lighter than 42Ca usually undergo [[beta plus decay to isotopes of potassium, and those heavier than 44Ca usually undergo [[beta minus decay to isotopes of [[scandium, although near the [[nuclear drip lines, [[proton emission and [[neutron emission begin to be significant decay modes as well. Like other elements, a variety of processes alter the relative abundance of calcium isotopes. The best studied of these processes is the mass-dependent [[Isotope fractionation|fractionation of calcium isotopes that accompanies the precipitation of calcium minerals such as [[calcite, [[aragonite and
apatite Apatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most co ...

apatite
from solution. Lighter isotopes are preferentially incorporated into these minerals, leaving the surrounding solution enriched in heavier isotopes at a magnitude of roughly 0.025% per atomic mass unit (amu) at room temperature. Mass-dependent differences in calcium isotope composition are conventionally 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.


History

Calcium compounds were known for millennia, although their chemical makeup was not understood until the 17th century.Greenwood and Earnshaw, p. 108 Lime as a [[lime (material)|building material and as [[lime plaster|plaster for statues was used as far back as around 7000 BC. The first dated [[lime kiln dates back to 2500 BC and was found in [[Khafajah, [[Mesopotamia. At about the same time, dehydrated
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk chalk, and drywall. A ma ...

gypsum
(CaSO4·2H2O) was being used in the [[Great Pyramid of Giza; this material would later be used for the plaster in the tomb of [[Tutankhamun. The [[ancient Romans instead used lime mortars made by heating
limestone Limestone is a common type of carbonate sedimentary rock. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (). Limestone forms when these minerals precipitate out of water con ...

limestone
(CaCO3); the name "calcium" itself derives from the Latin word ''calx'' "lime". [[Vitruvius noted that the lime that resulted was lighter than the original limestone, attributing this to the boiling of the water; in 1755, [[Joseph Black proved that this was due to the loss of [[carbon dioxide, which as a gas had not been recognised by the ancient Romans. In 1787, [[Antoine Lavoisier suspected that lime might be an oxide of a fundamental
chemical element 400px|The periodic table of the chemical elements In chemistry, an element is a pure substance consisting only of atoms that all have the same numbers of protons in their atomic nuclei. Unlike chemical compounds, chemical elements cannot be br ...

chemical element
. In his table of the elements, Lavoisier listed five "salifiable earths" (i.e., ores that could be made to react with acids to produce salts (''salis'' = salt, in Latin): ''chaux'' (calcium oxide), ''magnésie'' (magnesia, magnesium oxide), ''baryte'' (barium sulfate), ''alumine'' (alumina, aluminium oxide), and ''silice'' (silica, silicon dioxide)). About these "elements", Lavoisier speculated: Calcium, along with its congeners magnesium, strontium, and barium, was first isolated by
Humphry Davy Sir Humphry Davy, 1st Baronet (17 December 177829 May 1829) was a Cornish chemist and inventor who invented the Davy lamp and a very early form of arc lamp. He is also remembered for isolating, by using electricity, a series of elements for the ...

Humphry Davy
in 1808. Following the work of [[Jöns Jakob Berzelius and [[Magnus Martin af Pontin on
electrolysis In chemistry and manufacturing, electrolysis is a technique that uses direct electric current (DC) to drive an otherwise non-spontaneous chemical reaction. Electrolysis is commercially important as a stage in the separation of elements from natur ...

electrolysis
, Davy isolated calcium and magnesium by putting a mixture of the respective metal oxides with [[mercury(II) oxide on a [[platinum plate which was used as the anode, the cathode being a platinum wire partially submerged into mercury. Electrolysis then gave calcium–mercury and magnesium–mercury amalgams, and distilling off the mercury gave the metal. However, pure calcium cannot be prepared in bulk by this method and a workable commercial process for its production was not found until over a century later.


Occurrence and production

At 3%, calcium is the fifth [[Abundance of elements in Earth's crust|most abundant element in the Earth's crust, and the third most abundant metal behind
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has a ...

aluminium
and
iron Iron () is a chemical element with symbol Fe (from la|ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is by mass the most common element on Earth, right in front of o ...

iron
. It is also the fourth most abundant element in the [[lunar highlands. [[Sedimentary rocks|Sedimentary
calcium carbonate Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is t ...

calcium carbonate
deposits pervade the Earth's surface as fossilized remains of past marine life; they occur in two forms, the [[rhombohedral [[calcite (more common) and the [[orthorhombic [[aragonite (forming in more temperate seas). Minerals of the first type include
limestone Limestone is a common type of carbonate sedimentary rock. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (). Limestone forms when these minerals precipitate out of water con ...

limestone
, [[Dolomite (mineral)|dolomite, [[marble, [[chalk, and [[iceland spar; aragonite beds make up the [[Bahamas, the [[Florida Keys, and the [[Red Sea basins. [[Corals, [[sea shells, and [[pearls are mostly made up of calcium carbonate. Among the other important minerals of calcium are
gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/sidewalk chalk, and drywall. A ma ...

gypsum
(CaSO4·2H2O),
anhydrite Anhydrite, or anhydrous calcium sulfate, is a mineral with the chemical formula CaSO4. It is in the orthorhombic crystal system, with three directions of perfect cleavage parallel to the three planes of symmetry. It is not isomorphous with the ort ...

anhydrite
(CaSO4),
fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs scale of ...

fluorite
(CaF2), and
apatite Apatite is a group of phosphate minerals, usually referring to hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most co ...

apatite
([Ca5(PO4)3F]). The major producers of calcium are [[China (about 10000 to 12000 [[tonnes per year), [[Russia (about 6000 to 8000 tonnes per year), and the [[United States (about 2000 to 4000 tonnes per year). [[Canada and [[France are also among the minor producers. In 2005, about 24000 tonnes of calcium were produced; about half of the world's extracted calcium is used by the United States, with about 80% of the output used each year. In Russia and China, Davy's method of electrolysis is still used, but is instead applied to molten [[calcium chloride. Since calcium is less reactive than strontium or barium, the oxide–nitride coating that results in air is stable and [[lathe machining and other standard metallurgical techniques are suitable for calcium.Greenwood and Earnshaw, p. 110 In the United States and Canada, calcium is instead produced by reducing lime with aluminium at high temperatures.


Geochemical cycling

[[Calcium cycle|Calcium cycling 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. These ions are transported to the ocean where they react with dissolved CO2 to form
limestone Limestone is a common type of carbonate sedimentary rock. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (). Limestone forms when these minerals precipitate out of water con ...

limestone
(), which in turn settles to the sea floor where it is incorporated into new rocks. Dissolved CO2, along with [[carbonate and [[bicarbonate ions, are termed "[[Total inorganic carbon|dissolved inorganic carbon" (DIC). The actual reaction is more complicated and involves the bicarbonate ion (HCO) that forms when CO2 reacts with water at seawater [[pH: : + 2 → ([[Solid|s) + + At seawater pH, most of the CO2 is immediately converted back into . 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.


Uses

The largest use of metallic calcium is in [[steelmaking, due to its strong [[chemical affinity for oxygen and [[sulfur. Its oxides and sulfides, once formed, give liquid lime [[aluminate and sulfide inclusions in steel which float out; on treatment, these inclusions disperse throughout the steel and became small and spherical, improving castability, cleanliness and general mechanical properties. Calcium is also used in maintenance-free [[automotive battery|automotive batteries, in which the use of 0.1% calcium–[[lead alloys instead of the usual [[antimony–lead alloys leads to lower water loss and lower self-discharging. Due to the risk of expansion and cracking,
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has a ...

aluminium
is sometimes also incorporated into these alloys. These lead–calcium alloys are also used in casting, replacing lead–antimony alloys.Hluchan and Pomerantz, pp. 485–7 Calcium is also used to strengthen aluminium alloys used for bearings, for the control of graphitic [[carbon in [[cast iron, and to remove [[bismuth impurities from lead. Calcium metal is found in some drain cleaners, where it functions to generate heat and [[calcium hydroxide that [[Saponification|saponifies the fats and liquefies the proteins (for example, those in hair) that block drains. Besides metallurgy, the reactivity of calcium is exploited to remove [[nitrogen from high-purity
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in the Earth's atmosphere, at 0.934% (9340 ppmv). It is more than t ...

argon
gas and as a [[getter for oxygen and nitrogen. It is also used as a reducing agent in the production of [[chromium, [[zirconium, [[thorium, and [[uranium. It can also be used to store hydrogen gas, as it reacts with hydrogen to form solid [[calcium hydride, from which the hydrogen can easily be re-extracted. 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 44Ca/40Ca ratio in soft tissue rises and vice versa. 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 seawater, where 44Ca/40Ca tends to rise when the rate of removal of Ca2+ by mineral precipitation exceeds the input of new calcium into the ocean. 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 is never in a "steady state" with respect to calcium input and output. This has important climatological implications, as the marine calcium cycle is closely tied to the [[carbon cycle. Many calcium compounds are used in food, as pharmaceuticals, and in medicine, among others. For example, calcium and phosphorus are supplemented in foods through the addition of [[calcium lactate, [[calcium diphosphate, and [[tricalcium phosphate. The last is also used as a polishing agent in [[toothpaste and in [[antacids. [[Calcium lactobionate is a white powder that is used as a suspending agent for pharmaceuticals. In baking, [[calcium monophosphate is used as a [[leavening agent. [[Calcium sulfite is used as a bleach in papermaking and as a disinfectant, [[calcium silicate is used as a reinforcing agent in rubber, and [[calcium acetate is a component of [[liming rosin and is used to make metallic soaps and synthetic resins. Calcium is on the [[WHO Model List of Essential Medicines|World Health Organization's List of Essential Medicines.


Food sources

Foods rich in calcium include [[dairy products, such as [[yogurt and [[cheese, [[sardines, [[salmon, [[soy products, [[kale, and [[food fortification|fortified [[breakfast cereals. Because of concerns for long-term adverse side effects, including calcification of arteries and [[kidney stones, both the U.S. [[Institute of Medicine (IOM) and the [[European Food Safety Authority (EFSA) set [[Tolerable upper intake level|Tolerable Upper Intake Levels (ULs) for combined dietary and supplemental calcium. From the IOM, people of ages 9–18 years are not to exceed 3 g/day combined intake; for ages 19–50, not to exceed 2.5 g/day; for ages 51 and older, not to exceed 2 g/day. EFSA set the UL for all adults at 2.5 g/day, but decided the information for children and adolescents was not sufficient to determine ULs.


Biological and pathological role


Function

Calcium is an [[essential element needed in large quantities. The Ca2+ ion acts as an
electrolyte An electrolyte is a substance that produces an electrically conducting solution when dissolved in a polar solvent, such as water. The dissolved electrolyte separates into cations and anions, which disperse uniformly through the solvent. Electricall ...

electrolyte
and is vital to the health of the muscular, circulatory, and digestive systems; is indispensable to the building of bone; and supports synthesis and function of blood cells. For example, it regulates the contraction of muscles, nerve conduction, and the clotting of blood. As a result, intra- and extracellular calcium levels are tightly regulated by the body. Calcium can play this role because the Ca2+ ion forms stable [[coordination complexes with many organic compounds, especially
protein Proteins are large biomolecules or macromolecules that are comprised of one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

protein
s; it also forms compounds with a wide range of solubilities, enabling the formation of the [[skeleton. Sosa Torres, Martha; Kroneck, Peter M.H; "Introduction: From Rocks to Living Cells" pp 1-32 in "Metals, Microbes and Minerals: The Biogeochemical Side of Life" (2021) pp xiv + 341. Walter de Gruyter, Berlin. Editors Kroneck, Peter M.H. and Sosa Torres, Martha
DOI 10.1515/9783110589771-001
/ref>


Binding

Calcium ions may be complexed by proteins through binding the [[carboxyl groups of [[glutamic acid or [[aspartic acid residues; through interacting with [[phosphorylation|phosphorylated [[serine, [[tyrosine, or [[threonine residues; or by being [[chelation|chelated by γ-carboxylated amino acid residues. [[Trypsin, a digestive enzyme, uses the first method; [[osteocalcin, a bone matrix protein, uses the third. Some other bone matrix proteins such as [[osteopontin and [[bone sialoprotein use both the first and the second. Direct activation of enzymes by binding calcium is common; some other enzymes are activated by noncovalent association with direct calcium-binding enzymes. Calcium also binds to the [[phospholipid layer of the
cell membrane cell membrane vs. Prokaryotes The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates the interior of all cells from the outside ...

cell membrane
, anchoring proteins associated with the cell surface.Hluchan and Pomerantz, pp. 489–94


Solubility

As an example of the wide range of solubility of calcium compounds, [[monocalcium phosphate is very soluble in water, 85% of extracellular calcium is as [[dicalcium phosphate with a solubility of 2.0 [[Molar concentration|mM and the [[hydroxyapatite of bones in an organic matrix is [[tricalcium phosphate at 100 μM.


Nutrition

Calcium is a common constituent of [[multivitamin [[dietary supplements, but the composition of calcium complexes in supplements may affect its bioavailability which varies by solubility of the salt involved: [[calcium citrate, [[Calcium malate|malate, and [[Calcium lactate|lactate are highly bioavailable, while the [[Calcium oxalate|oxalate is less. Other calcium preparations include
calcium carbonate Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks as the minerals calcite and aragonite (most notably as limestone, which is a type of sedimentary rock consisting mainly of calcite) and is t ...

calcium carbonate
, [[calcium citrate malate, and [[calcium gluconate. The intestine absorbs about one-third of calcium eaten as the [[Radical ion|free ion, and plasma calcium level is then regulated by the [[kidneys.


Hormonal regulation of bone formation and serum levels

[[Parathyroid hormone and [[vitamin D promote the formation of bone by allowing and enhancing the deposition of calcium ions there, allowing rapid bone turnover without affecting bone mass or mineral content. When plasma calcium levels fall, cell surface receptors are activated and the secretion of parathyroid hormone occurs; it then proceeds to stimulate the entry of calcium into the plasma pool by taking it from targeted kidney, gut, and bone cells, with the bone-forming action of parathyroid hormone being antagonised by [[calcitonin, whose secretion increases with increasing plasma calcium levels.


Abnormal serum levels

Excess intake of calcium may cause [[hypercalcemia. However, because calcium is absorbed rather inefficiently by the intestines, high serum calcium is more likely caused by excessive secretion of parathyroid hormone (PTH) or possibly by excessive intake of vitamin D, both of which facilitate calcium absorption. All these conditions result in excess calcium salts being deposited in the heart, blood vessels, or kidneys. Symptoms include anorexia, nausea, vomiting, memory loss, confusion, muscle weakness, increased urination, dehydration, and metabolic bone disease. Chronic hypercalcaemia typically leads to [[calcification of soft tissue and its serious consequences: for example, calcification can cause loss of elasticity of [[vascular walls and disruption of laminar blood flow—and thence to [[Vulnerable plaque|plaque rupture and [[thrombosis. Conversely, inadequate calcium or vitamin D intakes may result in [[hypocalcemia, often caused also by inadequate secretion of parathyroid hormone or defective PTH receptors in cells. Symptoms include neuromuscular excitability, which potentially causes [[tetany and disruption of conductivity in cardiac tissue.


Kidney stones


Bone disease

As calcium is required for bone development, many bone diseases can be traced to the organic matrix or the [[hydroxyapatite in molecular structure or organization of bone. [[Osteoporosis is a reduction in mineral content of bone per unit volume, and can be treated by supplementation of calcium, vitamin D, and [[bisphosphonates. Inadequate amounts of calcium, vitamin D, or phosphates can lead to softening of bones, called [[osteomalacia.


Safety


Metallic calcium

Because calcium reacts exothermically with water and acids, calcium metal coming into contact with bodily moisture results in severe corrosive irritation. When swallowed, calcium metal has the same effect on the mouth, oesophagus, and stomach, and can be fatal.Rumack BH. POISINDEX. Information System Micromedex, Inc., Englewood, CO, 2010; CCIS Volume 143. Hall AH and Rumack BH (Eds) However, long-term exposure is not known to have distinct adverse effects.Hluchan and Pomerantz, pp. 487–9


See also


References


Bibliography

* * {{Authority control [[Category:Calcium| [[Category:Chemical elements [[Category:Alkaline earth metals [[Category:Dietary minerals [[Category:Dietary supplements [[Category:Reducing agents [[Category:Sodium channel blockers [[Category:World Health Organization essential medicines [[Category:WikiProject Elements pages using ENGVAR|GB