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Calcium (pronounced /ˈkælsiəm/) is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft grey alkaline earth metal, and is the fifth most abundant element by mass in the Earth's crust. Calcium is also the fifth most abundant dissolved ion in seawater by both molarity and mass, after sodium, chloride, magnesium, and sulfate.

Calcium is essential for living organisms, particularly in cell physiology, where movement of the calcium ion Ca2+ into and out of the cytoplasm functions as a signal for many cellular processes. As a major material used in mineralization of bones and shells, calcium is the most abundant metal by mass in many animals.

Notable characteristics
Chemically calcium is reactive and soft for a metal (though harder than lead, it can be cut with a knife with difficulty). It is a silvery metallic element that must be extracted by electrolysis from a fused salt like calcium chloride.[2] Once produced, it rapidly forms a grey-white oxide and nitride coating when exposed to air. It is somewhat difficult to ignite, unlike magnesium, but when lit, the metal burns in air with a brilliant high-intensity red light. Calcium metal reacts with water, evolving hydrogen gas at a rate rapid enough to be noticeable, but not fast enough at room temperature to generate much heat. In powdered form, however, the reaction with water is extremely rapid, as the increased surface area of the powder accelerates the reaction with the water. Part of the slowness of the calcium-water reaction results from the metal being partly protected by insoluble white calcium hydroxide. In water solutions of acids where the salt is water soluble, calcium reacts vigorously.

Calcium, with a specific mass of 1.55 g/cm3, is the lightest of the alkali earth metals; magnesium is heavier (1.74) and beryllium even more heavy (1.84) despite these two elements being lighter in atomic mass. From strontium on the alkali earth metals get heavier along with the atomic mass.

Calcium has a higher resistivity than copper or aluminium. Yet, weight for weight, allowing for its much lower density, it is a rather better conductor than either. However, its use in terrestrial applications is usually limited by its high reactivity with air.

Calcium salts are colorless from any contribution of the calcium, and ionic solutions of calcium (Ca2+) are colorless as well. Many calcium salts are not soluble in water. When in solution, the calcium ion to the human taste varies remarkably, 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.[3] In human nutrition, soluble calcium salts may be added to tart juices without much effect to the average palate.

Calcium is the fifth most abundant element by mass in the human body, where it is a common cellular ionic messenger with many functions, and serves also as a structural element in bone. It is the relatively high atomic-numbered calcium in the skeleton which causes bone to be radio-opaque. Of the human body's solid components after drying (as for example, after cremation), about a third of the total mass is the approximately one kilogram of calcium which composes the average skeleton (the remainder being mostly phosphorus and oxygen).

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 also occurs in igneous and metamorphic rocks chiefly in the silicate minerals: plagioclase, amphiboles, pyroxenes and garnets.

See also Calcium minerals.

Applications
Some uses are:

as a reducing agent in the extraction of other metals, such as uranium, zirconium, and thorium.
as a deoxidizer, desulfurizer, or decarbonizer for various ferrous and nonferrous alloys.
as an alloying agent used in the production of aluminium, beryllium, copper, lead, and magnesium alloys.
in the making of cements and mortars to be used in construction.
in the making of cheese, where calcium ions influence the activity of rennin in bringing about the coagulation of milk.

Calcium compounds
Calcium carbonate (CaCO3) used in manufacturing cement and mortar, lime, limestone (usually used in the steel industry); aids in production in the glass industry, 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 by being bubbled through a solution. It 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 in the manufacturing of plastics.
Calcium chloride (CaCl2) is used: in ice removal and dust control on dirt roads, in conditioner for concrete, as an additive in canned tomatoes, and to provide body for automobile tires.
Calcium cyclamate (Ca(C6H11NHSO3)2) was used as a sweetening agent but is no longer permitted for use because of suspected cancer-causing properties.
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) is used as common blackboard chalk, as well as, in its hemihydrate form being more well known as 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. Also carbonated-calcium deficient hydroxylapatite is the main mineral of which dental enamel and dentin are comprised.
元素简介
　　【中文名称】钙新生元乳钙上海伽蓝生物科技【英文名称】calcium
　　【结构或分子式】（Ca2+）
　　【相对分子量或原子量】40.078
　　【密度】1.55
　　【熔点】1115 K
　　(842 °C, 1548 °F)
　　【沸点】 1757 K
　　(1484 °C, 2703 °F)
　　【氧化态】：
　　Main Ca+2
　　Other
　　【性状】
　　银白色金属，质软。
　　莫氏硬度：1.75
　　【溶解情况】
　　溶于酸，常温下与水发生剧烈反应，生成氢氧化钙、氢气，氢氧化钙微溶于水。
　　Ca+H2O====Ca(OH)2+H2↑
　　【用途】
　　用作合金的脱氧剂、油类的脱水剂、冶金的还原剂、铁和铁合金的脱硫与脱碳剂以及电子管中的吸气剂等。
　　【制备或来源】
　　金属钙可由电解熔融的氯化钙而制得。也可用金属铝在真空和高温下还原石灰，再经精馏而得。
　　【其他】
　　原子体积:(立方厘米/摩尔)
　　25.9
　　元素在太阳中的含量:(ppm) (ppm是浓度单位,1ppm表示1单位溶剂中含有百万分之一单位的溶质)
　　70
　　元素在海水中的含量:(ppm)
　　390
　　地壳中含量：（ppm）
　　41000
　　电离能 (kJ /mol)
　　M - M+ 589.7
　　M+ - M2+ 1145
　　M2+ - M3+ 4910
　　M3+ - M4+ 6474
　　M4+ - M5+ 8144
　　M5+ - M6+ 10496
　　M6+ - M7+ 12320
　　M7+ - M8+ 14207
　　M8+ - M9+ 18191
　　M9+ - M10+ 20385
　　晶胞参数：
　　a = 558.84 pm
　　b = 558.84 pm
　　c = 558.84 pm
　　α = 90°
　　β = 90°
　　γ = 90°
　　原子序数：20，原子量40.078，化合价+2。
　　氧化物离解能（Do）：5.0（eV）
　　元素电离能（Ei）： 6.11（eV）
　　主要吸收线及其主要参数：
　　┌────┬───┬──┬──┬──┬───┬───┐
　　│λ(nm) │ f │ W │ F │ S* │ CL │ R·S │
　　├────┼───┼──┼──┼──┼───┼───┤
　　│422.7 │1.49 │ 0.7│A-A │0.06│0.005 │ │
　　│422.7 │ │ 0.7│N-A │0.03│0.005 │1.0 │
　　│239.9 │0.037 │ 0.7│N-A │20 │ │120 │
　　│445.5 │ │ 0.7│N-A │ │ │ │
　　└────┴───┴──┴──┴──┴───┴───┘
　　λ：波长
　　f：振子强度
　　W：单色器光谱通带
　　N-A（氧化亚氮-乙炔焰）
　　S*：元素的特征浓度（1%吸收灵敏度）
　　CL：元素的检测极限
　　R·S：同一元素主要吸收线间的相对灵敏度
　　F：火焰类型
　　元素名称：钙
　　元素符号：Ca
　　元素原子量：40.08
　　声音在其中的传播速率：（m/S）
　　3810
　　元素类型：金属元素
　　质子数：20
　　中子数：20
　　原子序数：20
　　所属周期：3
　　所属族数：IIA
　　电子层分布：2-8-8-2
　　电子排布式：1s22s22p63s23p64s2
　　晶体结构：晶胞为面心立方晶胞，每个晶胞含有4个金属原子。
　　发现人：戴维、贝采利乌斯、蓬丁 发现年代：1808年
　　钙的有关化学方程式：
　　Ca+2HCl=CaCl2+H2 ↑
　　N2+3Ca=Ca3N2
　　3C+CaO=CaC2+CO ↑
　　CO2+Ca(OH)2(过量)=CaCO3↓+H2O
　　2CO2(过量)+Ca(OH)2=Ca(HCO3)2
　　SO2+Ca(OH)2===CaSO3+H2O
　　SO3+Ca(OH)2=CaSO4↓+H2O
　　2HNO3+CaCO3=Ca（NO3）2+H2O+CO2 ↑
　　2Cl2+2Ca（OH）2=CaCl2+Ca（ClO）2+2H2O
　　CaH2+2H2O=Ca(OH)2+2H2 ↑
　　2HF+CaCl2=CaF2+2HCl
　　SiO2+CaO=CaSiO3
　　SiO2+CaCO3=CaSiO3+CO2 ↑
　　Ca(HCO3)2=CaCO3+H2O+CO2 ↑
　　CaCO3=CaO+CO2 ↑
发现过程
　　1808年5月，英国化学家戴维电解石灰与氧化汞的混合物，得到钙汞合金，将合金中的汞蒸馏后，就获得了银白色的金属钙。瑞典的贝采利乌斯、法国的蓬丁，使用汞阴极电解石灰，在阴极的汞齐中提出金属钙。
元素描述
　　银白色的轻金属。质软。密度1.54克/厘米3。熔点839±2℃。沸点1484℃。化合价+2。电离能6.113电子伏特。化学性质活泼，能与水、酸反应，有氢气产生。在空气在其表面会形成一层氧化物和氮化物薄膜，以防止继续受到腐蚀。加热时，几乎能还原所有的金属氧化物。
元素来源
　　在自然界分布广，以化合物的形态存在，如石灰石、白垩、大理石、石膏、磷灰石等；也存在于血浆和骨骼中，并参与凝血和肌肉的收缩过程。金属钙可由电解熔融的氯化钙而制得；也可用金属在真空中还原石灰，再经蒸馏而获得。
元素用途
　　用来作合金的脱氧剂，以及油类的脱水剂等。
   元素辅助资料
　　钙是在自然界中分布最广的十个元素之一，但由于它不易从化合物中还原成单质状态，所以迟迟未被发现。
　　长时期里，化学家们将从含碳酸钙的石灰石焙烧获得的钙的氧化物当作是不可再分割的物质。在1789年拉瓦锡发表的元素表中就列有它。但戴维不顾这些，在1808年开始对氧化钙进行电解。戴维刚开始选用的方法并不理想，所以无法将金属钙分离出来。到1808年5月，戴维从贝齐里乌斯和瑞典皇家医生蓬丁共同电解生石灰和水银的混合物取得钙的实验中获得了启发。他将湿润的生石灰和氧化汞按3比1的比例混合后，放置在一铂片上，与电池的正极相接，然后又在混合物中作一洼穴，灌入水银，插入一铂丝，与电池的负极相接，得到较大量钙汞合金。把钙汞合金经蒸馏后得到了银白色的金属钙。
　　从此钙被确定为元素，并被命名为calcium，元素符号是Ca。calcium来自拉丁文中表示生石灰的词calx。