Calcium

Calcium — Material Page

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Calcium (Ca, atomic number 20) is a soft, silvery-white alkaline earth metal in Group 2 of the periodic table, and the fifth most abundant element in Earth's crust (~4.1 wt%), where it occurs almost exclusively as carbonates (calcite, aragonite, dolomite), sulfates (gypsum, anhydrite), phosphates (apatite), and silicates. With a density of 1.55 g/cm³ and a face-centered cubic (FCC) crystal structure at room temperature (transitioning to BCC above 450 °C), calcium is the lightest of the alkaline earth metals after beryllium and magnesium. Its melting point of 842 °C and thermal conductivity of 201 W/m·K are both significantly higher than those of its Group 2 neighbors magnesium and strontium, reflecting its intermediate position in the group. Calcium has six stable isotopes, of which ⁴⁰Ca (96.94%) is overwhelmingly dominant — indeed ⁴⁰Ca is the most abundant isotope of any element heavier than argon in the solar system, produced in vast quantities by oxygen burning and silicon burning in massive stars. Its electrochemical standard potential of –2.87 V vs. SHE makes it one of the strongest reducing agents among common metals.

In metallurgy and chemical industry, elemental calcium's value lies almost entirely in its powerful reducing and scavenging capabilities rather than its structural properties. Calcium is the preferred metallothermic reductant for producing high-purity uranium, thorium, and rare earth metals from their halides — the Goldschmidt process — because it forms highly stable CaF₂ or CaCl₂ slag that separates cleanly from the target metal. In steelmaking, calcium wire injection (as Ca-Si or Ca-Fe-Si cored wire) is the standard method for deep desulfurization and oxide inclusion modification: calcium converts elongated MnS inclusions into spherical CaS and Ca-Al-O globular inclusions that do not initiate fatigue cracks in the rolling direction, dramatically improving the transverse toughness and machinability of clean steels. Calcium is also the key deoxidizer in lead battery grid alloys and the grain-refining agent added to aluminum alloys for improved castability.

Beyond metallurgy, calcium's most scientifically exciting current frontier is electrochemical energy storage, where Ca-metal anodes offer a compelling theoretical alternative to lithium-ion technology. With a volumetric capacity of 2,072 mAh/cm³ (versus 2,046 for Li), a standard potential of –2.87 V (close to Li's –3.04 V), natural abundance roughly 2,500× greater than lithium, and no supply chain concentration risk, calcium is the leading candidate for post-lithium battery chemistries. The primary challenge — identifying electrolytes that allow reversible Ca²⁺ stripping and plating without forming a blocking passivation layer — has seen rapid progress since 2015 with the discovery of Ca(BH₄)₂/THF and fluorinated borate electrolytes enabling room-temperature plating. The ⁴⁸Ca isotope is also central to nuclear physics: bombarding ²⁴⁸Cm with ⁴⁸Ca beams at JINR Dubna produced elements 114 (flerovium) through 118 (oganesson), completing Period 7 of the periodic table.

General Properties

Property	Value	Notes
Atomic Number	20	Group 2 (alkaline earth metal), Period 4; between magnesium (12) and strontium (38) in the same group; fifth most abundant element in Earth's crust
Atomic Mass	40.078 u	Six stable isotopes; ⁴⁰Ca (96.94%) dominates — the most abundant isotope of any element heavier than argon in the solar system, produced by oxygen burning in massive stars
Density (20 °C)	1.55 g/cm³	Lightest alkaline earth metal after Be (1.85) and Mg (1.74); less dense than aluminum (2.70); floats on most organic solvents; not used structurally due to high reactivity
Melting Point	842 °C (1,115 K)	Significantly higher than Mg (650 °C) and lower than Sr (777 °C) and Ba (727 °C); accessible in standard induction furnaces; handled as solid granules or turnings under inert atmosphere
Boiling Point	1,484 °C (1,757 K)	Wide liquid range (~640 °C); calcium vapor is used in vacuum distillation purification; calcium vapor pressure is significant above ~900 °C — relevant for metallothermic reduction process design
Thermal Conductivity	201 W/m·K	Unusually high for its density — comparable to aluminum (237 W/m·K) and much higher than Mg (156 W/m·K); reflecting efficient free-electron thermal transport in the FCC structure
Electrical Resistivity	33.6 nΩ·m (20 °C)	Good electrical conductor — better than tin (115 nΩ·m) and comparable to aluminum (28 nΩ·m); rarely exploited electrically due to chemical reactivity
Crystal Structure	FCC (α-Ca, RT); BCC (β-Ca, >450 °C)	FCC at room temperature; transforms to BCC above 450 °C; a further high-pressure phase (sc, simple cubic) exists above ~20 GPa — one of the few elements with a simple cubic ground state under pressure

Mechanical Properties

Property	Value	Notes
Hardness	Mohs ~1.75; ~17 HV	Soft enough to cut with a knife; harder than the alkali metals (Na, K) but softer than most structural metals; mechanically unimportant — calcium is used exclusively as a chemical reagent or alloying addition
Elastic (Young's) Modulus	20 GPa	Low stiffness — about 10% that of steel; comparable to lead (16 GPa); consistent with weak metallic bonding in the large FCC lattice
Poisson's Ratio	0.31	Typical for isotropic metallic systems; relevant to stress analysis of calcium wire injection cored wire deformation during steelmaking processes
Ductility	Moderate (can be extruded)	More ductile than magnesium at room temperature due to FCC slip systems; calcium can be extruded into wire — the form used for steelmaking injection and battery anode foil fabrication

Thermal & Environmental Properties

Property	Value	Notes
Reactivity with Air	Moderate — surface oxidation and nitriding	Forms CaO and Ca₃N₂ on prolonged exposure to air; reacts with atmospheric CO₂ and moisture to form CaCO₃ and Ca(OH)₂; bulk calcium oxidizes more slowly than Na or K but must be stored under mineral oil or inert gas for long-term preservation
Reactivity with Water	Reacts readily, producing H₂	Ca + 2H₂O → Ca(OH)₂ + H₂↑; the reaction is vigorous but not explosive unlike Na or K; calcium turnings react with cold water within seconds; calcium powder reacts rapidly and the evolved H₂ can ignite
Corrosion Resistance	Low — requires inert atmosphere storage	Standard potential of –2.87 V vs. SHE makes calcium highly anodic to all common structural metals; no passive oxide film provides protection in moist environments; supplied and stored in hermetically sealed containers under argon or mineral oil
Oxidation States	+2 (exclusively)	Ca²⁺ is the only stable state; d⁰ closed-shell configuration analogous to Mg²⁺; ionic radius 100 pm (6-coordinate) — larger than Mg²⁺ (72 pm) and smaller than Sr²⁺ (118 pm); responsible for preferential precipitation of Ca²⁺ as sparingly soluble carbonates, sulfates, and phosphates
Electrode Potential	–2.87 V vs. SHE (Ca²⁺/Ca)	Among the most negative standard electrode potentials of common metals; slightly less negative than Li (–3.04 V) and Na (–2.71 V); the basis of calcium's use as a metallothermic reductant and as an anode in next-generation calcium-ion battery research

Chemical Properties

Property	Value / Behavior	Notes
Surface Oxide	CaO (lime, cubic rock salt structure)	CaO melts at 2,613 °C and is one of the most thermally stable binary oxides; the standard building material additive (quicklime); reacts exothermically with water to form Ca(OH)₂ (slaked lime); standard flux and slag modifier in steelmaking and cement kiln chemistry
Carbonate Chemistry	CaCO₃ (calcite, aragonite); Ksp = 3.4 × 10⁻⁹	The most abundant calcium mineral form; dissolves in CO₂-saturated water (karst formation), reacting to form soluble Ca(HCO₃)₂; the basis of water hardness; thermally decomposes above 840 °C to CaO + CO₂ in lime kiln calcination — the world's largest intentional chemical transformation by mass
Phosphate Chemistry	Hydroxyapatite Ca₁₀(PO₄)₆(OH)₂	Hydroxyapatite is the mineral component of bone and tooth enamel (~65 wt% of bone dry mass); synthetic hydroxyapatite and calcium phosphate ceramics (β-TCP, biphasic CaP) are the standard materials for bone scaffolds and orthopedic coatings in biomedical implants
Reducing Power	Metallothermic reductant for REE, U, Th halides	ΔG of CaF₂ formation (–1,176 kJ/mol) is more negative than most metal fluorides, enabling calcium to reduce UF₄, ThF₄, and rare earth trifluorides to the metal; the Goldschmidt process produces gram-to-kilogram quantities of high-purity reactive metals not accessible by electrolysis

Identifier	Value
Symbol	Ca
Atomic Number	20
CAS Number	7440-70-2
UN Number	UN1401 (powder)
EINECS Number	231-179-5

Isotope	Type	Notes
⁴⁰Ca	Stable	96.941% natural abundance; I = 0; the most abundant isotope of any element heavier than argon in the solar system; produced by oxygen burning and silicon burning in massive stars; ⁴⁰K decays to ⁴⁰Ca (89%) as well as ⁴⁰Ar — the ⁴⁰K/⁴⁰Ca decay system is used in geochronology
⁴²Ca	Stable	0.647% natural abundance; I = 0; used in isotope dilution mass spectrometry (IDMS) as a spike isotope for high-precision calcium quantification in biological and geological samples
⁴³Ca	Stable	0.135% natural abundance; I = 7/2, NMR-active; ⁴³Ca NMR spectroscopy probes Ca²⁺ coordination environments in biominerals, cement hydration products, and calcium-binding proteins — though low sensitivity limits routine use
⁴⁴Ca	Stable	2.086% natural abundance; I = 0; the primary isotope used in MC-ICP-MS calcium isotope ratio measurements (δ⁴⁴/⁴⁰Ca); calcium isotope fractionation is a proxy for bone resorption, ocean pH, and carbonate diagenesis in geochemistry
⁴⁶Ca	Stable	0.004% natural abundance; rarest stable calcium isotope; enriched ⁴⁶Ca is used as a tracer in metabolic studies of calcium absorption and bone turnover in clinical nutrition research
⁴⁸Ca	Stable*	0.187% natural abundance; double beta decay candidate (t½ > 6 × 10¹⁹ yr); the heaviest stable-like calcium isotope; enriched ⁴⁸Ca beams at JINR Dubna were used to synthesize elements 114–118 (flerovium through oganesson) by bombardment of actinide targets, completing Period 7 of the periodic table
⁴¹Ca	Radioactive	t½ = 9.94 × 10⁴ yr (electron capture); cosmogenic nuclide produced by neutron activation of ⁴⁰Ca in rocks; used as a long-lived tracer for bone metabolism and calcium cycling in geological systems; AMS measurement allows detection at natural abundance levels of ~10⁻¹⁵
⁴⁵Ca	Radioactive	t½ = 162.6 days (β⁻); produced by neutron activation of ⁴⁴Ca in reactors; low-energy beta emitter (Emax = 258 keV); used as a radiotracer for calcium metabolism studies, bone resorption measurements, and soil calcium cycling experiments where long half-life and pure beta emission are advantageous

Scientific & Research Applications

Use Case	Form Typically Used	Description
Metallothermic Reduction of Reactive Metals	Ca granules, Ca turnings (99%+)	The Goldschmidt process uses calcium metal as the reductant to produce high-purity uranium (from UF₄), thorium (from ThF₄), and rare earth metals from their trifluorides or trichlorides. The strong thermodynamic driving force — ΔG of CaF₂ formation is –1,176 kJ/mol, more negative than virtually all metal fluorides — ensures complete reduction. The CaF₂ or CaCl₂ slag separates readily from the metal product by density difference.
Calcium-Ion Battery Research	Ca foil anodes, Ca powder, Ca-Sn alloy anodes	Calcium metal anodes are under intensive development as alternatives to lithium in next-generation batteries. Ca²⁺ has twice the charge per ion as Li⁺, and calcium's abundance and low cost are compelling. Research since 2015 has demonstrated reversible room-temperature Ca plating in fluorinated borate and Ca(BH₄)₂-based electrolytes. Cathode materials under investigation include Ca-intercalation oxides (CaMnO₂), organic carbonyl compounds, and sulfur composites.
Vacuum System Gettering	Ca wire, Ca granules, evaporated Ca films	Calcium's high affinity for O₂, N₂, CO, CO₂, and H₂O makes it an effective non-evaporable getter for maintaining ultra-high vacuum in electron tubes, particle accelerator beam pipes, and sealed vacuum devices. Calcium flash getters are activated by resistive heating and provide a large reactive surface area within a compact geometry.
Isotope Tracer Studies	Enriched ⁴²Ca, ⁴⁴Ca, ⁴⁵Ca solutions	Stable calcium isotopes (⁴²Ca, ⁴⁴Ca) are used as non-radioactive tracers in human metabolic studies of calcium absorption, bone mineral turnover, and dairy calcium bioavailability — measured by MC-ICP-MS with sub-0.1% isotope ratio precision. Radioactive ⁴⁵Ca is used in soil science and plant physiology to trace calcium mobility in rhizosphere and soil-plant transfer experiments.
Superheavy Element Synthesis	Enriched ⁴⁸Ca beam (accelerator target)	Enriched ⁴⁸Ca ion beams accelerated in the U400 cyclotron at JINR Dubna were used to bombard ²⁴²Pu, ²⁴³Am, ²⁴⁵Cm, ²⁴⁸Cm, ²⁴⁹Bk, and ²⁴⁹Cf targets, producing elements 114 (flerovium) through 118 (oganesson) in cold fusion reactions between 2000 and 2006. ⁴⁸Ca is uniquely suited for this role due to its doubly magic-adjacent nuclear structure (Z=20, N=28) giving high cross-sections relative to other projectiles.
Biomedical Imaging Contrast Agents	CaCO₃ nanoparticles, Ca-chelate complexes	Calcium carbonate nanoparticles are under investigation as biodegradable ultrasound contrast agents and drug delivery vehicles — they dissolve in the mildly acidic tumor microenvironment, releasing drugs and producing CO₂ microbubbles that enhance ultrasound echogenicity. ⁴⁵Ca and stable Ca isotopes are used to measure bone density, calcium absorption efficiency, and osteoporosis treatment efficacy in clinical metabolic studies.

Industrial & Commercial Applications

Sector	Form / Compound Used	Description
Steelmaking — Inclusion Modification	Ca-Si cored wire, Ca-Fe-Si cored wire (injection)	Calcium wire injection is the standard method for inclusion shape control in clean steel production. Calcium converts elongated MnS inclusions and alumina clusters into spherical calcium aluminosilicate globules (Ca-Al-O) that do not initiate fatigue cracks in the rolling direction. This dramatically improves transverse notch toughness, machinability, and HIC (hydrogen-induced cracking) resistance in line pipe and offshore structural steels. Typical addition rates: 0.1–0.3 kg Ca per tonne of steel.
Steelmaking — Desulfurization	CaO-CaF₂ flux, Ca-Mg powder injection	Calcium oxide and calcium carbide (CaC₂) are the primary reagents for hot metal and ladle desulfurization in integrated steelmaking. CaO reacts with dissolved sulfur to form stable CaS slag (ΔG = –540 kJ/mol at 1,600 °C), achieving sulfur levels below 20 ppm required for pipeline and automotive steels. Combined Ca-Mg injection achieves deeper desulfurization (<10 ppm S) in one treatment step.
Lead-Acid Battery Grid Alloys	Pb-Ca (0.03–0.10 wt% Ca), Pb-Ca-Sn alloy	Calcium addition to lead significantly reduces self-discharge rates in maintenance-free (VRLA and AGM) lead-acid batteries by suppressing hydrogen gassing and antimony-free grid corrosion. Pb-Ca-Sn alloys (0.05–0.10% Ca, 0.3–2.0% Sn) are the standard grid material in automotive SLI batteries, UPS systems, and stationary energy storage, having replaced Pb-Sb alloys in most consumer applications since the 1980s.
Aluminum Alloy Grain Refinement	Al-Ca master alloy (2–5% Ca)	Calcium additions of 0.01–0.1 wt% to aluminum alloys refine grain structure during solidification, improving castability and reducing hot tearing in complex die castings. Ca is also added to Mg-Al alloys to suppress high-temperature creep by forming a stable CaAl₂ intermetallic at grain boundaries — enabling use of Mg alloy engine components at temperatures where conventional Mg-Al alloys soften unacceptably.
Pyrotechnics	Ca metal powder, CaSi₂ (calcium silicide)	Calcium produces a characteristic brick-red to orange flame color (Ca⁺ emission at 616–646 nm) in pyrotechnic compositions. Calcium silicide (CaSi₂) is a safer and more stable pyrotechnic fuel than calcium metal powder, used in signal flares, railway detonators, and certain initiator compositions where a calcium spectral signature is required. Ca metal is also used as a heat-generating fuel in self-heating food and beverage containers.
Cement & Construction	CaO (quicklime), Ca(OH)₂ (hydrated lime), CaCO₃	While elemental calcium metal is not used in construction, its oxide and hydroxide are among the world's most produced chemicals. Portland cement clinker contains ~65 wt% CaO; world cement production exceeds 4 billion tonnes per year, making the calcination of CaCO₃ to CaO the largest intentional chemical process globally and accounting for ~8% of anthropogenic CO₂ emissions. Hydrated lime (Ca(OH)₂) is used in soil stabilization, water treatment pH adjustment, and flue gas desulfurization.

Purity	Main Use
99%	Laboratory research, vacuum technology, and chemical synthesis — suitable for metallothermic reduction of reactive metal halides, vacuum getter applications, and synthesis of calcium compounds where sub-1% impurities (primarily Mg, Sr) do not affect product quality

Synonym / Alternative Name	Context
Ca	Chemical symbol
Calcium metal	Standard commercial and regulatory designation for the elemental form; distinguished from calcium compounds (CaO, CaCO₃, Ca(OH)₂) in trade and regulatory contexts
Elemental calcium	General scientific term used in geochemistry and materials science to distinguish the pure metal from calcium-containing minerals and compounds
Calcio	Spanish and Italian language equivalent; the name derives from the Latin calx (lime, limestone), reflecting the element's discovery by Humphry Davy in 1808 through electrolysis of lime — the material known to builders since antiquity
Calcium (Kalk)	German language context; Kalk is the common German word for lime/limestone (from Latin calx), while Calcium is used for the element; the etymological connection underscores calcium's historical association with its carbonate mineral form
Kalzium	Alternative German spelling of the element name, used in older literature and some regulatory documents

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