Atomic number 20

Calcium

Ca
+ phase
− phase

Shells Orbitals 4s

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20 electrons in 4 shells: 2, 8, 8, 2.

Shells are a teaching model: an electron does not travel on a ring at a fixed distance.

Category: Alkaline earth metal
Period: 4
Group: 2
Block: s
Standard atomic weight: 40.078(4) Da
Electron configuration: [Ar] 4s²
Phase at 25 °C: solid
Melting point: 842 °C
Boiling point: 1484 °C
Density: 1.54 g/cm³
Electronegativity (Pauling): 1.0
Discovered: 1808, Humphry Davy

Neighbours in the table

K Sc

Of calcium’s twenty electrons, arranged 2, 8, 8 and 2 across four shells, the last pair tells the story of its chemistry. Those two occupy the 4s subshell outside an argon core, written [Ar] 4s². Both go together in a reaction, leaving Ca²⁺, an ion of 18 electrons arranged as argon’s are. Twenty protons now pull on eighteen electrons, so the ion is smaller than the atom and than K⁺, which has one proton fewer.

Calcium sits one place right of potassium in period 4, with one proton more. That extra pull shrinks the atom and holds its first electron more tightly: its first ionisation energy, the cost of removing one electron, is 6.113 eV against potassium’s 4.341. That higher price is why calcium reacts less readily than potassium. Down group 2 the same rule holds: calcium is larger than magnesium above it, and reactivity rises with size.

Los Alamos describes the metal as silvery and rather hard. In air it grows a white nitride coating; lit, it burns with a flame the same page calls yellow-red. The Romans made lime in the first century; the metal inside it waited until 1808. Berzelius and Pontin had electrolysed lime in mercury to make a calcium amalgam, an alloy with mercury, and Davy, learning of it, isolated the metal.

Where it is found

Calcium ranks fifth in abundance among the elements of the crust. Jefferson Lab, in PubChem’s record, puts the share at about 4.2 per cent, 4.15 × 10⁴ milligrams per kilogram; Los Alamos puts it above 3 per cent. A litre of seawater holds 4.12 × 10² milligrams. Calcium combines so readily with oxygen and water that the free element does not occur in nature.

Limestone, gypsum and fluorite are its common minerals. Limestone, marble and chalk are all calcium carbonate, and so are pearls, most mollusc shells and most of a hen’s eggshell. Water carrying dissolved carbon dioxide dissolves that carbonate, because a soluble hydrogen carbonate forms; that is how caves open in limestone and why water is hard. The hydrogen carbonate survives only in solution, so where a drip evaporates the carbonate returns as stalactites and stalagmites.

Pushing electrons back onto Ca²⁺ takes electrical energy, so the metal is made by electrolysis. Los Alamos gives electrolysis of the fused chloride, with calcium fluoride added to lower the melting point. Jefferson Lab describes another: aluminium displacing calcium from lime in hot, low-pressure containers.

Why it matters

Jefferson Lab notes little demand for the metal, because it reacts with so many common materials. As a reducing agent it frees thorium, uranium and zirconium from their compounds and strips oxygen, sulfur and carbon from alloys.

Calcium’s trade is in the carbonate. Heating limestone drives off carbon dioxide and leaves quicklime, calcium oxide. Slaked with water it becomes calcium hydroxide, which OpenStax calls the cheap, strong base of chemical processing.

US plants made about 16 million metric tons of lime in 2024, worth near 3.2 billion dollars. Steelmaking took the most; in a blast furnace the calcium oxide from limestone binds the ore’s silica into slag, the furnace’s waste. Before electric light, quicklime blocks heated by gas flames glowed intense white over theatre stages, the origin of “in the limelight”.

OpenStax’s anatomy text calls calcium the body’s most abundant mineral: about 1,200 g in a woman and 1,400 g in a man, most of it in bone and teeth. In bone it forms hydroxyapatite, a crystal of calcium phosphate and carbonate, which makes bone hard; collagen fibres keep it from being brittle. Of the calcium in blood, over half is bound to proteins; the free Ca²⁺ is what clotting, muscle contraction and the pacing of the heart need. The body cannot make calcium; the diet supplies it.

Properties

Properties of calcium Standard atomic weights from IUPAC/CIAAW (abridged, 2024); other values from PubChem's periodic table, kelvin converted to °C.
Property	Value	Unit	Source
Atomic number	20
Standard atomic weight	40.078(4) Da	Da	IUPAC/CIAAW 2024
Melting point	842 °C	°C	Los Alamos: 842 °C
Boiling point	1484 °C	°C	Los Alamos: 1484 °C
Density	1.54 g/cm³	g/cm³
Electronegativity (Pauling)	1.0
Electron configuration	1s² 2s² 2p⁶ 3s² 3p⁶ 4s²
Phase at 25 °C	solid

Safety

Calcium metal is classified Danger on PubChem’s record: in contact with water it gives off hydrogen, a flammable gas. CAMEO adds that the reaction’s heat can ignite that hydrogen, and that finely divided calcium can catch fire in air, a hazard confined to the powder. A large lump, under ordinary conditions, is unlikely to burn or to react strongly with water, though the metal burns skin and eyes on contact. That concerns laboratories and plants handling the element, not the calcium locked in bone or chalk; the PubChem Laboratory Chemical Safety Summary below is the sheet for the metal.

Questions

Why does calcium form a 2+ ion and not 1+ or 3+?

The price of each electron settles it. OpenStax's table of ionisation energies, converted to electronvolts, gives calcium 6.113 eV for the first electron and 11.871 for the second, then 50.913 for the third. That jump marks the edge of the argon core, whose electrons occupy smaller shells and are far harder to remove. Two electrons go for a modest outlay and a third would cost more than four times as much, so the ion stops at 2+.

Does calcium metal react with water?

Yes. Group 2 metals from magnesium down react with water, giving hydrogen gas and the metal hydroxide, and at room temperature barium reacts most vigorously. OpenStax reads the hydrogen as a sign that these metals give up electrons more readily than hydrogen does. PubChem's GHS entry for calcium therefore carries the signal word Danger, since the gas released is flammable. The International Chemical Safety Card for the powder says to store it dry, fireproof and under inert gas.

What colour does calcium burn in a flame test?

Red, though the sources reach for different words. OpenStax puts calcium and strontium together as giving shades of red, and Los Alamos says the burning metal has a yellow-red flame. The CAMEO entry on PubChem's safety record calls it crimson. OpenStax supplies the reason: the colour belongs to calcium's emission spectrum, the set of wavelengths its hot atoms give out, just as the alkali metals colour a flame with theirs.

Where does the name calcium come from?

From calx, the Latin word for lime. Los Alamos records that the Romans were making lime under that name in the first century. IUPAC's note in PubChem's element record adds that calx meant lime itself or the limestone the element was found in. The group name has its own reason: OpenStax explains that group 2 is called alkaline because the oxides of its heavier members give alkaline solutions in water.

Is calcium a metal?

Yes. PubChem classes it as a metal in group 2, the alkaline earth metals, and Los Alamos describes a silvery, rather hard solid. Metals conduct and bend because their outer electrons are easily removed, and calcium's two go readily. Few people ever see it, though. The element is never found free, since it combines with oxygen and water; the calcium in rock, shells and bone is the 2+ ion locked inside compounds.

Compounds containing calcium

CaCl₂ Calcium chloride 3 atoms in one formula unit of an ionic lattice.
CaF₂ Calcium fluoride 3 atoms in one formula unit of an ionic lattice.
CaCO₃ Calcium carbonate 5 atoms in one formula unit of an ionic lattice.
Ca(OH)₂ Calcium hydroxide 5 atoms in one formula unit of an ionic lattice.
CaSO₄ Calcium sulfate 6 atoms in one formula unit of an ionic lattice.