Calcium carbonate

Ca 1
C 1
O 3

Formula unit Lattice

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5 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; calcium carbonate has no molecular shape.

IUPAC name: calcium carbonate
Molar mass: 100.0860(90) g/mol
Atoms: 5
SMILES: C(=O)([O-])[O-].[Ca+2]
InChIKey: VTYYLEPIZMXCLO-UHFFFAOYSA-L
PubChem CID: 10112

Made of

Ca C O

Chalk, marble and limestone are forms of one ionic compound, and so are pearls, the shells of most molluscs and the bulk of a hen’s eggshell. Calcium carbonate, CaCO₃, pairs one calcium ion, Ca²⁺, with one carbonate ion, CO₃²⁻. Its molar mass is 100.09 g/mol, and it comes as a white powder or colourless crystals.

Most ionic solids dissolve readily in water, but OpenStax lists the carbonates of the alkaline earth metals among the exceptions. Only 14 mg of calcium carbonate dissolves in a litre of water at 25 °C. Water carrying dissolved carbon dioxide takes up far more, because soluble calcium hydrogen carbonate forms. Dilute acids dissolve it too, releasing carbon dioxide as bubbles.

Strong heat breaks it apart rather than melting it. PubChem’s data give about 825 °C for the split into calcium oxide and carbon dioxide, and a range of 700 to 900 °C for calcite. That reaction is the basis of lime making.

Structure and bonding

The drawing holds five atoms in two unconnected pieces. One is a lone calcium ion, charge +2. In the other, a carbon atom is bonded to three oxygen atoms: one by a double bond and two by single bonds, each single-bonded oxygen at −1.

That carbonate is one of three equivalent resonance forms, ways of placing the same electrons. The real ion is the average of the three, so all three C–O bonds are identical and the −1 charges belong to no particular oxygen. The carbonate ion itself is flat and trigonal planar, a shape of the ion, not the compound.

The pair is one formula unit of a lattice, not a free-standing particle: the solid is held together by electrostatic attraction between calcium and carbonate ions. OpenStax states a general rule for simple ions that behave as charged spheres: stable structures surround each ion with as many opposite charges as possible.

It crystallises in three forms: hexagonal calcite, orthorhombic aragonite and hexagonal vaterite, names for each crystal’s symmetry. Calcite forms below 30 °C and aragonite above it. Aragonite is metastable, long-lived but not the most stable form: dry heat of about 400 °C converts it to calcite for good.

Where it turns up

The NIOSH card, from the US National Institute for Occupational Safety and Health, finds it in limestone, chalk, marble, calcite, aragonite and oyster shells. Oysters and corals take calcium and carbonate out of seawater to build their hard parts, according to NOAA, the US National Oceanic and Atmospheric Administration.

Limestone caves show the compound dissolving and returning. Water carrying carbon dioxide turns the rock into soluble calcium hydrogen carbonate, which is how caves and sinkholes open. OpenStax names carbon dioxide as the agent; PubChem’s data add that ammonium salts also raise the solubility, while alkali hydroxide lowers it. Where drops of the solution evaporate, calcium carbonate is left behind as stalactites and stalagmites.

Indoors, it is the active ingredient of the antacid Tums. The Jefferson Lab text on PubChem’s calcium record adds white paint, cleaning powder and toothpaste.

Why it matters

Of all carbonates, OpenStax says, industry uses calcium carbonate and sodium carbonate in the greatest amounts. US production reported to the Environmental Protection Agency for 2023 fell between 4 billion and just under 5.5 billion pounds. Paint, paper and plastics take the main share of the market. Ground grades come straight from quarried chalk, limestone or marble.

Heat turns limestone into lime: in a kiln it gives off carbon dioxide and leaves calcium oxide, quicklime. The carbonate is also a raw material for Portland cement. In copper smelting, limestone serves as a flux, a material that helps carry impurities away. There it reacts with the ore’s silica, SiO₂, to give carbon dioxide and molten calcium silicate, CaSiO₃, part of the slag, a low-melting waste that separates from the metal.

In the stomach, the carbonate reacts with hydrochloric acid to give calcium chloride, carbon dioxide and water, which is how the tablet neutralises acid. In bone, calcium carbonate and calcium phosphate combine to make hydroxyapatite, the crystals that give bone its hardness and strength.

Properties

Properties of calcium carbonate Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 10112.
Property	Value	Unit	Source
Formula (Hill order)	CCaO₃
Molar mass	100.0860(90) g/mol	g/mol	IUPAC/CIAAW 2024
Calcium atoms	1
Carbon atoms	1
Oxygen atoms	3
PubChem molecular weight	100.09 g/mol	g/mol	PubChem

Safety

Calcium carbonate is essentially harmless in its everyday forms, from chalk to antacid tablets. Of 3535 company reports to the European Chemicals Agency, 3180 (about 90%) find it meets no hazard criteria of the Globally Harmonized System. The caution its safety data give is dust, which may irritate the eyes, skin and airways and cause coughing. The PubChem Laboratory Chemical Safety Summary and the NIOSH card are the sheets for anyone handling the powder at work.

Questions

Is calcium carbonate a molecule?

No. It is an ionic compound, a lattice of calcium ions, Ca²⁺, and carbonate ions, CO₃²⁻, held together by the electrostatic pull between opposite charges. Its formula states a one-to-one ratio of those ions rather than a particle with a shape. The carbonate ion inside it is a real bonded unit, though: flat and trigonal planar, with three carbon–oxygen bonds that experiments show to be identical.

Why does calcium carbonate fizz in vinegar?

Carbonates react with acids to give a salt of the metal, water and carbon dioxide gas, and the gas escaping as bubbles is the fizz. The EU food-additive specification tests for exactly this: the powder dissolves with effervescence in dilute acetic acid, the acid in vinegar, and in dilute hydrochloric and nitric acids. A drop of acid on limestone or marble shows the same bubbling.

How did chalk help invent fizzy water?

Carbonated drinks go back to 1767, when Joseph Priestley worked out a way of dissolving carbon dioxide in water. His gas came from chalk, which is calcium carbonate, treated with oil of vitriol, the old name for sulfuric acid. The acid released carbon dioxide from the carbonate, just as any acid does, and Priestley infused water with the gas to make carbonated water.

Why do shells dissolve as the ocean becomes more acidic?

Oysters, corals and other shell builders combine calcium and carbonate from seawater. As the ocean absorbs carbon dioxide, NOAA explains, the gas forms carbonic acid, which releases hydrogen ions; carbonate ions bond with them, so fewer remain for building shells. If the pH falls far enough, existing shells start to dissolve. In one experiment, the shells of small sea snails called pteropods slowly dissolved after 45 days in water set to the conditions projected for 2100.