Two bonding levels
Ionic between ions; covalent within carbonate.
The C–O bonding inside CO₃²⁻ is covalent and delocalized. Interactions between Ca²⁺ and carbonate ions organize the extended crystal.
Ionic solid · carbonate salt
Calcium carbonate
CaCO₃
Calcium carbonate is an ionic solid composed of Ca²⁺ cations and CO₃²⁻ anions in a 1:1 ratio. The carbonate ion is internally covalent and resonance-delocalized, while the solid as a whole is an extended ionic crystal rather than a collection of discrete CaCO₃ molecules.
Material distinction: limestone, chalk and marble are rocks or materials rich in calcium carbonate, not synonyms for one perfectly pure microscopic crystal.
Essentials
CaCO₃ is a formula unit for an ionic crystal.
The simplest correct picture separates calcium ions from carbonate ions. There is no need to invent a covalent Ca–CO₃ molecule to explain the 1:1 stoichiometric ratio.
Detail
The carbonate ion is trigonal planar and resonance-delocalized.
A single Lewis structure can draw one C=O and two C–O⁻ bonds, but the isolated carbonate ion is better understood through resonance: the three C–O positions are equivalent by symmetry.
Crystal polymorphs
CaCO₃ can crystallize in more than one structure.
Calcite and aragonite have the same chemical formula but different crystal structures. Formula alone therefore does not specify the complete solid-state arrangement.
Understand
Acid converts carbonate into carbon dioxide and water.
Carbonate consumes protons. The resulting carbonic-acid chemistry leads to CO₂ and H₂O, which is why carbonate minerals effervesce in acid.
Geochemical link
Solubility depends strongly on chemical environment.
Pure-water solubility is limited, but dissolved CO₂, pH and complexation can shift carbonate equilibria and change how readily calcium carbonate dissolves or precipitates.
Reference identifiers
Names and identifiers that pin down the chemical identity.
| Formula | CaCO₃ |
|---|---|
| CAS Registry Number | 471-34-1 |
| PubChem CID | 10112 |
| Standard InChI | InChI=1S/CH2O3.Ca/c2-1(3)4;/h(H2,2,3,4);/q;+2/p-2 |
Advanced
Push past the first model without losing the simple picture.
Are calcite and aragonite different compounds?
They have the same stoichiometric formula CaCO₃ but different crystal structures. They are polymorphs: different solid forms of the same composition.
Why is carbonate drawn with resonance?
No single conventional Lewis structure makes all three C–O positions equivalent. Resonance is a bookkeeping device for the delocalized electronic structure.
Is every white calcium-carbonate material pure CaCO₃?
No. Natural and commercial materials can contain other minerals, water, organic matter or processing additives.
Constituent elements