Carbon dioxide (${\mathrm{CO}}_2$)

Carbon dioxide is $\mathrm{C}{\mathrm{O}}_2$, a linear molecule with two carbon-oxygen double bonds. The individual bonds are polar, but their dipoles cancel in the symmetric equilibrium geometry. Some vibrations change the molecular dipole and therefore absorb infrared radiation.

Carbon dioxide is a product of many oxidation and combustion processes and a substrate for photosynthetic carbon fixation. Dissolved carbon dioxide participates in carbonate acid-base equilibria, so adding it to water can change pH without directly adding a strong acid.

A nonmolecular extended carbon–oxygen solid. A corner-sharing tetrahedral model has four O neighbours per C and two C neighbours per bridging O, preserving CO2 stoichiometry.

An amorphous nonmolecular form of carbon dioxide. It is distinct from molecular dry ice and from assuming a unique crystalline polymorph.^[ref]