Sesquicarbide

In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by the chemical bonds type as follows: (i) salt-like, (ii) covalent compounds, (iii) interstitial compounds, and (iv) "intermediate" transition metal carbides. Examples include calcium carbide (CaC₂), silicon carbide (SiC), tungsten carbide (WC) (often called simply carbide when referring to machine tooling), and cementite (Fe₃C), each used in key industrial applications. The naming of ionic carbides is not systematic.

Salt-like carbides are composed of highly electropositive elements such as the alkali metals, alkaline earth metals, and group 3 metals, including scandium, yttrium, and lanthanum. Aluminium from group 13 forms carbides, but gallium, indium, and thallium do not. These materials feature isolated carbon centers, often described as "C⁴⁻", in the methanides or methides; two-atom units, "C₂²⁻", in the acetylides; and three-atom units, "C₃⁴⁻", in the sesquicarbides. The graphite intercalation compound KC₈, prepared from vapour of potassium and graphite, and the alkali metal derivatives of C₆₀ are not usually classified as carbides.

Carbides of this class decompose in water producing methane. Three such examples are aluminium carbide Al
4C
3, magnesium carbide Mg
2C and beryllium carbide Be
2C.

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