In chemistry, a carbide is a compound composed of carbon and a less electronegative element. Carbides can be generally classified by chemical bonding type as follows: (i) salt-like, (ii) covalent compounds, (iii) interstitial compounds, and (iv) "intermediate" transition metal carbides. Examples include calcium carbide, silicon carbide, tungsten carbide (often called simply carbide), and cementite, each used in key industrial applications.
Salt-like carbides are composed of highly electropositive elements such as the alkali metals, alkaline earths, 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 naming of ionic carbides is not consistent and can be quite confusing.
Carbides of this class decompose in water producing methane. Two such examples are aluminium carbide Al4C3 and beryllium carbide Be2C.
Methanides in general chemical context refers to any compound that hydrolyzes to methane, which might include also salts with hydrogenated anions such as CH3−, CH2−
2, and CH−
3. However, according to IUPAC systematic naming conventions, only the latter is properly called "methanide". In theory one can describe compounds that contain the methyl group, with relatively large bond polarity between the carbon and non-hydrogen atom, as salts of this anion; however in truth most such compounds, if not all, are, in fact covalent.
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