Nitrides
In chemistry, a nitride is a compound of nitrogen where nitrogen has a formal oxidation state of −3. Nitrides are a large class of compounds with a wide range of properties and applications.
The nitride ion, N3−, is never encountered in protic solution because it is so basic that it would be protonated immediately. Its ionic radius is estimated to be 140 pm.
Like carbides, nitrides are often refractory materials owing to their high lattice energy which reflects the strong attraction of "N3−" for the metal cation. Thus, titanium nitride and silicon nitride are used as cutting materials and hard coatings. Hexagonal boron nitride, which adopts a layered structure, is a useful high-temperature lubricant akin to molybdenum disulfide. Nitride compounds often have large band gaps, thus nitrides are usually insulators or wide bandgap semiconductors, examples include boron nitride and silicon nitride. The wide band gap material gallium nitride is prized for emitting blue light in LEDs. Like some oxides, nitrides can absorb hydrogen and have been discussed in the context of hydrogen storage, e.g. lithium nitride.
Classification of such a varied group of compounds is somewhat arbitrary. Compounds where nitrogen is not assigned 3- oxidation state are not included, e.g. nitrogen trichloride, nor are ammonia and its many organic derivatives.
Only one alkali metal nitride is stable, the purple-reddish lithium nitride (Li3N), which forms when lithium burns in an atmosphere of N2.Sodium nitride has been generated, but remains a laboratory curiosity. The nitrides of the alkaline earth metals have the formula M3N2 are however numerous. Examples include Mg3N2, Be3N2, Ca3N2, and Sr3N2. The nitrides of electropositive metals (including Li, Zn, and the alkaline earth metals) readily hydrolyze upon contact with water, including the moisture in the air:
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