What property of lithium makes them forms the nitride, lithium nitride LiX3N whereas the other group 1 elements forms the azide, like sodium azide and potassium azide - NaNX3 and KNX3 respectively?
Does lithium azide exist? If so, what is its structure?
Answer
The formation of LiX3N can be explained thermodynamically. The enthalpy of formation of the NX3− ion is extremely endothermic (it involves three successive additions of electrons) and with the larger counterions NaX+ or KX+ the lattice energy (which is proportional to q+q−r++r−) is not sufficiently large to compensate for the large ΔfH of NX3−. The same argument explains why magnesium also forms a nitride.
According to my very brief research on the Internet, lithium azide does exist but is unstable. It can be prepared via reaction of NaNX3 and LiX2SOX4: see Acta Chem. Scand. 1957, 11, 581 for more details.
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