# Why does lithium from the nitride whereas the other alkali metal forms the azide?

What property of lithium makes them forms the nitride, lithium nitride $\ce{Li3N}$ whereas the other group 1 elements forms the azide, like sodium azide and potassium azide - $\ce{NaN3}$ and $\ce{KN3}$ respectively?

Does lithium azide exist? If so, what is its structure?

• This is an example where the application of HSAB is appropriate. Consider that the hardness of the metal cations of the alkali metals increases down the group. Also, consider that azide is a softer base compared to nitride. However, this explains it only qualitatively... – Tan Yong Boon Aug 18 '18 at 8:52

The formation of $\ce{Li3N}$ can be explained thermodynamically. The enthalpy of formation of the $\ce{N^3-}$ ion is extremely endothermic (it involves three successive additions of electrons) and with the larger counterions $\ce{Na+}$ or $\ce{K+}$ the lattice energy (which is proportional to $\frac{q_+ q_-}{r_+ + r_-}$) is not sufficiently large to compensate for the large $\Delta_\text{f}H$ of $\ce{N^3-}$. 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 $\ce{NaN3}$ and $\ce{Li2SO4}$: see Acta Chem. Scand. 1957, 11, 581 for more details.
• @Jan Wikipedia says it exists, but is unstable and readily decomposes into $\ce{Na + N2}$ - no surprises there. – orthocresol Oct 22 '16 at 14:37