Lithium is the only alkali metal to form a nitride directly. This is because of the following reasons:

⇒ Li⁺ is very small in size and so its size is the most compatible with the N^3– ion.

⇒ One more reason is that because of the diagonal relationship of Li and Mg. Lithium like magnesium forms a nitride while other alkali metals do not.

6Li (s) + N₂ (g) 2Li₃N (s)