Why only lithium form nitrides?

Answer 1

Lithium is the only alkali metal (group 1) to form a nitride when burnt in air. (This is however common with the alkaline earth metals (group2), e.g. Mg3N2 . Note that sodium nitride is known.

the 3 :1 ratio of elements in the group 1 nitrides leads to a high coordination number for the nitride ion in a lattice. Lithium has the smallest ionic radius of all of the group 1 metals. The structure of Li3N is unusual - each nitrogen is surrounded by eight lithium atoms, 6 in the same plane. The determination as to why one rection goes and an other doesn't is complex and requires an understanding of the thermodynamics, the so-called Born-Haber cycle. In VERY simple terms it is probably the stability of this odd structure that makes this compound stable whereas Na3N, K3N etc (where Na, K etc are bigger than Li) are not. Sodium nitride, Na3N is a strange solid- a semiconductor with ionic character- and has a different structure from Li3N with 6 coordinate nitrogen.

Answer 2

Lithium is more likely to form nitrides due to its lower ionization energy and smaller size compared to other alkali metals. This makes it easier for lithium to lose an electron and form ionic bonds with nitrogen atoms. Additionally, the high reactivity and stability of lithium nitride make it a preferred choice in certain applications.