The most stable trihalide of nitrogen is:

Order of stability: - ${\mathrm{NF}}_3>{\mathrm{NCl}}_3>{\mathrm{NBr}}_3>{\mathrm{NI}}_3$

NF_3​ is more stable than the other NX₃​ . This has been ascribed to repulsions
between lone pairs on the two rather proximate fluorine atoms. . The other NX₃​ molecules may be less stable than NF₃​ owing to congestion round the small
central nitrogen atom leading to non-bonded repulsive interactions between the halogens. This is particularly bad for large iodine atoms.