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Chemistry Background Information
Nitrogen triiodide is a contact explosive. The slightest touch or alpha particle will cause an instantaneous violent decomposition. NI3 is unstable because of the 3 highly strained nitrogen iodine bonds. Iodine is a very large atom with an atomic number of 53. Nitrogen is far smaller, its atomic number is 7. The structure of NI3 is trigonal pyramidal. The lone pair in the molecule naturally requires a 120 degree breadth in the structure, leaving the very large iodine atoms with very little room. The bonds between I and N become incredibly strained. Any disruption of the molecule, the tickle of a feather, will cause an explosion.

Comparing ammonia, NH3, and nitrogen triiodide, NI3, is useful because they have the same overall structure but behave very differently.

Bond energies provide quantitative information regarding the strength of bonds. Ammonia is relatively stable. The energy of dissociation for the N-H bond in ammonia is 393 KJ/mol. The energy of dissociation for the N-I bond in nitrogen triiodide is 159 KJ/mol. If a bond energy is large, the bond is strong because a large amount of energy is required to break the bond. NI3 has relatively weak bonds.

The enthalpy of formation for the decomposition reactions of NH3 and NI3 further demonstrates the difference between the two molecules. The reactions for their decomposition are as follows:

ΔH = -1820 KJ/mol 2NH3 → N2 + 3H2

ΔH = -275 KJ/mol 2NI3 → N2 + 3I2

Enthalpy of formation is found by subtracting the total bond energies of the bonds formed from those of the bonds broken. If the energy of the bonds broken is very large, then more energy is required to break the bonds. The decomposition of NH3 requires much more heat than NI3 . Both reactions are exothermic, but NI3 is drastically less exothermic. NI3 will more willingly decompose, or explode, because the energy of the bonds formed in its decomposition ( N2 + I2) only differ slightly from the energy of NI3 .

NitrogenTriiodide Structure

Additional Resources:
Table of Bond EnergiesNitrogen Triiodide
References/Useful Links:
1890 NI3 DirectionsWikipedia Article