[pzona69]

My teacher was showing us NI₃ a while back, and described its instability by saying that the nitrogen atoms were so much smaller than the iodines that it makes it hard for them to bond, which is why it is so unstable (obviously this is a simplified explanation).

A few weeks later I tried to buy iodine tincture at a drugstore, and they only sold it in these tiny 2oz bottles, so it ended up costing me over $10 for 6oz (it was for a school project). They told me you can only buy it in such small amounts because, among other things, it is used in homemade contact explosives.

This got me thinking about the NI₃. This is probably something I should know, but is this actually a general rule, that large atoms and small atoms form unstable compounds because of the size difference? Or is there something special about iodine?

[Arkcon]

Google around a bit, nitrogen tribromide and nitrogen trichloride are also interesting.  I use their names because research shows that their formulas are actually more complex than a simple trihalide.  At any rate, nitrogen trifluoride is stable, which leads me to believe this has as much to do with electronegativity, and the energy stored in the nitrogen-halide bond.