Nitrogen Triiodide (2016)

Summary

Nitrogen Triiodide by John Walker Space-filling model of Nitrogen Triiodide molecule. (Public domain image by Wikimedia user Benjah-bmm27.) One fine day during my years at engineering school I was walking down the stairs between two floors in the chemistry building. Bang! It sounded like a gunshot, and close at hand. I was alone in the stairwell, and a quick inventory showed no punctures, so I shrugged it off and continued down the stairs. Just one of those things…. A few steps later, bang! By the time I reached the landing, there had been two additional loud reports. It's then that I noticed there were dark splotchy purple-red stains on some of the stairs. I went back up a couple of stairs and deliberately stepped on one: bang, and a purple cloud shot from around my shoe and started to disperse. This was my first encounter with nitrogen triiodide (NI3), a compound formed by reacting elemental iodine with ammonium hydroxide. (The terminology for these chemicals can be confusing. Iodine is a crystalline solid at room temperature; the liquid “iodine” used to disinfect wounds is properly referred to as “tincture of iodine” and is composed of iodine and iodine salts dissolved in alcohol and water. Ammonia (NH3) is a gas at room temperature. The “ammonia” used as a cleaning agent is actually ammonium hydroxide, ammonia gas dissolved in water.) In nitrogen triiodide, due to the structure of the electron shells of nitrogen, the three iodine atoms all bond to one side of the nitrogen atom. Since the iodine atoms are about twice the size of the nitrogen, they don't “fit” well in the limited space, and this creates what is called “steric strain”: the iodine atoms repel one another and thus are only very loosely bonded to the central atom of nitrogen. This is a high energy configuration, whose energy can be dramatically reduced by rearranging the atoms in a sample of NI3 so that nitrogen atoms are bonded to one another to form nitrogen gas (N2) and elemental iodine. This leave...