When a feather drops on it, or a mosquito lands on top of nitrogen triiodide – Booom! – an instantaneous dark purple or violet explosion occurs. It’s because nitrogen triiodide is so unstable that it detonates even when it’s barely disturbed.
Three iodine atoms cluster around one side of a nitrogen atom. Being crowded around one end causes something known as bond strain as the atoms repel each other in a small space. Hence that the molecule is susceptible to falling apart, lets say, quite explosively.
NO3 visualized in 3D – Source: Wikipedia
You should definitely take a look at the awesome slow-mo video of triiodide blasting off, created by the blokes over at the Royal Institution.
