In organic chemistry, an "unsaturated" compound is one which contains at least at one "pi" bond--a "double" bond between two of its carbons which uses two electrons from each carbon instead of one. Determining how many pi bonds an unsaturated compound has--its "unsaturation number"--is tedious to do if you choose to draw the compound out by hand. If, on the other hand, you calculate this number using a simple formula chemists have devised, it will take you only a few moments.
Replace any halogens--such as bromine, iodine or chlorine--your compound has with hydrogens for the purpose of the calculation. For instance, if your compound is C6H6N3OCl, you would re-write it as C6H7N3O.
Disregard any oxygens your compound contains--these are not relevant to the degree of unsaturation calculation. You would now write the example compound as C6H7N3.
Subtract each nitrogen from a hydrogen. In this example, you can now represent the compound as C6H4.
Calculate the unsaturation number for your compound, which is now in the form CnHm, using this formula Ω = n - (m/2) + 1, where "Ω" is the degree of unsaturation--the number of pi bonds your compound has. For the example compound, C6H4, do this as follows: Ω = 6 - (4/2) + 1 = 6 - 2 + 1 = 5. The compound C6H6N3OCl therefore contains five double bonds.