Elements With Near-Perfect Atomic Weight

Atomic Mass is essentially a measure of the average of the different isotopes of a specific element. Therefore while some periodic tables round to cool round numbers they are not exactly exact as in all the combinations with that number of protons its a very slim probobilty they would round out to a round number. Oxygen for instance on my periodic table is listed as 15.999. So I would say exact is not the word for it.