most molecules probably do have an odour of some description. as simple elements as they are so small and the electron arrangements are so simple, it is likely that the detection threshold in a human olfactory system is just... none existant i guess. our sense of smell is very basic. There are a few theories as to why some molecules smell, for example... vibrations. Some researchers think that molecules vibrate at different frequencies and that olfactory cells contain molecules that vibrate at similar frequencies which causes an interaction and impulses to the brain etc. Also the electrolyte theory is a bit naff, but some think that when stink molecules enter olfac. cells it upsets the electrolyte balance and causes impulses to the brain again. The most widely accepted though is the size/shape fit theory i guess. The size shape and electron arrangement determines how the cells interact with the molecules. Oh and also the idea that the olfactory nerves extend into the mucous layer beneath the olfactory membrane and contain specific primary receptor proteins. These can bind to certain molecules and set of impulses. Like specific enzymes to chemicals, specific receptor proteins to certain molecules.