How does the smell of a compound come about, and is it possible to define a smell?
Smell is what you feel when molecules of some compound (and not any kind of radiation, mind you) touch the olfactory nerves deep inside your nose. It is just another property of a compound, like molecular weight, or melting point, or color, only different.


Unlike molecular weight, it can't be calculated from molecular formula by simple rules.
Unlike molecular weight or melting point, it can't even be adequately described with a single number.
Unlike all of the above, it can't be reliably predicted for a new compound, though much can be done by the way of analogy.


$\ce{H2Se}$ smells worse than $\ce{H2S}$, and $\ce{H2Te}$ is worse still. A disturbing characteristic of the latter, given by an anonymous expert in the field, I'd better keep hidden so as not to offend the public taste:


   Smell it, and you'll vomit farther than you can see.