Biophysicist Luca Turin explains the 'magic' of scent in his book The Secret of Scent,
"Molecules are what we smell: smell is our chemical sense, and we are so used to it that we do not stop to think about how amazing it is. Consider this: a bit of brain grows downwards through a bone shot through with holes and dangles in the breeze somewhere up your nose. Smells, unlike sound and light, do not act at a distance: if you smell something, it is because pieces of the smelly stuff are evaporating from whatever it is you are smelling - whether it is Ruis de Nuit or frying bacon - flying through the air and ending up in your nose. The fact that they are colorless makes it look like a mysterious agency is at work, but if you were to add perfume to colorless smoke you would soon realize that you only smell the stuff when the color reaches you. A complex smell is just what it says: a complicated mixture of different molecules, each adding its own odor, the sum total of which is the thing we call the smell. But what are these smelly molecules? ...molecules are assemblages of atoms. Atoms come in different flavors, or elements, and the elements differ in their ability to connect with each other. As was figured out about a hundred years ago, atoms are not unlike tiny solar systems, with a central sun (the nucleus) and planets around them (electrons). The nucleus is made up of positive charges and the electrons are negative. Equal numbers of negative and positive charges are required to give a nice neutral, well-behaved atom. The negative planets don't simply stack up in increasingly larger orbits, but instead form groups - groups of eight to be exact. It's as if in each orbit there was room for eight planets, then when that orbit is considered filled up, you start with the next one.
What is important about all of this is that the social behavior of atoms - the branch of science known as chemistry - depends largely on how many atoms there are in the outer orbit. It is as if atoms are more comfortable with filled orbits, and are constantly searching for partners to swap electrons and achieve peace. For example, if one atom has seven electrons in its outer circle, it behaves like a collector trying to fill that yawning gap on its shelf and snaps up any electron around. Similarly, if an atom has just started the next electron orbit, and has only one electron in it, it is quite happy to give that up to go back to the neat arrangement of a properly filled eight-electron orbit. As you can readily imagine, when this generous soul meets the manic collector, it's Love."
And so on.... a more scientific explanation of Luca Turin's theory of smell can be found here.
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