Here's the first set! (C means carbon, H means hydrogen, S means sulfur):
Do molecules that contain sulfur seem to smell nice? Of course, these are only a few of the millions of molecules that can contain sulfur, but frankly, most of them are pretty stinky!
Now look at the second group! (C means carbon, H means hydrogen, N means nitrogen):
Whoa! These are worse than the sulfur-containing molecules! In fact, most of the world's very worst-smelling molecules contain nitrogen. There's even one called "putrescene"! However, there are also many, many perfectly odor-free nitrogen-containing molecules!
Let's do some nicer-smelling examples (C means carbon, H means hydrogen, O means oxygen):
Now we're talking! These are odors that we don't mind sniffing! What features do you observe in common? See that group with an O next to a =O? The atom without a label is a carbon. That's called an ester group, and esters tend to have sweet, pleasant odors. Find the ester group in each molecule.
More nice molecules:
These are a group of molecules called terpenes. They have that hexagon thing at the top (which is sometimes broken, with only 5 carbons), and that funny fishtail part at the bottom. Terpenes tend to have sharp, but pleasant, odors. Notice that there are different things hanging off the rin in different places, and these are what make each molecule smell different.
Here's the last set! (C means carbon, H means hydrogen, O means oxygen):
All of these molecules are aldehydes, because they all have the HC=O part. See if you can find this aldehyde group in each example.
Now, please understand that natural scents usually have lots of molecules in them, not just one. However, in all of these examples, if you smelled this molecule, you would know immediately what it came from. The smell wouldn't be exactly like the natural scent, but you could recognize it.
One final comment - earlier this month, we talked about how the receptors in the nose reacted with the odor molecules, letting you smell them? Well, these are examples of those odor molecules. Now here's the cool thing - your nose's receptors can tell that these molecules have different sizes and shapes, and that's how you can tell smells apart! The receptors send slightly different signals to your brain, which knows that you should identify one kind of signal with bananas, and another kind with poop! And that's a very good thing!