Wikipedia). In other words, a molecule is when you have two or more atoms bound together in some way. One atom does not make a molecule - hydrogen on its own is just an atom, but combine it with another hydrogen and an oxygen atom and you've got at water molecule!
We can have small molecules - take something like sodium chloride - or large molecules - like DNA.
Click here for a post on our skin!) Fatty acids are considered fairly decent sized molecules - as you can see from this oleic acid molecule - and they don't tend to penetrate our skin. (That's why jojoba oil is considered unique in that it likes to penetrate our skin through the hair follicles!)
I say "they don't tend..." because there could be some current research I haven't seen that shows that some of the smaller fatty acids can penetrate our skin or there could be some information on penetration enhancers that I haven't read. I can't be completely certain and completely up to the minute on my science here!