Over at Uncertain Principles, Chad Orzel tries to explain the fundamental difference between physics and chemistry:
My take on this particular question is that there’s a whole hierarchy of (sub)fields, based on what level of abstraction you work at. The question really has to do with what you consider the fundamental building block of the systems you study.
Chad’s rough breakdown is fine as far as it goes. But it wouldn’t (in my experience) be a terribly accurate guide to discerning what (say) a physical chemist actually worked on in his or her research.
Chad describes the corresponding rung on the hierarchy like this:
This is the level of the overlapping fields of molecular physics and physical chemistry, which takes atoms as the essential particles and looks at how they fit together to make simple molecules. They don’t worry about the nuclei at all, really, and only a little bit about the electrons. It’s a tricky division to make, but if I had to make a stab at defining the essential difference between molecular physics and small-molecule chemistry, I would say that the physics side is mostly concerned with how small molecules are put together and how they stay together, while chemists are more interested in how small molecules react with each other and swap pieces back and forth.
Mmmm … kind of.
I was a physical chemist who worked on mechanistic studies of oscillating chemical reactions. I’m not sure that I took “atoms as the essential particles” to a greater or lesser extent than did my colleagues working in organic chemistry or in biophysical chemistry. Also, I’m not sure all the molecules and ions in my system would count as “small”. Sure, they’re small compared to proteins, but big compared to diatomic and triatomic species.
But maybe this difference of opinion is, in itself, illustrating a point of difference between chemistry and physics.
Chad (the physicist) sees the division as a matter of the size and type of the building blocks in your system. My own view is that the differences between chemistry and physics have less to do with the objects of study and more to do with the particular theoretical, methodological, and instrumental “toolbox” one brings to the job of studying those objects.
When I mentioned this to Chad, he replied, “The objects being studied play a large part in determining the appropriate methodology and theoretical approach for dealing with them.”
That’s true, but at the boundaries, a chemist and a physicist will look at the same stuff using different theoretical and methodological toolboxes.
They may borrow tools from each other, but they won’t always use them in ways that the folks they borrowed them from would envision (or completely endorse). Maybe some of this is related to physicists generally using their tools in a scaling-up-from-atoms way and chemists generally using their tools in a scaling-down-from-molecules way. But some of it may have more to do with the sorts of problems people in each field seem themselves as responsible for explaining — something I think may not always track building-block size perfectly.