What's the Difference Between Physics and Chemistry?

An off-line question from someone at Seed:

Fundamentally, what is the difference between chemistry and physics?

There are a bunch of different ways to try to explain the dividing lines between disciplines. 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.

At the most fundamental level, you have particle physics and high-energy nuclear physics, which sees everything in terms of quarks and leptons, which are put together to form mesons and hadrons, including the protons and neutrons that we're used to.

The next level up would be low-energy nuclear physics, which deals with protons and neutrons as the essential building blocks, and looks at how they're put together to make nuclei. They don't discard the quark model of nucleons, but it would be calculationally intractable to deal with the individual quarks, so they treat protons and neutrons as given (more or less), and look at how they are arranged.

Next up is atomic physics, which takes nuclei and electrons as given, and looks at how they're put together to form atoms. We don't really worry that much about how the protons and neutrons are arranged in the nucleus, save for where that affects how the electrons are arranged (in things like the hyperfine structure of atoms, which depends on the nuclear spin).

Next is where you start to make the transition between physics and chemistry. 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.

The next level is what most people think of when you say "chemistry," which is dealing with complex molecules. Here, the fundamental entities are groups of atoms-- hydroxyl this and ester that and sulfide and azide and all the rest. They look at how small molecules are put together to form large ones.

From here, you've got two different branches, but the same scale-based hierarchy continues. If you consider big molecules as your basic units, and look at how they combine in small numbers to make more complicated structures, then you're getting into biochemistry. The next level is cell biology, then you get into the study of whole organisms, and eventually into neuroscience and psychology and on into things that aren't really science any more.

On the other branch, if you pack enough molecules of the same type together, it goes back to being physics, in the condensed matter/ solid state regime. There, you treat huge numbers of nuclei and electrons in a statistical sort of way-- you take the bulk structure as a given, and ask how the electrons are, on average, distributed through the system. The fundamental units in this case are collections of vast numbers of atoms and electrons.

And, of course, if you go to a large enough condensed matter system, it just becomes mechanics, in which you treat huge agglomerations of atoms and molecules as solid objects, and look at their motion in response to bulk forces from other huge agglomerations of atoms. When the solid objects become big enough, it becomes astronomy, and then cosmology.

That's my personal Grand Unified Theory of the sciences, anyway.


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"They don't worry about the nuclei at all, really, and only a little bit about the electrons."

Oh come on, give us a litle more credit than that. The average chemist may cringe at the mention of a Born-Oppenheimeer wavefunction, but rest assured physical chemists are very familiar with that kind of thing. And nowadays, quite a few physical chemists are also familiar with non-adiabatic corrections to the Born-Oppenheimer approximation. There's been a lot of work in recent years in conical intersections and the effects of Berry's phase in reaction dynamics.

To turn the table around, I bet a lot of physicists would run away from composing wavefunctions out of Slater determinants, and the ones who wouldn't would be more likely to sneer at the thought and rather work with fancy Grassmann fields anyway.

Also, a great many chemists are familiar with spectroscopic techniques such as NMR, EPR, Raman scattering and the like as tools for extracting information about molecular structures. It is rather disingenuous to think that chemists could use such techniques without at least a decent backgound in AMO physics.

That's essentially what I've been saying on this topic for years, so you are obviously correct!

Don't worry about the electrons? That is about all I worried about in organic chemistry, were are the electrons going and what are they doing.

And, of course, if you go to a large enough condensed matter system, it just becomes mechanics... When the solid objects become big enough, it becomes astronomy, and then cosmology.

Ya left out the geology step in the middle there, Chad.

I mostly agree with #1 on BO being at the Physics/Chemistry boundary, B.O. also being associated with the classical statement that Physics sparks but Chemistry stinks.

As wikipedia begins:

The Born-Oppenheimer (BO) approximation is ubiquitous in quantum chemical calculations of molecular wavefunctions. It consists of two steps.

In the first step the nuclear kinetic energy is neglected, that is, the corresponding operator T_n is subtracted from the total molecular Hamiltonian. In the remaining electronic Hamiltonian H_e the nuclear positions enter as parameters. The electron-nucleus interactions are not removed and the electrons still "feel" the Coulomb potential of the nuclei clamped at certain positions in space. (This first step of the BO approximation is therefore often referred to as the clamped nuclei approximation.)

In the second step of the BO approximation the nuclear kinetic energy T_n (containing partial derivatives with respect to the components of R) is reintroduced and the Schrödinger equation for the nuclear motion... is solved. This second step of the BO approximation involves separation of vibrational, translational, and rotational motions. This can be achieved by application of the Eckart conditions. The eigenvalue E is the total energy of the molecule, including contributions from electrons, nuclear vibrations, and overall rotation and translation of the molecule.

The exceptions to Born-Oppenheimer are themselves interesting, including some genuine Chemistry on surfaces, and some molecular biology.

I'm not claiming that organic chemists don't know anything about nuclear structure or atomic spectroscopy-- that would be foolish. The claim is that their primary concern is with things at a higher level of abstraction.

It's the same with my own field of atomic physics. There are plenty of people working in atomic physics who have an excellent understanding of nuclear physics. I've worked with several of them. Their primary concern as atomic physicists though, is with the arrangement of the electrons in the atoms, and not with how the protons and neutrons are arranged in the nucleus, except insofar as that affects things like the hyperfine structure and internuclear interactions.

Atomic physicists should certainly know something about nuclear physics, in the same way that nuclear physicists should know something about spectroscopy, and particle physicists should know something about band structure. But the study of atomic physics can be defined, roughly, as being that part of physics that cares primarily about the study of individual atoms and their behavior.

me: And, of course, if you go to a large enough condensed matter system, it just becomes mechanics... When the solid objects become big enough, it becomes astronomy, and then cosmology.

NJ: Ya left out the geology step in the middle there, Chad.

Good point.
It's mechanics, then geology, then planetary science, then astronomy, then cosmology.

The Phys/Chem boundary is not a straight line, but either fuzzy or fractal.

Where, Chad, do you put exotic atoms (i.e. hadrons other than protons or neutrons in the nucleus), muonium, positronium? And where do they go in the Periodic Table? The recently detected molecule dipositronium belongs to which discipline?

Without details, I hereby invoke Saint Linus Pauling for defining Chermistry.

A physicist hands you a paper describing the answer. A chemist hands you a vial containing the answer. (A social advocate demands legislation condemning the question.)

When did anthracene ever have so much fun in a short reaction sequence?
Phys. Rev. 134 A1416-A1424 (1964) says it should be a room temp supercon (when doped). Easy to make via ADMET. Easy to derivatize as a lyotropic liquid crystal for spinning into miles of supercon wire. Shouldn't somebody do the chemistry and find out?

Jonathan@8: rather than adding epicycles to the model, we can set aside anything hard-to-categorize and call it "interdisciplinary". Sic transit astroparticle physics, cosmochemistry, plasma physics, exotic atoms, biogeochemistry, and so on.

In my field, organic chemistry, most people usually do not have to calculate anything. It is not that they would be lazy but it is extremely difficult to model things accurately. It is sufficient in most cases to use qualitative explanations, to get the work done. There is a computational branch, people working on chemical calculations of simplified systems ab initio but it is very hard thing and not very useful - real life problems get out of hand quickly. Just taking in account the influence of a solvent is non-trivial. And even the "ab intio" calculations lots of things are introduced by hand, the MO sets used for calculations are chosen on empirical considerations and so on.

With spectroscopic methods: most chemists don't build the instruments, they just learn how to use them. The spectroscopists have to understand whats going on inside and if they are really good, how to program a new puls sequence for NMR. These people are just few, and their position is more like doing experimental physics as a specialised service for chemist.

My daughter asked me for help with her high school chemistry. I looked at the book chapter--it was about what used to be called "valences" but when I was in high school were "oxygen numbers" but apparently are "valences" again--and very early on they started talking about quantum numbers. Well I had heard of them because I read a lot of nontechnical books about physics, but they're certainly not anything they taught us in high school chemistry OR physics.

If the public schools are dumbing down the curriculum as so many people are complaining, why do my kids' textbooks always make me feel so stupid?

By HennepinCountyLawyer (not verified) on 03 Dec 2007 #permalink

I wanted to mention geology as well, but I'd put it a little lower down, chemistry splitting to biology on one side and geology on the other. Looking at it this way, physics is a huge field, it touches on just about every other science. Usually I descibe other sciences in this scale based way, and physics as the study of energy. But its been a while since I was 'almost' a physicist.

So hypothetically speaking. If a theorist/computationalist (with chemistry degrees) was trying to get employed at a SLAC to teach and pursue research in molecular physics and quantum optics with an eye towards single-molecule spectroscopy and photochemistry, should they apply to physics or chemistry departments? The 'hierarchy' is killing 'em.

By Cry for me (not verified) on 03 Dec 2007 #permalink

So hypothetically speaking. If a theorist/computationalist (with chemistry degrees) was trying to get employed at a SLAC to teach and pursue research in molecular physics and quantum optics with an eye towards single-molecule spectroscopy and photochemistry, should they apply to physics or chemistry departments?

Hypothetically, I would say that if you would be comfortable teaching physics (especially intro physics), you should go ahead and apply to both. It doubles your hypothetical options.

If you wouldn't be comfortable teaching physics classes, hypothetically, then only apply to chemistry departments.

I would say that chemistry is a specialization in a narrow subset of physics, emphasizing empirically-gained knowledge about the properties of matter.

By Caledonian (not verified) on 03 Dec 2007 #permalink

Just having fun here. ;-)

Offhand, I'd say that, with the exception of fluid mechanics, physics tends to be a dry science. Chemistry is the wet science; biology is the wet and messy science.

Caledonian: ah yes, don't forget to preach from your lofty physics vantage to all us less scientists. You'd do Lord Kelvin proud.

Re: Caledonian's comment. My high school physics and chemistry teachers (both taught AP, but I dropped out of the AP chem half-way through and switched to regular chem) had an ongoing rivalry. There were two related articles in one issue of the school newspaper, written by each of them: The Top Ten Reasons Chemistry is Better Than Physics; The Top Ten Reasons Physics is Better Than Chemistry.

One item from each list stands out, though I can't recall the exact phrasing. The chem teacher said that chemistry was the central science, and therefore physics was just a branch of chemistry; the phys teacher said that physics was the fundamental science, and therefore chemistry was just an example of physics. They were (are) both great teachers, and it was a great article.

Anyway, it's true that chemistry historically began as an empirical science with no real understanding of an underlying explanatory framework, though it did make predictions based on laws: generalizations of observed regularities. The categorization of certain types of substances, i.e., acids and bases and their reactions with each other and other substances, Mendeleev's version of the Periodic Table, which predicted undiscovered elements, etc.

It's easy to say now that the underlying theory can be explained in terms of physics, but that doesn't mean that physicists can retroactively take credit for the huge body of empirical data and laws in chemistry's history and the modern work being churned out, just because some physical discoveries have turned out to be relevant to our understanding of how chemical reactions work. The connections between different disciplines gives us hope that we're on the right track, putting together a consistent view of the universe, but we wouldn't have that without people working at all levels of scale. It's just like paleontology and evolutionary biology. Both fed into each other but paleontology hasn't been subsumed into the other science.

There was a story about young Teller visiting a chemistry group in Chicago just when they were trying to figure out the best isolation technique for obtaining the minuscule traces of plutonium from irradiated material. As they were testing various precipitation techniques to see if Pu would concentrate in the precipitate, Teller exasperated them by claiming that he could calculate any concievable chemistry problem from the first principles...

I happen to be enrolled in a chemical physics PhD program and I am still trying to figure out the difference between chemical physics and physical chemistry.

I think Dr. Free-Ride has the best answer so far: the difference is cultural, not scientific. Both chemistry and physics have progressed so much since 1926 with the advent of quantum theory that it is really, really hard to draw a line in the sand today. It is therefore unfortunate (and unproductive) that the vast majority of physics and chemistry are still taught from the trenches of 19th century knowledge.

Some recent advances (in addition to spectroscopy and non-adiabatic dynamics that I've mentioned above) that I think muddy the sand further are the techniques of quantum chemistry (ab initio wavefunction-based methods), density functional theory, atoms-in-molecules theory (a la Bader) based on quantum mechanics with open boundary conditions, and terahertz spectroscopy.

It seems that when it comes to DFT, there is a *very* interesting twist on how differently chemists and physicists apply DFT. DFT practitioners seem to be divided between Kohn-Sham methods and plane-wave methods, which seem to fall along partisan lines of chemistry vs. physics. Just ask a DFT practitioner about RKS/B3LYP/6-31+G** or Bethe-Salpeter equations; chances are either one will provide you with about 5 seconds of blank stares.

And then there are the "conceptual DFT" people who associate strongly with the seminal book of Parr and Yang (1995) and the review article by Geerlings, De Proft and Langenaeker Chem. Rev.; 2003; 103(5) pp 1793 - 1874, that try to axomatize and make mathematically rigorous the chemical concepts of electronegativity (electron donating/withdrawing tendencies of atoms), chemical hardness, and the like.

I'd be happy to elaborate, if anyone else is interested.

Another cultural note: I've noticed that what passes for physical chemistry in the US is often considered the turf of atomic and molecular physicists in Europe. I'm thinking of spectroscopy and computational quantum mechanics in particular. Not sure what to make of that.

In reference to #20, the earliest quote to that effect that I know of can be attributed to Dirac, which goes like "the underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are thus completely known, and the difficulty is only that that the exact application of these laws leads to equations much too complicated to be solvable."

If Teller could really predict precipitation reactions from first principles, then we should make copies of his brain as soon as possible, and the entire community of computational chemists should promptly start looking for new careers.

"Chemie ist das, was knallt und stinkt, Physik ist das, was nie gelingt."

"Chemistry is that which bangs and stinks, physics is that which one can never succeed [in getting it to work]." German proverb referring to school experiments.

By David MarjanoviÄ, OM (not verified) on 04 Dec 2007 #permalink

Over here (University of Vienna), spectroscopy counts as analytical chemistry...

By David MarjanoviÄ (not verified) on 04 Dec 2007 #permalink

Caledonian: ah yes, don't forget to preach from your lofty physics vantage to all us less scientists. You'd do Lord Kelvin proud.

Ah, the delicate ego of the insecure.

Physics encompasses the study of all properties of matter; chemistry is necessarily a specialization within it. But the impracticality (and in many cases current impossibility) of deriving chemical predictions from first principles is what makes chemistry so distinct as a discipline unto itself. As opposed to, say, optics. Optics is also a subfield within the larger category of physics, but is simple enough that it never needed to reach the level of specialization required for chemistry.

Just as aerodynamics should theoretically be derivable from quantum mechanics, and in reality includes lots and lots of empirically-derived findings that we can't derive from the basic physics, lots of chemistry cannot be derived from the basic physics in practice.

The real issue is that we're comparing a lesser category with a greater one, which is like comparing apples and roast turkeys.

By Caledonian (not verified) on 04 Dec 2007 #permalink

# 26 | Caledonian | December 4, 2007 8:58 AM

Another: "Caledonian: ah yes, don't forget to preach from your lofty physics vantage to all us less scientists. You'd do Lord Kelvin proud."

Caledonian: "Ah, the delicate ego of the insecure."

Do I smell a physicist?

Caledonian: "Physics encompasses the study of all properties of matter; chemistry is necessarily a specialization within it."

That's what physicists say; that's not what they do. They still manage to leave 90-odd percent of the study of 'the proporties of matter' to other fields. It's almost as if they *couldn't* deal with such things.

However, I'm sure that Caledonian will assure us all that it's really the case that physicists *could*, but don't want to lower themselves.

To your comment on Born-Oppenheimeer wavefunction. A very intelligent Physicist will moan at the mention of "Lobry-de Bruyn-van-Alberda-van-Ekenstein transformation" in carbohydrate chemistry.LOL.

I love physics, maybe not as much as I love Chemistry, But there are no hard Distinctions between the objectives of both sciences in understanding Matter.

Let both physics and chemists team up and bully the Biologist. xD.

"What's the Difference Between Physics and Chemistry?"


Much Chemistry (because of history, convenience, cost, and us as "made of meat") is in aqueous solution, which limits the temperature range. Again, I say "much" rather than all, as colliding molecular beams for Femtochemistry indicate. But Physics happily spans a range from nanokelvin to Planck temperature. Too hot for molecules? Hell, we like it too hot for atoms, too hot for protons, too hot to distinguish one force from another.

Also different: the iconic Science Fiction authors. Chemistry has Isaac Asimov and Harry Stubbs (Hal Clement). Physics has Dave Brin, Greg Benford, Catherine Asaro, San Schmidt, and Greg Egan. I've left out many other related names, as well as the concentration of Astronomy experts in Science Fiction, Engineering (harkening back to the Hugo Gernsback radio-hacker era, Arthur C. Clarke, et al), and Biology (including the brilliantly self-taught such as Greg Bear).