I Blame the Bohr Model
Tuesday July 14, 2009
The other day I mentioned that I had added an index of 11th grade chemistry topics and that I was hyperlinking content so you could learn high school chemistry online. This has involved creating some new content for topics I normally wouldn't cover. The Bohr Model is a case in point. It's considered important for students to learn, but I personally feel it gives them the wrong idea about how electrons behave. They aren't out there happily circling the nucleus. I mean, they might be, but they could be passing through the nucleus or exist pretty much anywhere in the vicinity of the atom. The Bohr Model addressed fundamental flaws in the Rutherford model and successfully explained the Ryberg formula, but most students don't leave high school caring much about the spectral lines of hydrogen. They do have this notion about what an atom is and how it looks and behaves. Usually that is based on the Bohr Model. It's a good model and all... it's just not right.
Comments
My college-level general biology course and textbook described atoms using the Bohr model. At the time it didn’t seem to make much difference, but now that I’ve had several chemistry courses, I guess it’s a bit surprising. Sure makes the atom a whole lot easier to picture, though.
With every level of chemistry one can learn to love or hate the Bohr model. It does a great job of illustrating the relative energy levels of electrons, but violates one of the most principle of principles in quantum mechanics – that of uncertainty! However, it is an undispensable tool for learning quantum theory.
I teach the Bohr model to my college students, but I take pains to point out that while it does a good job explaining the hydrogen emission spectrum, it’s useless for helium or more complex atoms. It’s an important ’stepping stone’ to a deeper understanding of atomic structure, but that’s all. (Teachers call this ’scaffolding’.)
The Bohr model is too significant to pass up. If you cant teach it without explaining the limitations of the Bohr model and explaining more accurate models, then you aren’t teaching
The Bohr model strictly applies to 1-electron models of the atom. Simply adding this comment to its explication (with some general comments on exchange and correlation energy) gives a nice introduction into why it takes 4 quantum numbers to “describe” an electron.
For lower-level courses, the Bohr model is also the best and simplest way to show energy quantization. Much harder to visualize a change in the average radius of a probability distribution …
Amen! And if students keep using this stupid (wrong!) Newtonian model of time, how are they ever going to learn to synchronize with a GPS satellite?
Seriously, the idea that a model is not reality is one of the fundamental concepts you want your science students to understand. Likewise, they need to learn how a newer and better model subsumes the correct predictions of the older model.
sorry, i agree with the blogger. unless one HAS to teach the bohr model because some later class will assume the student knows it (perpetuation of mistakes), it is just as easy to teach quantized wave modes in a confined volume. do we teach elephants on turtles when we teach cosmology? we could even do away with newton and go straight to QED and quantum statistics.
The Bohr model is an excellent educational starting point. Unfortunately, we find ourselves stepping off into the unknown the moment we invoke a quantum mechanical description. What is a wavefunction? It is merely a mathematical object. It has no basis in physical reality. Chemists call wavefunctions “orbitals”. If we don’t like the Bohr model, then why are Chemists insistent on referring to “electron orbitals”?
The underlying problem is that most teachers (sorry to say, and with all due respect) simply do not think deeply about the subject of fundamental physics any more. Quantum mechanics does not provide the only description of the atom. It may not even provide any real description at all (it just seems to!). QM, DFT and other first-principles and ab initio approaches all fail. Why is this? Why aren’t we going back to turning point moments in scientific history like the moment when Bohr realized that he just wasn’t getting anywhere with his classical model of the atom, threw up his arms (figuratively, perhaps), and helped initiate the scientific pursuits that helped shape what we now know as quantum mechanics (version 1.0). We’re not on QM-version 2.0, and we still haven’t gotten any better answers. What is “spin” (in the quantum mechanical sense)? Is the uncertainty principle a cold hard fact of reality? Is the exclusion principle a general hard-fast rule (NO!!) or just an ad-hoc rule of thumb that matches up nicely with experiments? What’s the physical basis for exclusion?
Einstein never liked QM simply because it does nothing to get us any closer to an understanding of “the Old One” (i.e. the underlying meaning of things in the universe).
Why aren’t we digging deeper into these questions? Where are the professors good enough to instill urgency, creativity and integrity of thought in today’s students? Where are the free-thinkers who are not afraid to knock Einstein and Bohr off their god-like pedestals? And why do we treat such historical scientific figures with so much respect in the first place — to the extent that many people are frankly afraid to raise their hand and ask “a stupid question”.
It’s time to question this whole thing called science once again and see what new things we can learn about “the Old One”.