February 9, 2012 | 10
One of the standout anecdotes in Carl Zimmer’s most excellent compilation, Science Ink (a.k.a. My Favorite Science Book of 2011 And Possibly Ever) occurs in the first few pages:
“A former student [physics major] got a tattoo of a cartoon atom on the back of one of his legs. He told me that the first day after he got it, he went to rugby practice, and was showing it to someone when one of the seniors on the team (also a physics major) walked by. The senior looked at it, and said, ‘Oh, please. The Bohr model?’ And walked off.”
Oh, snap! Guess that poor underclassman got told! And he must live with the shame of his naive physics knowledge on his skin permanently (barring modification or tattoo removal treatments). Welcome to Hipster Physics!
Seriously, though, this is not the first time a physicist has complained about the much-maligned Bohr model of the atom. It’s like a rite of passage, the day you learn that the eye-catching little diagram of a small nucleus orbited by electrons you see all around — from the logo of the US Atomic Energy Commission, to the scene changes in episodes of The Big Bang Theory — simply isn’t the most accurate model for the atom anymore among “serious” scientists (or science writers). And espousing it is grounds for mockery, usually in the form of polite snickers and chuckling condescension from those “in the know.”
Clearly I am not a hipster, because I love the Bohr model, and will staunchly defend its use — at least in popular physics books for general audiences, and introductory courses for undergraduates. Sure, it’s been superseded since Niels Bohr first proposed it in 1913, as our understanding of the quantum world has advanced. I’m not advocating its return to cutting-edge physics research. But when it comes to outreach, it’s the perfect entry-level model for atomic structure.
Let’s get into the Wayback machine and go back to the dawn of the 20th century, just after J.J. Thomson had discovered the electron, and proposed his “plum pudding” model of the atom (see below). Bear in mind that for centuries, physicists had fought the very idea of an atom, despite the fact that Democritus had been an “atomist” two thousand years earlier. (The Time Lord likes to point out that, in this respect, the chemists were way ahead of the physicists; they accepted the existence of the atom much earlier.)
Thomson initially called his mysterious little particles “corpuscles,” and suggested that they were the primary components of an atom: a collection of negatively charged “plums” immersed in a positively-charged “soup,” or “pudding.” But then, in 1909, Ernest Rutherford went and discovered the atomic nucleus via a classic scattering experiment involving gold foil. Such an effect (scattering of alpha particles) occurred because there was a hard, dense center to atomic structure.
Thomson’s plum pudding model was handily discarded, and in its stead, Rutherford proposed something more akin to the planets orbiting the sun in our solar system. The nucleus serves as a “sun” at the center, and is positively charged, while the electrons are “planets” and negatively charged, moving about the nucleus in circular orbits.
It was pretty close to the popular design we’re familiar with today, but it violated classical physics in a very important way: if the Rutherford model were correct, the electrons would emit radiation as they orbited, such that over time, the electrons would spiral inward and collapse into the nucleus. All atoms would be inherently unstable. Since they weren’t, obviously something else was going on.
Furthermore, the frequency of the radiation would increase as the electron spiraled inward, because the orbit would get smaller and the electron would move ever-faster. That just didn’t happen. And this model didn’t agree with electrical discharge experiments demonstrating that atoms only emit light (electromagnetic radiation) in discrete frequencies, leading to Max Planck proposing “quanta” in 1900, thereby launching a revolution in physics.
Phew! Clearly, Rutherford’s model needed to be brought in line with the nascent field of quantum mechanics before it could be truly viable. Enter a young Danish upstart named Niels Bohr, who’d come to Rutherford’s lab via a postdoc with Thomson after earning his PhD in physics from the University of Copenhagen. Bohr set about adapting Rutherford’s model to accommodate the need for discrete units of energy (the quanta).
The model he came up with is the one we know and love today (often termed the Rutherford-Bohr model), in which electrons move about the atomic nucleus in circular orbits, just as in Rutherford’s model. But those orbits have set discrete energies, and those energies are related to an orbit’s size: the lowest energy, or “ground state,” is associated with the smallest orbit. Whenever an electron changes speed or direction (according to the Bohr model), it emits radiation in the specific frequencies associated with particular orbitals.
Diss the Bohr model all you like — that innovation snagged its creator the 1922 Nobel Prize in Physics. As Sheldon Cooper would say, “Bazinga!”
Yeah, okay, it’s not perfect. The biggest issue is that it violates the Uncertainty Principle (which wasn’t even formulated until 1927). Remember, the principle states that you can’t correctly pinpoint both a particle’s position and momentum (energy) at the same time, and in the Bohr model, you’ve got electrons with both known orbits and well-defined radii.
(There’s also other shortcomings related to predictions about the spectra of larger atoms and the relative intensities of spectral lines, yadda, yadda, yadda, but we’re focusing on the most major objections for the sake of simplicity. John and Jane Q. Public are not lying awake at night quibbling over the Zeeman effect.)
And technically, the electrons don’t really “move” around the nucleus in orbits. Erwin Schroedinger (of the famous cat paradox) was the one who proved that electrons are really waves (although they show up as particles when you perform an experiment to determine its position), and those waves are stationary.
Sure, you can check to see where an electron is, but each time you do, it will show up in a different position — not because it’s moving, but because of the superposition of states. The electron doesn’t have a fixed position until you look at it and the wave function collapses. (However, if you make a ton of measurements and plot the various positions of the electron, eventually you’ll get a ghostly orbit-like pattern such as the one depicted above.)
That’s why Schroedinger’s atomic model dispenses with orbits in favor of energy levels, which is what physicists really care about anyway. It still shares some similar concepts with the Bohr model. For instance, if an atom heats up (i.e., is energized), its electrons move to higher levels. As they cool and fall back to their normal ground state, the excess energy has to go somewhere, so it’s emitted as photons, which our eyes perceive as light. And those photons possess frequencies that match the change in energy levels, in keeping with earlier experiments.
Confused yet? No wonder! To understand why physicists discarded the Bohr model, you’ve got to delve into the mind-bending intricacies of quantum mechanics, and explain all kinds of things the average person likely has never encountered in any real depth: wave functions, uncertainty, superposition of states, spectral lines, and so on.
That’s why I prefer the Bohr model to introduce non-scientists to the basics of atomic structure. It gets across the basic concepts (discrete intervals and why light is emitted in specific units of frequency), and offers the neophyte a handy visualization via the analog of the atom as small-scale solar system. There’s plenty of opportunity to enhance someone’s understanding later as they progress in their basic physics knowledge — education happens in stages, not all at once. In fact, the Bohr model offers the perfect opening to talk about some of those more advanced ideas.
So don’t y’all be dissin’ my beloved Bohr model!