Bohr’s molecular model, a century later

Despite the great power and scope of modern computers, there remains yearning for intuitive, pedagogically appealing ways to understand electronic structures in atoms, molecules, and more complicated systems. Niels Bohr’s century-old model, in which electrons parade around atomic nuclei much like planets orbit the Sun, retains such appeal. In its historical context, the Bohr model managed to elucidate the hydrogen-atom spectrum and thereby gave great impetus to the birth of quantum mechanics.^1,2,3,4 But it proved incompatible with quantum theory, which found that the ground state of the H atom was spherically symmetric and had zero orbital angular momentum, contrary to Bohr’s assumption of a planar, circular electron orbit with angular momentum h/2π, specified by Planck’s constant. Even so, today the Bohr model has valid roles in describing highly excited Rydberg atoms,⁵ cavity quantum electrodynamics,⁶ and quasi-Rydberg states in...