Happy 100th birthday to the Bohr atom

Danish physicist Niels Bohr, whose model of atoms helped explain the spectrum of light emitted and absorbed by different elements, as illustrated by the spectrum emitted by the Sun.

AB Lagrelius & Westphal, via Wikipedia (Niels Bohr photo); N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF (solar spectrum)

Niels Bohr's model of the hydrogen atom—first published 100 years ago and commemorated in a special issue of Nature—is simple, elegant, revolutionary, and wrong. Well, "wrong" isn't exactly accurate—incomplete or preliminary are better terms. The Bohr model was an essential step toward an accurate theory of atomic structure, which required the development of quantum mechanics in the 1920s. Even in its preliminary state, the model is good enough for many calculations in astronomy, chemistry, and other fields, saving the trouble of performing often-complex calculations with the Schrödinger equation. This conceptual and mathematical simplicity keeps the Bohr model relevant.

Despite a century of work, atomic physics is not a quiet field. Researchers continue to probe the structure of atoms, especially in their more extreme and exotic forms, to help understand the nature of electron interactions. They've created anti-atoms of antiprotons and positrons to see if they have the same spectra as their matter counterparts or even to see if they fall up instead of down in a gravitational field. Others have made huge atoms by exciting electrons nearly to the point where they break free, and some have made even more exotic "hollow atoms," where the inner electrons of atoms are stripped out while the outer electrons are left in place.

Bohr and his legacy

The Bohr atomic model is familiar to many: a dense nucleus of positive charge with electrons orbiting at specific energies. Because of that rigid structure (in contrast to planets, which can orbit a star at any distance), atoms can only absorb and emit light of certain wavelengths, which correspond to the differences in energy levels within the atom. Bohr neatly solved the problem of that feature of the hydrogen spectrum and (along with contributions by other physicists) a few more complex atoms. Even though the Bohr model was unable to provide quantitative predictions for many atomic phenomena, it did explain the general behavior of atoms—specifically why each type of atom and molecule has its own unique spectrum.