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.

Bohr also established the difference between strictly electronic phenomena—emission and absorption of light, along with ionization—from radioactivity, which is a nuclear process. Bohr's model assumed a very simple thing: electrons behave like particles in Newtonian physics but are confined to specific orbits.

Like the Copernican model of the Solar System, which opened up a mental space for later researchers to develop the view we have today, the Bohr model solved a major problem in understanding atoms, establishing a conceptual framework on which the mathematical structure of quantum mechanics would be built.

Neils Bohr was a Danish physicist, but he worked extensively in other countries, collaborating with scientists from many nations. The work that made his name, for example, was based on postdoctoral research in England with J.J. Thompson, who discovered the electron, and Ernest Rutherford, who established the existence of atomic nuclei. Bohr famously continued to work on quantum theory throughout his life and made significant contributions to nuclear physics in the early days of that field. He also helped physicists escape Europe during the Nazi persecution of "undesirable" groups, including Jews and those perceived as working on "Jewish science." (Hollywood, it's time for a Bohr movie.)

Atoms and electrons in the 21st century

One important implication of Bohr's research was that electrons behave differently within atoms and materials than they do in free space. As Nobel laureate Frank Wilczek pointed out in his comment in the Nature retrospective, we're still learning exactly how interactions shape electrons—and how electrons' properties result in different behavior at high and low energies.

Bohr's original model conceived of electrons as point particles moving in circular orbits, but later work by the likes of Louis de Broglie and Erwin Schrödinger showed that the structure of atoms arose naturally if electrons have wavelike properties. Bohr was one who grappled with the meaning of the wave-particle duality in the 1920s, something modern experiments are still probing in creative ways.

Similarly, atomic physics is pushing into new frontiers, thanks to both gentler and more aggressive probing techniques. Rydberg atoms—atoms in which the electrons are excited nearly to the ionization point—are physically very large, some almost big enough to be macroscopic. These aren't very stable since such a loose connection between electron and atom means nearly anything can strip the electron away. However, the weaker interactions also mean a Rydberg atom behaves nearly like Bohr's model predicted: a miniature solar system with a planetary electron. This sort of system allows physicists precise control of atomic properties.

Another exciting area of research involves ionization, but of a different sort than usual. As Bohr's model indicated, electrons fall into orbits, and those closest to the nucleus are harder to strip from the atom than those farther out. Thus, ionization typically happens when electrons are removed from the outermost orbits. However, the pulsed X-ray laser at SLAC can remove just the inner electrons, leaving an ion with a thin shell of electrons outside—what SLAC physicist Linda Young referred to as a "hollow atom." This is an unstable configuration since the electrons prefer to be in a lower energy state, so hollow atoms rapidly collapse, emitting bursts of high-energy photons and electrons. Some researchers are considering potential applications for these energy outbursts.

Bohr's model was the beginning, but the story of atoms he helped write is still ongoing. The tale is (you knew it was coming) far from Bohring.