Many of us are familiar with the Bohr atom: a simple model with a nucleus and planet-like electrons orbiting in circular paths. It’s a useful picture, even though it’s not complete. Bohr proposed it in 1913, but it took about ten more years for physicists to work out why it worked — and to refine it into the quantum-mechanical picture of atoms we have today. However, we’re still probing the structure of atoms, especially the really bizarre behaviors under extreme conditions. Bohr’s contributions are still relevant today.

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. [Read more…]

A century of the Bohr atom