Niels Bohr (1885 - 1962) was a young post-doctoral fellow from Denmark working with Ernest Rutherford in England during these heady days of discovery. The experimental feature plaguing their minds was the structure of the atom. How could the electrons orbit the nucleus and not lose energy? Why was this configuration stable?

Bohr developed these ideas into a theory of atomic structure. He knew from Balmer's work that the energy levels were related to integers according to his phenomenological equation. These integers became Bohr's quantum numbers. Since the energy was known to scale inversely with the square of these quantum numbers and also inversely with the square of the angular momentum, he suggested that the angular momentum was directly proportional to the quantum number. Furthermore, since Planck's constant h had the same units as angular momentum, Bohr theorized that the angular momentum was quantized in units of h/2π. This quantization could not be proven on classical grounds or even justified, but Bohr showed that when he made these assumptions, he could predict the spectrum of hydrogen with unprecedented accuracy. Look here for a brief presentation of the derivation of his theory.

The atom that Bohr envisioned had these stable orbits because of quantization of the angular momentum. The resulting image, such as at right, is the one with which the world is most familiar when discussing atomic structure, namely, small electron balls whizzing around a central nucleus. But why was it quantized? And his theory provided no means for calculating the intensities of these atomic lines - only their positions. And of course there was no concept of chemical bonding or any further explanations beyond the hydrogen atom. And yet, his success was indisputable. Where was the connection?