© William P. Reinhardt.
Bohr's classical orbits astonishingly allowed him to predict the actual energy levels, and spectra, of the electron in the one-electron hydrogen atom, but what happens when there are two electrons? In 1921, Irving Langmuir found periodic orbits of two electrons moving with respect to a fixed nucleus (which is not drawn to scale here). In these periodic orbits, the repelling electrons bounce back and forth along the black paths shown above. However, this orbit is completely unstable. If one of the electrons is slightly disturbed, the electrons immediately follow chaotic trajectories such as the ones shown by the green and purple lines. One electron is ejected from the atom, and the other remains in a stable orbit around the nucleus. As helium atoms are the most stable atoms known, and by no means self-ionize, this indicates that classical motion of the electrons is only a good starting point for understanding the nature of atomic structure and spectra for one-electron atoms and ions. In fact, we actually need quantum mechanics to fully understand the hydrogen atom, and most definitely to understand even the most basic properties of the helium atom. (Unit: 6)