This has been a chapter of ideas, and very strange ideas they were when they first appeared: light waves that came in bundles, beams of electrons that were also waves, atoms that didn't radiate like they were supposed to by classical theory, and atoms that jumped discontinuously from one energy state to another, with the intermediate energies being forbidden.

These bundles of light energy were called light quanta, from the same root word that gave us "quantity." Atoms that could exist only in certain energy states, and which could jump from one state to another by absorbing or emitting quanta of electromagnetic energy, were termed quantized atoms, and the new mechanics based on such ideas was quantum mechanics.

The simplest quantum theory was that of Niels Bohr, which we have been examining in this chapter. His theory explained the hydrogen atom perfectly, but that was all it explained. Any atom or ion with more than one electron was too complicated for the Bohr theory, and all attempts to patch the theory failed. Schrodinger took the next step, around 1926, with his improved quantum theory, wave mechanics. This was the missing key to atomic structure. Wave mechanics led to an explanation for the structure and properties of all the atoms, and for interatomic bonding in molecules as well. This is the subject of the next chapter.