The theory of quantum mechanics was discovered in 1925 and 1926. This was an improvement on the old quantum theory. The old quantum theory was an approximate theory which sometimes worked in a very remarkable way, and sometimes failed. But quantum mechanics — so far as chemistry is concerned — quantum mechanics is the basic theory, and there is nothing wrong with it. It works.

I realized in 1926 already that quantum mechanics could be applied to answer many additional questions about the nature of the chemical bonds, the structure of molecules and crystals. So, during the next ten years I was able to apply quantum mechanics to chemical problems in very productive ways, changing the whole basic nature of chemistry in such a way that essentially all chemists make use of these new ideas, along with the old structural chemistry that had been developed much earlier.