At first my reaction was: "No way, it has to be a lot more recent than a 100 years." But then I realized that 1920s was a 100 years ago. The "History" section

At first my reaction was: "No way, it has to be a lot more recent than a 100 years." But then I realized that 1920s was a 100 years ago. The "History" section of this page was interesting: https://en.wikipedia.org/wiki/Molecular_orbital_theory https://en.wikipedia.org/wiki/Molecular_orbital_theory It apparently took most of 30s and 40s for those theories to mature, and the ideas of the then new quantum mechanics to be applied to molecules.