Transforming dreams into reality: a fairy-tale wedding of chemistry with quantum mechanics

As the atomic theory of matter became more established and accepted by the majority of the scientific community, chemists began to dream about the possibility of understanding the physical and chemical properties of different substances in terms of their microscopic composition. Identifying the constituent elements of a substance and their respective proportions allowed chemists to characterize it by a “ chemical formula .” It did not take long for chemists to realize that substances could have different properties even though they had the same “chemical formula” and to conclude that it was not enough to identify the constituent elements of a substance. It was essential to determine how the atoms of those elements were arranged spatially. Thus, chemists began to realize that the physical and chemical properties of a substance were closely related to its “ molecular structure .” The dream now seemed less impossible and more fascinating: how to arrange a certain group of atoms spatially in order to obtain a molecule with specific physical and chemical properties? However, in addition to knowing the spatial arrangement of the atoms in a molecule, it was necessary to know how these atoms are connected, that is, what the “ chemical structure ” of the molecule was and what was the origin of the “ chemical bond ”. The end of the century before last and the first quarter of the last century were the scenery of a fantastic revolution in our knowledge of the atomic structure of matter. Benefiting from this development, chemists began to have at their disposal powerful instruments for the microscopic analysis of the macroscopic properties of chemical systems: quantum mechanics as well as classical and quantum statistical mechanics. Finally, in the last 40 years, with the availability of personal computers and sophisticated multifunctional “software”, the dream began to become reality, and the development of what we now call Quantum Chemistry had an enormous impact on chemistry. However, in order to have a clear measure of the extent of this impact, it is essential to reconstruct the environment in which Chemistry found itself at the end of the 19th century, with all its difficulties and uncertainties. Only then will the reader be able to understand the incredible change brought about by the application of quantum mechanics to Chemistry. This reconstruction will be done by following the evolution of three concepts that form the pillars of modern chemistry: molecular structure, chemical structure and chemical bonding.