Roberto Todeschini

The concept of molecular structure is one the most important concepts in the development of the scientific knowledge of the XX century. As a matter of fact the reasoning based on the molecular structure has been the main engine for the great development of physical chemistry, molecular physics, organic chemistry, quantum chemistry, chemical synthesis, polymer chemistry, medicinal chemistry, etc. By definition, a system is complex when its behaviour as a whole is not derivable from the properties of its parts: a molecule, together with its imbedded concept of molecular structure, exactly fulfils these conditions. In fact, molecule properties do not depend only on the properties of the component atoms but also on their mutual connections: it is in principle a holistic system, i.e. its emergent properties cannot be derived as the sum of the properties of its parts, but they are inherent to the whole molecule organisation and stability. As a consequence of its complexity, molecular structure cannot be represented by a unique formal model; several molecular representations can represent the same molecule, depending on the level of the underlying theoretical approach and these representations are often not derivable from each other.