About the relationships between chemistry and physics

OLIMPIA LOMBARDI

Bristol

Workshop; Charla invitada: Realisation and Composition across the Sciences; 2021

University of Bristol

One of the hottest topics in the philosophy of chemistry is thequestion about the relationships between chemistry and physics. Since thebeginnings of the twentieth century, the extraordinary success of quantummechanics promoted the idea that chemistry can be reduced to physics. It wasonly in the last decades of the century that some philosophers began tochallenge that idea, pointing out that many chemical concepts cannot be easilyexplained by quantum mechanics. Since then, the controversy between those whofavor reductionist views and those who advocate for a kind of theoreticalindependence of chemistry has permeated the discussions in the field of the philosophyof chemistry.The central concept in this debate is that ofmolecular structure, due to its essential role in chemistry: molecularstructure is the main element factor in the explanation of reactivity. It has evenbeen claimed to be so fundamental that to explain molecular structure is prettymuch to explain the whole of chemistry. The problem is that the concept seemsto find no comfortable place in the theoretical framework of quantum mechanics.The purpose of this talk is to take a fresh look at the problem of molecularstructure on the basis of a clarification of the problem at issue and thedistinction between different meanings of the term ?molecular structure?.The first step is, then, twofold:‒  I will distinguish between whatI will call ?the problem of reduction? and ?the problem of elimination.? In ourcase, the problem of reduction is the question of whether the concept ofmolecular structure can be explained exclusively by means of the concepts andthe laws of quantum mechanics. The problem of elimination is the question ofwhether scientists can dispense with the concept of molecular structure whenpursuing their explanatory and predictive goals. My point here is that apositive answer to the second problem is not yet a positive answer to the first.‒  I will distinguish between twosenses of ?molecular structure.? In molecular chemistry, the term refers to thespatial arrangement of the atoms in the molecule and the chemical bonds thathold them together. In quantum chemistry, a moleculeis a system in physical space, composed by nuclei surrounded by inertinner-shell electrons, placed in definite positions, and bonds are explained interms of interacting ?bonding? electrons; the structure of this system isdefined by these elements in their spatial arrangement.The second step is to discuss some issues usuallyrelated to the problem of molecular structure in the light of the abovedistinctions:‒  The problem of theBorn-Oppenheimer approximation: whether this is an approximation compatiblewith the principles of quantum mechanics and to what extent it might bedispensable.‒  The problem of symmetry:whether quantum mechanics can account for certain asymmetric properties ofmolecules and to what extent this answers the problem of molecular chemistry.‒  The problem of isomerism: whetherquantum mechanics can account for the existence of the experimentally observedisomers.