Visualising the Invisible: Atomic Molecular Models in 19th Century Chemistry

– Article by Kathryn Ashe

This week’s CPS saw the return of Hasok Chang to UCL.  Prof Chang currently works with the Department of History and Philosophy of Science at Cambridge University, but has spent 16 years of his career working at UCL and has been the speaker at CPS meetings many times.  This time he covered a new topic: how did we work out the molecular structure of water?  And is a theory less important because it is not true?

This topic took us on a tour of the changing philosophies in Chemistry throughout the 19^th Century.  We started with Dalton, who left school at the age of eleven but whose perseverance in studying science lead to his realisation that the molecular formula of water is HO: as there was no method of directly observing molecules, Dalton believed the simplest possible solution must apply.  Around the same time, Avogadro’s work led him to believe that the molecular formula of water was H₂O – although he only achieved this by adding caveats to his original hypothesis when it didn’t work when applied to all compounds.  This theory, however, violated the basic laws of physics known at the time: positively charged hydrogen and negatively charged oxygen could obviously form a compound, but two atoms of hydrogen would surely repel each other.  These opposing ideas lead to two different philosophies in science: realism (good theories should truthfully explain even unobservable things) and positivism (science should only deal with the observable).  Unfortunately, the latter theory was adopted by some of the eminent scientists of the day: Dumas in France claimed that atoms cannot exist because we have no experience of them, and consequently all references to atoms and atomic theory was removed from French textbooks.

The answer to this conundrum came from organic chemistry.  At that time, many new compounds were being synthesised and discovered – but there was no system for classifying them or for discovering how they fitted together.  Modelling was used in organic chemistry to try to understand these problems; whether the model related to the truth or not was unimportant.  Related to this was substitution theory: hydrogen and chlorine could be substituted for each other in different compounds, but like before this contravened what was known at the time about the charges of atoms.  Type theory, developed by Williamson (at UCL!), provided a model which fitted with his observations of etherification (aka the ‘inverse Jesus’ reaction of turning alcohol into water).  Water, ethanol and diethyl ether all belong to the same ‘type’, with each hydrogen attached to the oxygen being consecutively replaced with an ethyl group.

Physical molecular models were created soon afterwards: Hofmann created the first ball and stick models out of croquet balls (albeit in only two dimensions).  Meanwhile Kekulé, despite realising the structure of benzene, was not consistent in the accuracy of his beliefs.  He used sausage-shaped molecular models, as he thought that well-formed molecules should stack up nicely; he also said that the existence of atoms should not concern chemists.

After this tour of chemical philosophy through history, we were left pondering two important questions: is success a sign of truth? And if not, what is?  As always, the evening ended with wine and refreshments in the Nyholm Room, which we could enjoy with our new knowledge of the importance of wine in chemistry: optical isomers were discovered when Pasteur realised there were two types of tartaric acid in his wine, whilst etherification was first recorded in 1275 by Ramon Llull who probably discovered it by seeing what happened when he added acid to wine.