The French geologist Alexandre-Émile Béguyer de Chancourtois was the first scientist to see the periodicity of elements when they were arranged in order of their atomic weights. Credited with being the original discoverer of the periodicity of elements and the originator of the three-dimensional method of element arrangement and representation. He drew the elements as a continuous spiral around a metal cylinder divided into 16 parts. The atomic weight of oxygen was taken as 16 and was used as the standard against which all the other elements were compared. Tellurium was situated at the centre, prompting vis tellurique, or telluric screw. Despite de Chancourtois' work, his publication attracted little attention from chemists around the world. The reason being that De Chancourtois's original diagram was left out of the publication, making the paper hard to comprehend. The paper also dealt mainly with geological concepts, and did not suit the interests of many chemistry experts. It was not until 1869 that Dmitri Mendeleyev's periodic table attracted attention and gained widespread scientific acceptance. The original (unique) telluric screw is now hosted at the museum de l'École des Mines. Sources: Jacques Touret "Dans l'ombre de ses maîtres", Travaux Du comité Français d'histoire de la géologie - Troisième série - T.XX (2006) Science and Society picture library All periodic tables website Wikipedia article on Alexandre-Émile Béguyer de Chancourtois

August Wilhelm Hofmann was the first to introduce physical representations of molecules into lectures during his Friday Evening Discourses presentation "On the Combining Power of Atoms" at London's Royal Institution of Great Britain in . He introduced a colored set of four croquet balls to represent atoms (hydrogen, oxygen, chlorine and nitrogen), implanted with a fixed number of sticks corresponding to each atom's valence. Thus we distinguish the chlorine atom as univalent, the atom of oxygen as bivalent, that of nitrogen as trivalent, and lastly the carbon atom as quadrivalent. - this I believe I can show you by a very simple contrivance. I will on this occasion, with your permission, select my illustration from that most delightful of games croquet. Let the croquet balls represent our atoms, and let us distinguish the atoms of different elements by different colours. The white balls are hydrogen, the green ones chlorine atoms; the atoms of fiery oxygen are red, those of nitrogen, blue; the carbons atoms, lastly, are naturally represented by black balls. But we have, in addition, to exhibit the different combining powers of these atoms. This we accomplish by screwing into the balls a number of metallic arms (tubes and pins), which correspond respectively to the combining powers of the atoms represented, and which, while constituting an additional feature of distinction, enable us at the same time to join the balls and to rear in this manner a kind of mechanical structures in imitation of the atomic edifices to be illustrated." (Extracts from Proceedings of the Royal Institution, 4, 401-430) Interestingly, while physical representations, Hofmann had the models built so that they were two-dimensional representations of molecules. It's only in the mid 70s that 3-dimensional models emerged. Sources: J. Michael McBride Models and Structural Diagrams in the 1860s Wikipedia August Wilhelm von Hofmann

Mechanical arrangement adapted to illustrate structure in the non-saturated hydrocarbons by the chemist James Dewar. The model is made of bars, clamped together so as to allow free motion. In order to make the combination look like an atom, a thin round disc of blackened brass can be placed under the central nut. At the ends of the arms are holes to connect one carbon atom with another by means of a nut. The structure at the bottom right of his figure is now called "Dewar benzene". I bring before the Society a simple mechanical arrangement adapted to illustrate structure in the non-saturated hydrocarbons. This little device is the mechanical representative of Dr. C. Brown's well-known graphic notation. A series of narrow thin bars of brass of equal length are taken, and every two of the bars clamped in the centre by a nut, so as to admit of free motion the one on the other. Such a combination represents a single carbon atom with its four places of attachment. In order to make the combination look like an atom, a thin round disc of blackened brass can be placed under the central nut. At the ends of the arms are holes to connect one carbon atom with another by means of a nut. The filling up of the places of attachment may be effected by slipping on the arms round discs of brass having a groove attached, and placing the symbol of the chemical element on the round projection. A carbon atom would then look like the following diagram. By J. Dewar, Esq. (from Proceedings of the Royal Society of Edinburgh, VI, Session 1866-67, pp. 82-86) Dewar sent Kekulé his brass strip molecular modeling kit in 1866, and Kekulé used it to hypothesize possible molecular structures for compunds. In 1867, Kekulé used Dewar's models to come up with a reasonable (by today's standards) structure for mesitylene (1,3,5-trimethylbenzene, C9H12). Note that in the drawing, little dots appear in in the middle of some of the bonds. They appear because the drawings are of a real model, and the various atoms were screwed together at the dots. Also note that the acetone molecules in the model appear not to be symmetrical, with one C-C bond bent and the other one straight. However, the bent bond is only an illusion created by the flatness of the paper; in Kekuleé's 3-D model, all of the C-C bonds are straight. James Deward is famous for his Researches in low temperature phenomena. See [3] to learn more. Sources: J. Michael Mc Bride Models and Structural Diagrams in the 1860s J. Dewar, Esq., Proceedings of the Royal Society of Edinburgh, VI, Session 1866-67, pp. 82-86 Today in Science History website

Van’t Hoff disseminated his stereochemical ideas to leading chemists of the day by sending them 3-D paper models of tetrahedral molecules, like these now housed in the Leiden Museum. There might be some difficulty in following my reasoning. I felt this myself, and I have made use of cardboard figures to facilitate the representation. Not wanting to require too much of the reader I will gladly send him the complete collection of all these objects Sources: Van der Spek, Trienke M. Selling a Theory: The Role of Molecular Models in JH van't Hoff's Stereochemistry Theory. Annals of science 63.02 (2006): 157-177. Gross Ari, Form and Function: Seeing, Knowing, and Reasoning with Diagrams in the Practice of Science, 2013. PhD Thesis. University of Toronto Chemical Heritage Foundation website Image courtesy O. Bertrand Ramsay