Molecular model

A molecular model, in this article, is a physical model that represents molecules and their processes. The creation of mathematical models of molecular properties and behaviour is molecular modelling, and their graphical depiction is molecular graphics, but these topics are closely linked and each uses techniques from the others. In this article, 'molecular model' will primarily refer to systems containing more than one atom and where nuclear structure is neglected. The electronic structure is often also omitted or represented in a highly sophisticated way. A molecular model, in this article, is a physical model that represents molecules and their processes. The creation of mathematical models of molecular properties and behaviour is molecular modelling, and their graphical depiction is molecular graphics, but these topics are closely linked and each uses techniques from the others. In this article, 'molecular model' will primarily refer to systems containing more than one atom and where nuclear structure is neglected. The electronic structure is often also omitted or represented in a highly sophisticated way. Physical models of atomistic systems have played an important role in understanding chemistry and generating and testing hypotheses. Most commonly there is an explicit representation of atoms, though other approaches such as soap films and other continuous media have been useful. There are several motivations for creating physical models: The construction of physical models is often a creative act, and many bespoke examples have been carefully created in the workshops of science departments. There is a very wide range of approaches to physical modelling, and this article lists only the most common or historically important. The main strategies are: Models encompass a wide range of degrees of precision and engineering: some models such as J.D. Bernal's water are conceptual, while the macromodels of Pauling and Crick and Watson were created with much greater precision. Molecular models have inspired molecular graphics, initially in textbooks and research articles and more recently on computers. Molecular graphics has replaced some functions of physical molecular models, but physical kits continue to be very popular and are sold in large numbers. Their unique strengths include: In the 1600s, Johannes Kepler speculated on the symmetry of snowflakes and also on the close packing of spherical objects such as fruit (this problem remained unsolved until very recently). The symmetrical arrangement of closely packed spheres informed theories of molecular structure in the late 1800s, and many theories of crystallography and solid state inorganic structure used collections of equal and unequal spheres to simulate packing and predict structure. John Dalton represented compounds as aggregations of circular atoms, and although Johann Josef Loschmidt did not create physical models, his diagrams based on circles are two-dimensional analogues of later models. August Wilhelm von Hofmann is credited with the first physical molecular model around 1860 (Fig. 1). Note how the size of the carbon appears smaller than the hydrogen. The importance of stereochemistry was not then recognised and the model is essentially topological (it should be a 3-dimensional tetrahedron). Jacobus Henricus van 't Hoff and Joseph Le Bel introduced the concept of chemistry in space—stereochemistry in three dimensions. van 't Hoff built tetrahedral molecules representing the three-dimensional properties of carbon.