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Inchemistry, theball-and-stick model is amolecular model of achemical substance which displays both thethree-dimensional position of theatoms and thebonds between them.[1] The atoms are typically represented byspheres, connected by rods which represent the bonds.Double andtriple bonds are usually represented by two or three curved rods, respectively, or alternately by correctly positioned sticks for thesigma andpi bonds. In a good model, the angles between the rods should be the same as theangles between the bonds, and the distances between the centers of the spheres should be proportional to the distances between the correspondingatomic nuclei. Thechemical element of each atom is often indicated by the sphere's color.[2]
In a ball-and-stick model, the radius of the spheres is usually much smaller than the rod lengths, in order to provide a clearer view of the atoms and bonds throughout the model. As a consequence, the model does not provide a clear insight about the space occupied by the model. In this aspect, ball-and-stick models are distinct fromspace-filling (calotte) models, where the sphere radii are proportional to theVan der Waals atomic radii in the same scale as the atom distances, and therefore show the occupied space but not the bonds.
Ball-and-stick models can be physical artifacts orcomputer models asmolecular graphics. The former are usually built from molecular modeling kits, consisting of a number ofcoil springs or plastic or wood sticks, and a number of plastic balls with pre-drilled holes. The sphere colors commonly follow theCPK coloring. Some university courses on chemistry require students to buy such models as learning material.
In 1865, German chemistAugust Wilhelm von Hofmann was the first to make ball-and-stick molecular models. He used such models in lecture at theRoyal Institution of Great Britain.
Specialist companies manufacture kits and models to order. One of the earlier companies was Woosters atBottisham,Cambridgeshire, UK. Besides tetrahedral, trigonal and octahedral holes, there were all-purpose balls with 24 holes. These models allowed rotation about the single rod bonds, which could be both an advantage (showing molecular flexibility) and a disadvantage (models are floppy). The approximate scale was 5 cm perångström (0.5 m/nm or 500,000,000:1), but was not consistent over all elements.
The Beeverses Miniature Models company inEdinburgh (now operating as Miramodus) produced small models beginning in 1961[3] usingPMMA balls andstainless steel rods. In these models, the use of individually drilled balls with precise bond angles and bond lengths enabled large crystal structures to be accurately created in a light and rigid form.
