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In
chemistry, the ball-and-stick model is a
molecular model of a
chemical substance which is to display both the
three-dimensional position of the
atoms and the
bonds between them. The atoms are typically represented by
spheres, connected by rods which represent the bonds. Double and triple bonds are usually represented by two or three curved rods, respectively, or alternately by correctly positioned sticks for the
sigma and
pi bonds. In a good model, the angles between the rods should be the same as the
angles between the bonds, and the distances between the centers of the spheres should be proportional to the distances between the corresponding
atomic nuclei. The
chemical element of each atom is often indicated by the sphere's color.
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 from
space-filling (calotte) models, where the sphere radii are proportional to the
Van der Waals atomic radii in the same scale as the atom distances, and therefore show the occupied space but not the bonds.
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Ball-and-stick models can be
physics|physical artifacts or virtual
computer models. The former are usually built from
molecular modeling kits, consisting of a number of
coil springs or plastic or wood sticks, and a number of plastic balls with pre-drilled holes. The sphere colors commonly follow the
CPK coloring. Some university courses on chemistry require students to buy such models as learning material.
History
thumb|left|Hofmann's 1865 ball-and-stick model of
methane (CH
4). Later discoveries disproved this geometry.
In 1865, German chemist
August Wilhelm von Hofmann was the first to make ball-and-stick molecular models. He used such models in lecture at the
Royal Institution of Great Britain.
Specialist companies manufacture kits and models to order. One of the earlier companies was Woosters at
Bottisham,
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 per
ångström (0.5 m/nm or 500,000,000:1), but was not consistent over all elements.
The
Beevers Miniature Models company in
Edinburgh (now operating as
Miramodus) produced small models beginning in 1961
using
PMMA balls and
stainless 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.
See also
*
VSEPR theory
References
{{DEFAULTSORT:Ball-And-Stick Model
Category:Molecular modelling