Square antiprism
| Uniform Square antiprism | |
|---|---|
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| Type | Prismatic uniform polyhedron |
| Elements | F = 10, E = 16 V = 8 (χ = 2) |
| Faces by sides | 8{3}+2{4} |
| Schläfli symbol | s{2,4} |
| Wythoff symbol | | 2 2 4 2 | 2 8 |
| Coxeter-Dynkin |      |
| Symmetry | D4d, [2+,8], (2*4) |
| References | U77(b) |
| Dual | Tetragonal trapezohedron |
| Properties | convex |
 Vertex figure 3.3.3.4 |
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In geometry, the square antiprism is the second in an infinite set of antiprisms formed by an even-numbered sequence of triangle sides closed by two polygon caps. It is also known as an anticube.[1]
If all its faces are regular, it is a semiregular polyhedron.
When eight points are distributed on the surface of a sphere with the aim of maximising the distance between them in some sense, then the resulting shape corresponds to a square anti-prism rather than a cube. Different examples include maximising the distance to the nearest point, or using electrons to maximise the sum of all reciprocals of squares of distances.
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[edit] Molecules with square antiprism geometry
According to the VSEPR theory of molecular geometry in chemistry, which is based on the general principle of maximizing the distances between points, a square antiprism is a favoured geometry when eight pairs of electrons surround a central atom. One molecule with this geometry is the octafluoroxenate(VI) ion (XeF82-) in the salt nitrosonium octafluoroxenate(VI); however, the molecule is distorted away from the idealized square antiprism.[2]
In addition, the element sulfur forms octatomic S8 molecules as its most stable allotrope. The S8 molecule has a structure based on the square antiprism, in which the eight atoms occupy the eight vertices of the antiprism, and the eight triangle-triangle edges of the antiprism correspond to single covalent bonds between sulfur atoms.
[edit] Related polyhedra
As an antiprism, the square antiprism belongs to a family of polyhedra that includes the octahedron (which can be seen as a triangle-capped antiprism), the pentagonal antiprism, the hexagonal antiprism, and the octagonal antiprism.
The gyroelongated square pyramid is a Johnson solid (specifically, J10) constructed by replacing one square of a square antiprism with a square pyramid. Similarly, the gyroelongated square bipyramid (J17) is a deltahedron (a polyhedron whose faces are all equilateral triangles) constructed by replacing both squares of a square antiprism with a square pyramid.
The snub disphenoid (J84) is another deltahedron, constructed by replacing the two squares of a square antiprism by pairs of equilateral triangles. The snub square antiprism (J85) can be seen as a square antiprism with a chain of equilateral triangles inserted around the middle. The sphenocorona (J86) and the sphenomegacorona (J88) are other Johnson solids that, like the square antiprism, consist of two squares and an even number of equilateral triangles.
[edit] In architecture
The main building block of the One World Trade Center being built at the site of the old World Trade Center destroyed on September 11, 2001 has the shape of a square antiprism.
[edit] See also
[edit] Notes
- ^ Holleman-Wiberg. Inorganic Chemistry, Academic Press, Italy, p. 299. ISBN: 0-12-352651-5.
- ^ Peterson, W. .; Holloway, H. .; Coyle, A. .; Williams, M. . (Sep 1971). "Antiprismatic Coordination about Xenon: the Structure of Nitrosonium Octafluoroxenate(VI)". Science 173 (4003): 1238–1239. Bibcode 1971Sci...173.1238P. doi:10.1126/science.173.4003.1238. ISSN 0036-8075. PMID 17775218.
[edit] External links
- Weisstein, Eric W., "Antiprism" from MathWorld.
- Square Antiprism interactive model
- Virtual Reality Polyhedra www.georgehart.com: The Encyclopedia of Polyhedra
- VRML model
- Conway Notation for Polyhedra Try: "A4"
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