Octahedron
Regular octahedron  

(Click here for rotating model)  
Type  Platonic solid 
Elements  F = 8, E = 12 V = 6 (χ = 2) 
Faces by sides  8{3} 
Conway notation  O aT 
Schläfli symbols  {3,4} 
r{3,3} or  
Face configuration  V4.4.4 
Wythoff symbol  4  2 3 
Coxeter diagram  
Symmetry  O_{h}, BC_{3}, [4,3], (*432) 
Rotation group  O, [4,3]^{+}, (432) 
References  U_{05}, C_{17}, W_{2} 
Properties  regular, convexdeltahedron 
Dihedral angle  109.47122° = arccos(−1/3) 
3.3.3.3 (Vertex figure) 
Cube (dual polyhedron) 
Net 
In geometry, an octahedron (plural: octahedra) is a polyhedron with eight faces, twelve edges, and six vertices. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at each vertex.
A regular octahedron is the dual polyhedron of a cube. It is a rectified tetrahedron. It is a square bipyramid in any of three orthogonal orientations. It is also a triangular antiprism in any of four orientations.
An octahedron is the threedimensional case of the more general concept of a cross polytope.
A regular octahedron is a 3ball in the Manhattan (ℓ_{1}) metric.
Contents
Regular octahedronEdit
DimensionsEdit
If the edge length of a regular octahedron is a, the radius of a circumscribed sphere (one that touches the octahedron at all vertices) is
and the radius of an inscribed sphere (tangent to each of the octahedron's faces) is
while the midradius, which touches the middle of each edge, is
Orthogonal projectionsEdit
The octahedron has four special orthogonal projections, centered, on an edge, vertex, face, and normal to a face. The second and third correspond to the B_{2} and A_{2} Coxeter planes.
Centered by  Edge  Face Normal 
Vertex  Face 

Image  
Projective symmetry 
[2]  [2]  [4]  [6] 
Spherical tilingEdit
The octahedron can also be represented as a spherical tiling, and projected onto the plane via a stereographic projection. This projection is conformal, preserving angles but not areas or lengths. Straight lines on the sphere are projected as circular arcs on the plane.
Orthographic projection  Stereographic projection 

Cartesian coordinatesEdit
An octahedron with edge length √2 can be placed with its center at the origin and its vertices on the coordinate axes; the Cartesian coordinates of the vertices are then
 ( ±1, 0, 0 );
 ( 0, ±1, 0 );
 ( 0, 0, ±1 ).
In an x–y–z Cartesian coordinate system, the octahedron with center coordinates (a, b, c) and radius r is the set of all points (x, y, z) such that
Area and volumeEdit
The surface area A and the volume V of a regular octahedron of edge length a are:
Thus the volume is four times that of a regular tetrahedron with the same edge length, while the surface area is twice (because we have 8 rather than 4 triangles).
If an octahedron has been stretched so that it obeys the equation
the formulas for the surface area and volume expand to become
Additionally the inertia tensor of the stretched octahedron is
These reduce to the equations for the regular octahedron when
Geometric relationsEdit
The interior of the compound of two dual tetrahedra is an octahedron, and this compound, called the stella octangula, is its first and only stellation. Correspondingly, a regular octahedron is the result of cutting off from a regular tetrahedron, four regular tetrahedra of half the linear size (i.e. rectifying the tetrahedron). The vertices of the octahedron lie at the midpoints of the edges of the tetrahedron, and in this sense it relates to the tetrahedron in the same way that the cuboctahedron and icosidodecahedron relate to the other Platonic solids. One can also divide the edges of an octahedron in the ratio of the golden mean to define the vertices of an icosahedron. This is done by first placing vectors along the octahedron's edges such that each face is bounded by a cycle, then similarly partitioning each edge into the golden mean along the direction of its vector. There are five octahedra that define any given icosahedron in this fashion, and together they define a regular compound.
Octahedra and tetrahedra can be alternated to form a vertex, edge, and faceuniform tessellation of space, called the octet truss by Buckminster Fuller. This is the only such tiling save the regular tessellation of cubes, and is one of the 28 convex uniform honeycombs. Another is a tessellation of octahedra and cuboctahedra.
The octahedron is unique among the Platonic solids in having an even number of faces meeting at each vertex. Consequently, it is the only member of that group to possess mirror planes that do not pass through any of the faces.
Using the standard nomenclature for Johnson solids, an octahedron would be called a square bipyramid. Truncation of two opposite vertices results in a square bifrustum.
The octahedron is 4connected, meaning that it takes the removal of four vertices to disconnect the remaining vertices. It is one of only four 4connected simplicial wellcovered polyhedra, meaning that all of the maximal independent sets of its vertices have the same size. The other three polyhedra with this property are the pentagonal dipyramid, the snub disphenoid, and an irregular polyhedron with 12 vertices and 20 triangular faces.^{[1]}
Uniform colorings and symmetryEdit
There are 3 uniform colorings of the octahedron, named by the triangular face colors going around each vertex: 1212, 1112, 1111.
The octahedron's symmetry group is O_{h}, of order 48, the three dimensional hyperoctahedral group. This group's subgroups include D_{3d} (order 12), the symmetry group of a triangular antiprism; D_{4h} (order 16), the symmetry group of a square bipyramid; and T_{d} (order 24), the symmetry group of a rectified tetrahedron. These symmetries can be emphasized by different colorings of the faces.
Name  Octahedron  Rectified tetrahedron (Tetratetrahedron) 
Triangular antiprism  Square bipyramid  Rhombic fusil 

Image (Face coloring) 
(1111) 
(1212) 
(1112) 
(1111) 
(1111) 
Coxeter diagram  =  

Schläfli symbol  {3,4}  r{3,3}  s{2,6} sr{2,3} 
ft{2,4} { } + {4} 
ftr{2,2} { } + { } + { } 
Wythoff symbol  4  3 2  2  4 3  2  6 2  2 3 2 

Symmetry  O_{h}, [4,3], (*432)  T_{d}, [3,3], (*332)  D_{3d}, [2^{+},6], (2*3) D_{3}, [2,3]^{+}, (322) 
D_{4h}, [2,4], (*422)  D_{2h}, [2,2], (*222) 
Order  48  24  12 6 
16  8 
NetsEdit
It has eleven arrangements of nets.
DualEdit
The octahedron is the dual polyhedron to the cube.
FacetingEdit
The uniform tetrahemihexahedron is a tetrahedral symmetry faceting of the regular octahedron, sharing edge and vertex arrangement. It has four of the triangular faces, and 3 central squares.
Octahedron 
Tetrahemihexahedron 
Irregular octahedraEdit
The following polyhedra are combinatorially equivalent to the regular polyhedron. They all have six vertices, eight triangular faces, and twelve edges that correspond oneforone with the features of a regular octahedron.
 Triangular antiprisms: Two faces are equilateral, lie on parallel planes, and have a common axis of symmetry. The other six triangles are isosceles.
 Tetragonal bipyramids, in which at least one of the equatorial quadrilaterals lies on a plane. The regular octahedron is a special case in which all three quadrilaterals are planar squares.
 Schönhardt polyhedron, a nonconvex polyhedron that cannot be partitioned into tetrahedra without introducing new vertices.
 Bricard octahedron, a nonconvex selfcrossing flexible polyhedron
Other convex octahedraEdit
More generally, an octahedron can be any polyhedron with eight faces. The regular octahedron has 6 vertices and 12 edges, the minimum for an octahedron; irregular octahedra may have as many as 12 vertices and 18 edges.^{[2]} There are 257 topologically distinct convex octahedra, excluding mirror images. More specifically there are 2, 11, 42, 74, 76, 38, 14 for octahedra with 6 to 12 vertices respectively.^{[3]}^{[4]} (Two polyhedra are "topologically distinct" if they have intrinsically different arrangements of faces and vertices, such that it is impossible to distort one into the other simply by changing the lengths of edges or the angles between edges or faces.)
Some better known irregular octahedra include the following:
 Hexagonal prism: Two faces are parallel regular hexagons; six squares link corresponding pairs of hexagon edges.
 Heptagonal pyramid: One face is a heptagon (usually regular), and the remaining seven faces are triangles (usually isosceles). It is not possible for all triangular faces to be equilateral.
 Truncated tetrahedron: The four faces from the tetrahedron are truncated to become regular hexagons, and there are four more equilateral triangle faces where each tetrahedron vertex was truncated.
 Tetragonal trapezohedron: The eight faces are congruent kites.
Octahedra in the physical worldEdit
Octahedra in natureEdit
 Natural crystals of diamond, alum or fluorite are commonly octahedral, as the spacefilling tetrahedraloctahedral honeycomb.
 The plates of kamacite alloy in octahedrite meteorites are arranged paralleling the eight faces of an octahedron.
 Many metal ions coordinate six ligands in an octahedral or distorted octahedral configuration.
 Widmanstätten patterns in nickeliron crystals
Octahedra in art and cultureEdit
 Especially in roleplaying games, this solid is known as a "d8", one of the more common polyhedral dice.
 In the film Tron (1982), the character Bit took this shape as the "Yes" state.
 If each edge of an octahedron is replaced by a oneohm resistor, the resistance between opposite vertices is 1/2 ohm, and that between adjacent vertices 5/12 ohm.^{[5]}
 Six musical notes can be arranged on the vertices of an octahedron in such a way that each edge represents a consonant dyad and each face represents a consonant triad; see hexany.
Tetrahedral TrussEdit
A framework of repeating tetrahedrons and octahedrons was invented by Buckminster Fuller in the 1950s, known as a space frame, commonly regarded as the strongest structure for resisting cantilever stresses.
Related polyhedraEdit
A regular octahedron can be augmented into a tetrahedron by adding 4 tetrahedra on alternated faces. Adding tetrahedra to all 8 faces creates the stellated octahedron.
tetrahedron  stellated octahedron 

The octahedron is one of a family of uniform polyhedra related to the cube.
Uniform octahedral polyhedra  

Symmetry: [4,3], (*432)  [4,3]^{+} (432) 
[1^{+},4,3] = [3,3] (*332) 
[3^{+},4] (3*2)  
{4,3}  t{4,3}  r{4,3} r{3^{1,1}} 
t{3,4} t{3^{1,1}} 
{3,4} {3^{1,1}} 
rr{4,3} s_{2}{3,4} 
tr{4,3}  sr{4,3}  h{4,3} {3,3} 
h_{2}{4,3} t{3,3} 
s{3,4} s{3^{1,1}} 
= 
= 
= 
= or 
= or 
=  




 
Duals to uniform polyhedra  
V4^{3}  V3.8^{2}  V(3.4)^{2}  V4.6^{2}  V3^{4}  V3.4^{3}  V4.6.8  V3^{4}.4  V3^{3}  V3.6^{2}  V3^{5} 
It is also one of the simplest examples of a hypersimplex, a polytope formed by certain intersections of a hypercube with a hyperplane.
The octahedron is topologically related as a part of sequence of regular polyhedra with Schläfli symbols {3,n}, continuing into the hyperbolic plane.
*n32 symmetry mutation of regular tilings: {3,n}  

Spherical  Euclid.  Compact hyper.  Paraco.  Noncompact hyperbolic  
3.3  3^{3}  3^{4}  3^{5}  3^{6}  3^{7}  3^{8}  3^{∞}  3^{12i}  3^{9i}  3^{6i}  3^{3i} 
TetratetrahedronEdit
The regular octahedron can also be considered a rectified tetrahedron – and can be called a tetratetrahedron. This can be shown by a 2color face model. With this coloring, the octahedron has tetrahedral symmetry.
Compare this truncation sequence between a tetrahedron and its dual:
Family of uniform tetrahedral polyhedra  

Symmetry: [3,3], (*332)  [3,3]^{+}, (332)  
{3,3}  t{3,3}  r{3,3}  t{3,3}  {3,3}  rr{3,3}  tr{3,3}  sr{3,3} 
Duals to uniform polyhedra  
V3.3.3  V3.6.6  V3.3.3.3  V3.6.6  V3.3.3  V3.4.3.4  V4.6.6  V3.3.3.3.3 
The above shapes may also be realized as slices orthogonal to the long diagonal of a tesseract. If this diagonal is oriented vertically with a height of 1, then the first five slices above occur at heights r, 3/8, 1/2, 5/8, and s, where r is any number in the range 0 < r ≤ 1/4, and s is any number in the range 3/4 ≤ s < 1.
The octahedron as a tetratetrahedron exists in a sequence of symmetries of quasiregular polyhedra and tilings with vertex configurations (3.n)^{2}, progressing from tilings of the sphere to the Euclidean plane and into the hyperbolic plane. With orbifold notation symmetry of *n32 all of these tilings are Wythoff constructions within a fundamental domain of symmetry, with generator points at the right angle corner of the domain.^{[6]}^{[7]}
*n32 orbifold symmetries of quasiregular tilings: (3.n)^{2}  

Construction 
Spherical  Euclidean  Hyperbolic  
*332  *432  *532  *632  *732  *832...  *∞32  
Quasiregular figures 

Vertex  (3.3)^{2}  (3.4)^{2}  (3.5)^{2}  (3.6)^{2}  (3.7)^{2}  (3.8)^{2}  (3.∞)^{2} 
Trigonal antiprismEdit
As a trigonal antiprism, the octahedron is related to the hexagonal dihedral symmetry family.
Uniform hexagonal dihedral spherical polyhedra  

Symmetry: [6,2], (*622)  [6,2]^{+}, (622)  [6,2^{+}], (2*3)  
{6,2}  t{6,2}  r{6,2}  t{2,6}  {2,6}  rr{6,2}  tr{6,2}  sr{6,2}  s{2,6}  
Duals to uniforms  
V6^{2}  V12^{2}  V6^{2}  V4.4.6  V2^{6}  V4.4.6  V4.4.12  V3.3.3.6  V3.3.3.3 
Family of uniform antiprisms n.3.3.3  

Polyhedron  
Tiling  
Config.  V2.3.3.3  3.3.3.3  4.3.3.3  5.3.3.3  6.3.3.3  7.3.3.3  8.3.3.3  9.3.3.3  10.3.3.3  11.3.3.3  12.3.3.3  ...∞.3.3.3 
Square bipyramidEdit
Polyhedron  

Coxeter  
Tiling  
Config.  V2.4.4  V3.4.4  V4.4.4  V5.4.4  V6.4.4  V7.4.4  V8.4.4  V9.4.4  V10.4.4 
See alsoEdit
ReferencesEdit
 ^ Finbow, Arthur S.; Hartnell, Bert L.; Nowakowski, Richard J.; Plummer, Michael D. (2010). "On wellcovered triangulations. III". Discrete Applied Mathematics. 158 (8): 894–912. doi:10.1016/j.dam.2009.08.002. MR 2602814.
 ^ [1]
 ^ Counting polyhedra
 ^ "Archived copy". Archived from the original on 17 November 2014. Retrieved 14 August 2016.CS1 maint: Archived copy as title (link)
 ^ Klein, Douglas J. (2002). "ResistanceDistance Sum Rules" (PDF). Croatica Chemica Acta. 75 (2): 633–649. Retrieved 30 September 2006.
 ^ Coxeter Regular Polytopes, Third edition, (1973), Dover edition, ISBN 0486614808 (Chapter V: The Kaleidoscope, Section: 5.7 Wythoff's construction)
 ^ Two Dimensional symmetry Mutations by Daniel Huson
External linksEdit
 Encyclopædia Britannica. 19 (11th ed.). 1911. .
 Weisstein, Eric W. "Octahedron". MathWorld.
 Klitzing, Richard. "3D convex uniform polyhedra x3o4o  oct".
 Editable printable net of an octahedron with interactive 3D view
 Paper model of the octahedron
 K.J.M. MacLean, A Geometric Analysis of the Five Platonic Solids and Other SemiRegular Polyhedra
 The Uniform Polyhedra
 Virtual Reality Polyhedra The Encyclopedia of Polyhedra
 Conway Notation for Polyhedra Try: dP4
Fundamental convex regular and uniform polytopes in dimensions 2–10
 

A_{n}  B_{n}  I_{2}(p) / D_{n}  E_{6} / E_{7} / E_{8} / F_{4} / G_{2}  H_{n}  
Triangle  Square  pgon  Hexagon  Pentagon  
Tetrahedron  Octahedron • Cube  Demicube  Dodecahedron • Icosahedron  
5cell  16cell • Tesseract  Demitesseract  24cell  120cell • 600cell  
5simplex  5orthoplex • 5cube  5demicube  
6simplex  6orthoplex • 6cube  6demicube  1_{22} • 2_{21}  
7simplex  7orthoplex • 7cube  7demicube  1_{32} • 2_{31} • 3_{21}  
8simplex  8orthoplex • 8cube  8demicube  1_{42} • 2_{41} • 4_{21}  
9simplex  9orthoplex • 9cube  9demicube  
10simplex  10orthoplex • 10cube  10demicube  
nsimplex  northoplex • ncube  ndemicube  1_{k2} • 2_{k1} • k_{21}  npentagonal polytope  
Topics: Polytope families • Regular polytope • List of regular polytopes and compounds 