Triangular bipyramid

In geometry, the triangular bipyramid is the hexahedron with six triangular faces, constructed by attaching two tetrahedra face-to-face. The same shape is also called the triangular dipyramid or trigonal bipyramid. If these tetrahedra are regular, all faces of triangular bipyramid are equilateral. It is an example of a deltahedron and of a Johnson solid.

Many polyhedra are related to the triangular bipyramid, such as new similar shapes derived in different approaches, and the triangular prism as its dual polyhedron. The many applications of triangular bipyramid include the trigonal bipyramid molecular geometry that describes its atom cluster, the solution of Thomson problem, and the representation of color order systems by the eighteenth century. The triangular bipyramid has a graph constructed by involving the wheel graph of a square pyramid.

Construction and properties
Like other bipyramids, the triangular bipyramid can be constructed by attaching two tetrahedra face-to-face. These tetrahedra cover their triangular base, such that the resulting polyhedron has six triangles, five vertices, and nine edges. The triangular bipyramid is said to be right if the tetrahedra are symmetrically regular and both of their apices are on the line passing through the center of base; otherwise, it is oblique.

According to Steinitz's theorem, a graph can be represented as the skeleton of a polyhedron if it is planar and 3-connected graph. In other words, the edges of that graph do not cross but only intersect at the point, and one of any two vertices leaves a connected subgraph when removed. The triangular bipyramid is represented by a graph with nine edges, constructed by adding one vertex connecting to three other vertices of wheel graph representing a square pyramid.

Like other right bipyramids, the triangular bipyramid has three-dimensional point group symmetry, the dihedral group $$ D_{3h} $$ of order twelve: the appearance of the triangular bipyramid is unchanged as it rotated by one-, two-thirds, and full angle around the axis of symmetry (a line passing through two vertices and base's center vertically), and it has mirror symmetry relative to any bisector of the base; it is also symmetrical by reflecting it across a horizontal plane.

In Johnson solid
If the tetrahedra are regular, then all edges of the triangular bipyramid are equal in length, forming equilateral triangular faces. A polyhedron with only equilateral triangles as faces is called a deltahedron. There are only eight different convex deltahedra, one of which is the triangular bipyramid with regular faces. More generally, the convex polyhedron in which all of the faces are regular is the Johnson solid, and every convex deltahedron is a Johnson solid. The triangular bipyramid with the regular faces is among numbered the Johnson solids as $$ J_{12} $$, the twelfth Johnson solid.

A triangular bipyramid's surface area is six times that of all triangles. In the case of edge length $$ a $$, its surface area is: $$ \frac{3\sqrt{3}}{2}a^2 \approx 2.598a^2. $$ Its volume can be calculated by slicing it into two tetrahedra and adding their volume. In the case of edge length $$ a $$, this is: $$ \frac{\sqrt{2}}{6}a^3 \approx 0.238a^3. $$

The dihedral angle of a triangular bipyramid can be obtained by adding the dihedral angle of two regular tetrahedra. The dihedral angle of a triangular bipyramid between adjacent triangular faces is that of the regular tetrahedron, 70.5&deg;. In the case of the edge where two tetrahedra are attached, the dihedral angle of adjacent triangles is twice that, 141.1&deg;.

Related polyhedra
Some types of triangular bipyramids may be derived in different ways. For example, the Kleetope of polyhedra is a construction involving the attachment of pyramids; in the case of the triangular bipyramid, its Kleetope can be constructed from triangular bipyramid by attaching tetrahedra onto each of its faces, covering and replacing them with other three triangles; the skeleton of resulting polyhedron represents the Goldner–Harary graph. Another type of triangular bipyramid is by cutting off all of its vertices; this process is known as truncation.

The bipyramids are the dual polyhedron of prisms, for which the bipyramids' vertices correspond to the faces of the prism, and the edges between pairs of vertices of one correspond to the edges between pairs of faces of the other; dual it again gives the original polyhedron itself. Hence, the triangular bipyramid is the dual polyhedron of the triangular prism, and vice versa. The triangular prism has five faces, nine edges, and six vertices, and it has the same symmetry as the triangular bipyramid.

Applications
The Thomson problem concerns the minimum-energy configuration of charged particles on a sphere. One of them is a triangular bipyramid, which is a known solution for the case of five electrons, by placing vertices of a triangular bipyramid inscribed in a sphere. This solution is aided by the mathematically rigorous computer.

In the geometry of chemical compound, the trigonal bipyramidal molecular geometry may be described as the atom cluster of the triangular bipyramid. This molecule has a main-group element without an active lone pair, as described by a model that predicts the geometry of molecules known as VSEPR theory. Some examples of this structure are the phosphorus pentafluoride and phosphorus pentachloride in the gas phase.

In the study of color theory, the triangular bipyramid was used to represent the three-dimensional color order system in primary color. The German astronomer Tobias Mayer presented in 1758 that each of its vertices represents the colors: white and black are, respectively, the top and bottom vertices, whereas the rest of the vertices are red, blue, and yellow.