Pentagonal pyramid

Pentagonal pyramid
Type	Pyramid Johnson J1 – J2 – J3
Faces	5 triangles 1 pentagon
Edges	10
Vertices	6
Vertex configuration	$5\times (3^{2}\times 5)+1\times 3^{5}$
Symmetry group	$C_{5\mathrm {v} }$
Dihedral angle (degrees)	In Johnson solid: triangle-to-triangle: 138.19° triangle-to-pentagon: 37.37°
Dual polyhedron	self-dual
Properties	convex, elementary
Net

In geometry, pentagonal pyramid is a pyramid with a pentagon base and five triangular faces, having a total of six faces. It is categorized as Johnson solid if all of the edges are equal in length, forming equilateral triangular faces and a regular pentagonal base. The pentagonal pyramid can be found in many polyhedrons, including their construction. It also occurs in stereochemistry in pentagonal pyramidal molecular geometry.

Properties

A pentagonal pyramid has six vertices, ten edges, and six faces. One of its faces is pentagon, a base of the pyramid; five others are triangles. Five of the edges make up the pentagon by connecting its five vertices, and the other five edges are known as the lateral edges of the pyramid, meeting at the sixth vertex called the apex. A pentagonal pyramid is said to be regular if its base is circumscribed in a circle that forms a regular pentagon, and it is said to be right if its altitude is erected perpendicularly to the base's center.

Like other right pyramids with a regular polygon as a base, this pyramid has pyramidal symmetry of cyclic group $C_{5\mathrm {v} }$: the pyramid is left invariant by rotations of one, two, three, and four in five of a full turn around its axis of symmetry, the line connecting the apex to the center of the base. It is also mirror symmetric relative to any perpendicular plane passing through a bisector of the base. It can be represented as the wheel graph $W_{5}$; more generally, a wheel graph $W_{n}$ is the representation of the skeleton of a $n$-sided pyramid. It is self-dual, meaning its dual polyhedron is the pentagonal pyramid itself.

3D model of a pentagonal pyramid

When all edges are equal in length, the five triangular faces are equilateral and the base is a regular pentagon. This pyramid has the property of Johnson solid $J_{2}$, a convex polyhedron in which all of its faces are regular polygons. The dihedral angle between two adjacent triangular faces is approximately 138.19° and that between the triangular face and the base is 37.37°. It is elementary polyhedra, meaning it cannot be separated by a plane to create two small convex polyhedrons with regular faces. Given that $a$ is the length of all edges of the pentagonal pyramid. A polyhedron's surface area is the sum of the areas of its faces. Therefore, the surface area of a pentagonal pyramid is the sum of the four triangles and one pentagon area. The volume of every pyramid equals one-third of the area of its base multiplied by its height. That is, the volume of a pentagonal pyramid is one-third of the product of the height and a pentagonal pyramid's area. In the case of Johnson solid, its surface area $A$ and volume $V$ are: $${\begin{aligned}A&={\frac {a^{2}}{2}}{\sqrt {{\frac {5}{2}}\left(10+{\sqrt {5}}+{\sqrt {75+30{\sqrt {5}}}}\right)}}\approx 3.88554a^{2},\\V&={\frac {5+{\sqrt {5}}}{24}}a^{3}\approx 0.30150a^{3}.\end{aligned}}$$

Applications

In polyhedron

Pentagonal pyramids can be found in a small stellated dodecahedron

Pentagonal pyramids can be found as components of many polyhedrons. Attaching its base to the pentagonal face of another polyhedron is an example of the construction process known as augmentation, and attaching it to prisms or antiprisms is known as elongation or gyroelongation, respectively. An example polyhedron is the pentakis dodecahedron, constructed from the dodecahedron by attaching the base of pentagonal pyramids onto each pentagonal face. The small stellated dodecahedron is a dodecahedron stellated by pentagonal pyramids.

Some Johnson solids are constructed by either augmenting pentagonal pyramids or augmenting other shapes with pentagonal pyramids: elongated pentagonal pyramid $J_{9}$, gyroelongated pentagonal pyramid $J_{11}$, pentagonal bipyramid $J_{13}$, elongated pentagonal bipyramid $J_{16}$, augmented dodecahedron $J_{58}$, parabiaugmented dodecahedron $J_{59}$, metabiaugmented dodecahedron $J_{60}$, and triaugmented dodecahedron $J_{61}$. Relatedly, the removal of a pentagonal pyramid from polyhedra is an example known as diminishment; metabidiminished icosahedron $J_{62}$ and tridiminished icosahedron $J_{63}$ are the examples in which their constructions begin by removing pentagonal pyramids from a regular icosahedron.

Stereochemistry

In stereochemistry, an atom cluster can have a pentagonal pyramidal geometry. This molecule has a main-group element with one active lone pair, which can be described by a model that predicts the geometry of molecules known as VSEPR theory. An example of molecule with this structure include iodosyl pentafluoride dianion IOF²⁻
₅.

References

• Ball, W. W. R.; Coxeter, H. S. M. (1987). Mathematical Recreations and Essays. Dover Publications.
• Baran, E. (2000). "Mean amplitudes of vibration of the pentagonal pyramidal XeOF⁻
  ₅ and IOF²⁻
  ₅ anions". Journal of Fluorine Chemistry. 101: 61–63. doi:10.1016/S0022-1139(99)00194-3.
• Berman, Martin (1971). "Regular-faced convex polyhedra". Journal of the Franklin Institute. 291 (5): 329–352. doi:10.1016/0016-0032(71)90071-8. MR 0290245.
• Calter, Paul A.; Calter, Michael A. (2011). Technical Mathematics. John Wiley & Sons. ISBN 978-0-470-53492-2.
• Çolak, Zeynep; Gelişgen, Özcan (2015). "New Metrics for Deltoidal Hexacontahedron and Pentakis Dodecahedron". Sakarya University Journal of Science. 19 (3): 353–360. doi:10.16984/saufenbilder.03497.
• Grgić, Ivan; Karakašić, Mirko; Ivandić, Željko; Glavaš, Hrvoje (2022). "Maintaining the Descriptive Geometry's Design Knowledge". In Glavaš, Hrvoje; Hadzima-Nyarko, Marijana; Karakašić, Mirko; Ademović, Naida; Avdaković, Samir (eds.). 30th International Conference on Organization and Technology of Maintenance (OTO 2021): Proceedings of 30th International Conference on Organization and Technology of Maintenance (OTO 2021). International Conference on Organization and Technology of Maintenance. doi:10.1007/978-3-030-92851-3.
• Hartshorne, Robin (2000). Geometry: Euclid and Beyond. Undergraduate Texts in Mathematics. Springer-Verlag. ISBN 9780387986500.
• Johnson, Norman W. (1966). "Convex polyhedra with regular faces". Canadian Journal of Mathematics. 18: 169–200. doi:10.4153/cjm-1966-021-8. MR 0185507. S2CID 122006114. Zbl 0132.14603.
• Kappraff, Jay (2001). Connections: The Geometric Bridge Between Art and Science (2nd ed.). World Scientific. ISBN 981-02-4585-8.
• Petrucci, Ralph H.; Harwood, William S.; Herring, F. Geoffrey (2002). General Chemistry: Principles and Modern Applications. Vol. 1. Prentice Hall. ISBN 9780130143297.
• Pisanski, Tomaž; Servatius, Brigitte (2013). Configuration from a Graphical Viewpoint. Springer. doi:10.1007/978-0-8176-8364-1. ISBN 978-0-8176-8363-4.
• Polya, G. (1954). Mathematics and Plausible Reasoning: Induction and analogy in mathematics. Princeton University Press.
• Rajwade, A. R. (2001). Convex Polyhedra with Regularity Conditions and Hilbert's Third Problem. Texts and Readings in Mathematics. Hindustan Book Agency. doi:10.1007/978-93-86279-06-4. ISBN 978-93-86279-06-4.
• Slobodan, Mišić; Obradović, Marija; Ðukanović, Gordana (2015). "Composite Concave Cupolae as Geometric and Architectural Forms" (PDF). Journal for Geometry and Graphics. 19 (1): 79–91.
• Smith, James T. (2000). Methods of Geometry. John Wiley & Sons. ISBN 0-471-25183-6.
• Uehara, Ryuhei (2020). Introduction to Computational Origami: The World of New Computational Geometry. Springer. doi:10.1007/978-981-15-4470-5. ISBN 978-981-15-4470-5.
• Wohlleben, Eva (2019). "Duality in Non-Polyhedral Bodies Part I: Polyliner". In Cocchiarella, Luigi (ed.). ICGG 2018 - Proceedings of the 18th International Conference on Geometry and Graphics: 40th Anniversary - Milan, Italy, August 3-7, 2018. International Conference on Geometry and Graphics. Springer. doi:10.1007/978-3-319-95588-9. ISBN 978-3-319-95588-9.

External links

• Weisstein, Eric W., "Pentagonal pyramid" ("Johnson solid") at MathWorld.
• Virtual Reality Polyhedra www.georgehart.com: The Encyclopedia of Polyhedra ( VRML model)