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trigonal bipyramidal arrangement
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Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.
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trigonal bipyramidal arrangement

molecular shape
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Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.
Figure 1: The four stable geometric structures of the seven-atom cluster of argon, in order of increasing energy: (A) A pentagonal bipyramid. (B) A regular octahedron with one face capped by the seventh atom. (C) A regular tetrahedron with three of its faces capped by other atoms. (D) A trigonal bipyramid with two of its faces capped by other atoms; although this has the highest energy of the four structures, it is very close in energy to the tricapped tetrahedron.

The term "trigonal bipyramidal arrangement" refers to a specific molecular geometry where a central atom is bonded to five other atoms. This arrangement minimizes electron pair repulsions, leading to a stable molecular shape.1

In this geometry, the central atom is surrounded by five electron pairs. Two of these pairs are positioned along an axis, above and below the central atom, while the remaining three pairs lie in a plane around the central atom, forming an equilateral triangle. This creates a structure that is essentially two pyramids joined at their bases.1

Examples of molecules exhibiting this geometry include phosphorus pentachloride (PCl5). While some molecules can form triangular shapes, like cyclopropane, these are often unstable due to ring strain. Other molecules, such as boron trifluoride (BF3), have a trigonal planar geometry, with atoms arranged in a flat triangle around a central atom.12

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2 Britannica Sources
  1. Are There Triangular Molecules?
  2. chemical bonding - Applying VSEPR theory to simple molecules

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