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This is true because the lone pair occupies more space near the central atom (A) than does a bonding pair of electrons. An equatorial lone pair is repelled by only two bonding pairs at 90°, whereas a hypothetical axial lone pair would be repelled by three bonding pairs at 90° which would make the molecule unstable.
In chemistry, a trigonal bipyramid formation is a molecular geometry with one atom at the center and 5 more atoms at the corners of a triangular bipyramid. [1] This is one geometry for which the bond angles surrounding the central atom are not identical (see also pentagonal bipyramid), because there is no geometrical arrangement with five terminal atoms in equivalent positions.
The pentagonal bipyramid is a case where bond angles surrounding an atom are not identical (see also trigonal bipyramidal molecular geometry). [1] [page needed] This is one of the three common shapes for heptacoordinate transition metal complexes, along with the capped octahedron and the capped trigonal prism. [2] [3] [page needed]
Carbon dioxide (CO 2) has two polar C=O bonds, but the geometry of CO 2 is linear so that the two bond dipole moments cancel and there is no net molecular dipole moment; the molecule is nonpolar. In methane , the bonds are arranged symmetrically (in a tetrahedral arrangement) so there is no overall dipole.
This would result in the geometry of a regular tetrahedron with each bond angle equal to arccos(− 1 / 3 ) ≈ 109.5°. However, the three hydrogen atoms are repelled by the electron lone pair in a way that the geometry is distorted to a trigonal pyramid (regular 3-sided pyramid) with bond angles of 107°.
The geometry is common for certain main group compounds that have a stereochemically-active lone pair, as described by VSEPR theory. Certain compounds crystallize in both the trigonal bipyramidal and the square pyramidal structures, notably [Ni(CN) 5 ] 3− .
In an AX 3 E 2 molecule, the two lone pairs occupy two equatorial positions, and the three ligand atoms occupy the two axial positions as well as one equatorial position. The three atoms bond at 90° angles on one side of the central atom, producing the T shape. [2] The trifluoroxenate(II) anion, XeF −
The atoms in SF 4 are arranged in a see-saw shape, with the sulfur atom at the center. One of the three equatorial positions is occupied by a nonbonding lone pair of electrons. Consequently, the molecule has two distinct types of F ligands, two axial and two equatorial. The relevant bond distances are S–F ax = 164.3 pm and S–F eq = 154.2 pm