In chemistry, a trigonal pyramid is a molecular geometry with one atom at the apex and three atoms at the corners of a trigonal base, resembling a tetrahedron (not to be confused with the tetrahedral geometry). When all three atoms at the corners are identical, the molecule belongs to point group C<sub>3v</sub>. Some molecules and ions with trigonal pyramidal geometry are the pnictogen hydrides (XH<sub>3</sub>), xenon trioxide (XeO<sub>3</sub>), the chlorate ion, , and the sulfite ion, . In organic chemistry, molecules which have a trigonal pyramidal geometry are sometimes described as sp<sup>3</sup> hybridized. The AXE method for VSEPR theory states that the classification is AX<sub>3</sub>E<sub>1</sub>.

thumb|right|150px|[[Phosphine, an example of a molecule with a trigonal pyramidal geometry.]]

Trigonal pyramidal geometry in ammonia

The nitrogen in ammonia has 5 valence electrons and bonds with three hydrogen atoms to complete the octet. This would result in the geometry of a regular tetrahedron with each bond angle equal to arccos(−)&nbsp;≈&nbsp;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°. In contrast, boron trifluoride is flat, adopting a trigonal planar geometry because the boron does not have a lone pair of electrons. In ammonia the trigonal pyramid undergoes rapid nitrogen inversion.

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Image:Ammonia-with-lone-pairs-3D-balls.png|The electron pair arrangement of ammonia is tetrahedral: the two lone electrons are shown in yellow, the hydrogen atoms in white

Image:Ammonia-3D-balls-A.png|The molecular geometry can be inferred from the electron pair arrangement, showing that ammonia has trigonal pyramidal geometry.

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See also

  • VSEPR theory#AXE method
  • Molecular geometry

References

  • Chem| Chemistry, Structures, and 3D Molecules
  • Indiana University Molecular Structure Center
  • Interactive molecular examples for point groups
  • Molecular Modeling
  • Animated Trigonal Planar Visual