Trigonal pyramidal molecular geometry

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. When all three atoms at the corners are identical, the molecule belongs to point group C_3v. One example of a molecule with a trigonal pyramidal geometry is ammonia (NH₃). Some molecules and ions with trigonal pyramidal geometry include the xenon trioxide molecule, XeO₃, the chlorate ion, ClO₃⁻, the sulfite ion, SO₃²⁻, and the phosphite ion, PO₃³⁻. In organic chemistry, molecules which have a trigonal pyramidal geometry are sometimes described as sp³ hybridized. The AXE method for VSEPR theory states that the classification is AX₃E.

Trigonal pyramidal geometry in ammonia

The nitrogen molecule 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 cos⁻¹(−⅓) ≈ 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.^[1]

See also

• VSEPR theory#AXE method
• Molecular geometry

References

External links

• Chem| Chemistry, Structures, and 3D Molecules
• Indiana University Molecular Structure Center
• Point Group Symmetry| Point Group Symmetry Interactive Examples
• Molecular Modeling
• Animated Trigonal Planar Visual