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Trigonal pyramidal molecular geometry
Examples	NH3, PCl3
Point group	C3v
Steric number	4
Coordination number	3
Bond angle(s)	<109.5º, ≈107º

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^{citation needed}. 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^{citation needed}.

Trigonal pyramidal geometry in ammonia

The nitrogen atom 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 inversion¹.

References

See also

• AXE method
• Molecular geometry

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

Wikipedia content modification information:

• This page was last modified on 2 September 2008, at 01:17.

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