Trigonal pyramidal molecular geometry

Trigonal pyramidal molecular geometry
Examples	NH3, PCl3, XeO3
Point group	C3v
Steric number	4
Coordination number	3
Bond angle(s)	107.28°

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. Some molecules and ions with trigonal pyramidal geometry are ammonia (NH₃), xenon trioxide, XeO₃, the chlorate ion, ClO₃⁻, and the sulfite ion, SO₃²⁻. 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 AX₃E₁.

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 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]

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  The electron pair arrangement of ammonia is tetrahedral: the two lone electrons are shown in yellow, the hydrogen atoms in white

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  The molecular geometry can be inferred from the electron pair arrangement, showing that ammonia has trigonal pyramidal geometry.

See also

Phosphine, an example of a molecule with a trigonal pyramidal geometry.

• VSEPR theory#AXE method
• Molecular geometry

References

External links

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