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Trigonal pyramid (chemistry)

  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 C3V. 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−.

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 repulsed 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°. Contrast to boron trifluoride with a flat trigonal planar geometry because boron does not have a lone pair of electrons.

In ammonia the trigonal pyramid undergoes rapid nitrogen inversion.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Trigonal_pyramid_(chemistry)". A list of authors is available in Wikipedia.
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