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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. 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 SO₃^2-, and the phosphite ion PO₃^3-.

[edit] 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(-1/3) ≈ 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.