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.

Trigonal pyramidal geometri