Trigonal pyramidal molecular geometry
# Trigonal pyramidal molecular geometry

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, resembling a tetrahedron (not to be confused with the tetrahedral geometry). 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 the pnictogen hydrides $(XH_3),$ xenon trioxide $(XeO_3),$ the chlorate ion, $ClO^−$ $_3,$ and the sulfite ion, $SO^{2−}$ $_3.$ In organic chemistry, molecules which have a trigonal pyramidal geometry are sometimes described as $sp^3$ hybridized. The AXE method for VSEPR theory states that the classification is $AX_3E_1.$

## 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 equal to $cos^{−1}(−1/3)$ ≈ 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.

* The electron pair arrangement of ammonia is tetrahedral: the two lone electrons are shown in yellow, the hydrogen atoms in white
* The molecular geometry can be inferred from the electron pair arrangement, showing that ammonia has trigonal pyramidal geometry.
