Trigonal pyramidal molecular geometry
Examples	NH3
Point group	C3v
Coordination number	3
Bond angle(s)	90°<θ<109.5°
μ (Polarity)	>0

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 (XH₃), xenon trioxide (XeO₃), the chlorate ion, ClO⁻
₃, and the sulfite ion, SO²⁻
₃. 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 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.^[1]

• 
  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.