Trigonal pyramidal molecular geometry

{{Short description|Configuration of atoms within a molecule}} {{redir|Trigonal pyramid|the shape|Triangular pyramid}} {{Infobox molecular geometry | Image_File=Pyramidal-3D-balls.png| Symmetry_group= C_3v | Atom_direction=3 | Bond_angle= 90° to 109.5° | mu=>0 | Examples= NH₃ }}

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, {{chem|ClO|3|−}}, and the sulfite ion, {{chem|SO|3|2−}}. 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. [[Image:Phosphine-3D-vdW.png|thumb|right|150px|Phosphine, an example of a molecule with a trigonal pyramidal geometry.]]

== 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 arccos(−{{sfrac|1|3}})&nbsp;≈&nbsp;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.<ref name="Cleeton">{{cite journal|author1-link=Claud E. Cleeton|first1=C. E. |last1=Cleeton |author2-link=Neal H. Williams|first2=N. H. |last2=Williams |date=1934 |title=Electromagnetic Waves of 1.1 cm Wave-Length and the Absorption Spectrum of Ammonia|journal=Phys. Rev. |volume=45 |issue=4 |page=234 |doi=10.1103/PhysRev.45.234|bibcode=1934PhRv...45..234C}}</ref>

<gallery> Image:Ammonia-with-lone-pairs-3D-balls.png|The electron pair arrangement of ammonia is tetrahedral: the two lone electrons are shown in yellow, the hydrogen atoms in white Image:Ammonia-3D-balls-A.png|The molecular geometry can be inferred from the electron pair arrangement, showing that ammonia has trigonal pyramidal geometry. </gallery>

==See also== *VSEPR theory#AXE method *Molecular geometry

==References== {{reflist}}

==External links== *[http://www.3dchem.com/3D Chem| Chemistry, Structures, and 3D Molecules]{{dead link|date=December 2017 |bot=InternetArchiveBot |fix-attempted=yes }} *[https://web.archive.org/web/20011224193355/http://www.iumsc.indiana.edu/IUMSC/ Indiana University Molecular Structure Center] * [http://www.staff.ncl.ac.uk/j.p.goss/symmetry/Molecules_l3d.html Interactive molecular examples for point groups] *[https://web.archive.org/web/20080120023822/http://chemlab.truman.edu/CHEM121Labs/MolecularModeling1.htm Molecular Modeling] *[http://intro.chem.okstate.edu/1314F97/Chapter9/3BP.html Animated Trigonal Planar Visual]

{{MolecularGeometry}}

Category:Molecular geometry