In
chemistry
Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...
, a trigonal pyramid is a
molecular geometry
Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that det ...
with one
atom
Atoms are the basic particles of the chemical elements. An atom consists of a atomic nucleus, nucleus of protons and generally neutrons, surrounded by an electromagnetically bound swarm of electrons. The chemical elements are distinguished fr ...
at the
apex and three atoms at the corners of a
trigonal
In crystallography, the hexagonal crystal family is one of the six crystal family, crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the tr ...
base, resembling a
tetrahedron
In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...
(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 (XeO3), the
chlorate ion, , and the
sulfite ion, . In
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the science, scientific study of the structure, properties, and reactions of organic compounds and organic matter, organic materials, i.e., matter in its various forms that contain ...
, molecules which have a trigonal pyramidal geometry are sometimes described as
sp3 hybridized. The
AXE method for
VSEPR theory states that the classification is AX
3E
1.
Trigonal pyramidal geometry in ammonia
The
nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
in
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
has 5
valence electrons and bonds with three hydrogen atoms to complete the
octet. This would result in the geometry of a regular
tetrahedron
In geometry, a tetrahedron (: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular Face (geometry), faces, six straight Edge (geometry), edges, and four vertex (geometry), vertices. The tet ...
with each bond angle equal to
arccos(−) ≈ 109.5°. However, the three hydrogen atoms are repelled by the electron
lone pair
In chemistry, a lone pair refers to a pair of valence electrons that are not shared with another atom in a covalent bondIUPAC ''Gold Book'' definition''lone (electron) pair''/ref> and is sometimes called an unshared pair or non-bonding pair. Lone ...
in a way that the geometry is distorted to a trigonal
pyramid
A pyramid () is a structure whose visible surfaces are triangular in broad outline and converge toward the top, making the appearance roughly a pyramid in the geometric sense. The base of a pyramid can be of any polygon shape, such as trian ...
(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.
ammonia
Ammonia is an inorganic chemical compound of nitrogen and hydrogen with the chemical formula, formula . A Binary compounds of hydrogen, stable binary hydride and the simplest pnictogen hydride, ammonia is a colourless gas with a distinctive pu ...
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.
See also
*
VSEPR theory#AXE method
*
Molecular geometry
Molecular geometry is the three-dimensional arrangement of the atoms that constitute a molecule. It includes the general shape of the molecule as well as bond lengths, bond angles, torsional angles and any other geometrical parameters that det ...
References
External links
Chem, Chemistry, Structures, and 3D MoleculesIndiana University Molecular Structure Center
{{MolecularGeometry
Molecular geometry