Trigonal pyramid (chemistry)
Encyclopedia
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−. 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 AX3E.
molecule in ammonia
has 5 valence electron
s 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 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
.
Chemistry
Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, a trigonal pyramid is a molecular geometry
Molecular geometry
Molecular geometry or molecular structure is the three-dimensional arrangement of the atoms that constitute a molecule. It determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity.- Molecular geometry determination...
with one atom at the apex
Apex (geometry)
In geometry, an apex is the vertex which is in some sense the highest of the figure to which it belongs.*In an isosceles triangle, the apex is the vertex where the two sides of equal length meet, opposite the unequal third side....
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
Molecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
C3v. One example of a molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
with a trigonal pyramidal geometry is ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
(NH3). Some molecules and ions with trigonal pyramidal geometry include the xenon trioxide
Xenon trioxide
Xenon trioxide is an unstable compound of xenon in its +6 oxidation state. It is a very powerful oxidizing agent, and liberates oxygen from water slowly , accelerated by exposure to sunlight. It is dangerously explosive upon contact with organic materials...
molecule, XeO3, the chlorate
Chlorate
The chlorate anion has the formula ClO. In this case, the chlorine atom is in the +5 oxidation state. "Chlorate" can also refer to chemical compounds containing this anion; chlorates are the salts of chloric acid. "Chlorate", when followed by a roman numeral in parentheses, e.g...
ion, ClO3−, the sulfite
Sulfite
Sulfites are compounds that contain the sulfite ion SO. The sulfite ion is the conjugate base of bisulfite. Although the acid itself is elusive, its salts are widely used.-Structure:...
ion, SO32−, and the phosphite
Phosphite
A phosphite is a salt of phosphorous acid. The phosphite ion is a polyatomic ion with a phosphorus central atom where phosphorus has an oxidation state of +3...
ion, PO33−. In organic chemistry, molecules which have a trigonal pyramidal geometry are sometimes described as sp3 hybridized
Orbital hybridisation
In chemistry, hybridisation is the concept of mixing atomic orbitals to form new hybrid orbitals suitable for the qualitative description of atomic bonding properties. Hybridised orbitals are very useful in the explanation of the shape of molecular orbitals for molecules. It is an integral part...
. The AXE method for VSEPR theory states that the classification is AX3E.
Trigonal pyramidal geometry in ammonia
The nitrogenNitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
molecule in ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
has 5 valence electron
Valence electron
In chemistry, valence electrons are the electrons of an atom that can participate in the formation of chemical bonds with other atoms. Valence electrons are the "own" electrons, present in the free neutral atom, that combine with valence electrons of other atoms to form chemical bonds. In a single...
s and bonds with three hydrogen atoms to complete the octet
Octet rule
The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low The octet rule is a chemical rule of thumb that states that atoms of low (The octet rule is a chemical rule of thumb that states that atoms of low (...
. This would result in the geometry of a regular tetrahedron
Tetrahedron
In geometry, a tetrahedron is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral", and is one of the Platonic solids...
with each bond angle cos−1(−⅓)
Inverse trigonometric function
In mathematics, the inverse trigonometric functions are the inverse functions of the trigonometric functions with suitably restricted domains .The notations sin−1, cos−1, etc...
≈ 109.5°. However, the three hydrogen atoms are repelled by the electron lone pair
Lone pair
In chemistry, a lone pair is a valence electron pair without bonding or sharing with other atoms. They are found in the outermost electron shell of an atom, so lone pairs are a subset of a molecule's valence electrons...
in a way that the geometry is distorted to a trigonal pyramid
Pyramid
A pyramid is a structure whose outer surfaces are triangular and converge at a single point. The base of a pyramid can be trilateral, quadrilateral, or any polygon shape, meaning that a pyramid has at least three triangular surfaces...
(regular 3-sided pyramid) with bond angles of 107°. In contrast, boron trifluoride
Boron trifluoride
Boron trifluoride is the chemical compound with the formula BF3. This pungent colourless toxic gas forms white fumes in moist air. It is a useful Lewis acid and a versatile building block for other boron compounds.-Structure and bonding:...
is flat, adopting a trigonal planar
Trigonal planar
In chemistry, trigonal planar is a molecular geometry model with one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. In an ideal trigonal planar species, all three ligands are identical and all bond angles are 120°. Such species belong to...
geometry because the boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
does not have a lone pair of electrons.
In ammonia the trigonal pyramid undergoes rapid nitrogen inversion
Nitrogen inversion
In chemistry, a nitrogen compound like ammonia in a trigonal pyramid geometry undergoes rapid nitrogen inversion whereby the molecule turns inside out. This interconversion is a room temperature process because the energy barrier is relatively small. Contrast this to phosphine which does not show...
.