Chemical bonding model
Encyclopedia
Chemical bonding
models are theoretical models used to explain atomic bonding structure, molecular geometry
, properties, and reactivity of physical matter.
ing interactions including covalent, ionic
, and metallic bonding among others. The theories associated with bonding are often developed around the covalent bonds and extended to ionic and metallic bonding. There are a variety of active theories or models associated with covalent bonding, the building block of molecules. These theories make various approximations rendering each them useful for describing different nuances of common molecular bonding.
and d-orbital splitting to students. As a result, material based on these theories is still included in most courses and most standardized tests.
an early bonding theory that has developed into Modern valence bond theory
. VBT views bonds as weakly coupled orbitals with each atom sharing a valence electron in a manner governed by the octet
or 18 electron rule. Lewis structures are a representation of VBT's most basic bonding while molecular geometry is derived from orbital hybridization. Orbital hybridization is often taught with VESPR Theory despite significant differences in the underlying theory.
Valence shell electron pair repulsion (VSEPR) Theory
The simplest and most primitive of the theories currently taught. Describes molecular geometry through the repulsion of electron fields which include bonds and lone pairs. It does not require any application of orbital shape.
Crystal Field Theory (CFT)
This approximation begins with the geometries of the d orbitals derived from quantum mechanics. Ligands with their electron density are assumed to destabilize the metal d orbitals they interact with raising their energy while the remain d-orbitals drop in energy to balance the overall change in energy.
Ligand Field Theory (LFT)
Considered a hybrid of CFT and MO Theory or simple an approximate application of MO Theory to transition metal
complexes.
Molecular Orbital (MO) Theory
A current and often applied model of molecular bonding. MO Theory assumes that bonds are derived from a linear combination of atomic orbitals
. In this linear combination each pair of atomic orbitals involved in bonding results in a bonding and anti-bonding orbital. The destabilized of orbitals of CFT are now seen as anti-bonding component of orbitals that have overall been stabilized through bonding interactions.
applies components of bonding models to simulate various chemical phenomenon associated with bonding. Computational chemistry also extends beyond covalent bonding to investigate the interactions of groups of molecules and higher order structure. In these systems the chemical bond is often simplified and approximated to reduce computing time.
Chemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
models are theoretical models used to explain atomic bonding structure, 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...
, properties, and reactivity of physical matter.
Overview
There are a variety of known chemical bondChemical bond
A chemical bond is an attraction between atoms that allows the formation of chemical substances that contain two or more atoms. The bond is caused by the electromagnetic force attraction between opposite charges, either between electrons and nuclei, or as the result of a dipole attraction...
ing interactions including covalent, ionic
Ionic bond
An ionic bond is a type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. Ionic bonds are formed between a cation, which is usually a metal, and an anion, which is usually a nonmetal. Pure ionic bonding cannot exist: all ionic compounds have some...
, and metallic bonding among others. The theories associated with bonding are often developed around the covalent bonds and extended to ionic and metallic bonding. There are a variety of active theories or models associated with covalent bonding, the building block of molecules. These theories make various approximations rendering each them useful for describing different nuances of common molecular bonding.
Molecular Bonding Theories
Modern bonding theories such as VBT, LFT, and MO Theory assumes that bonds are formed by atoms sharing electrons in directional orbitals. This by all accounts represents reality accurately. A more simplified or primitive model such as VSEPR Theory presupposes no orbital directionality. CFT actually treats electrons of the two atoms as repulsive as the two atoms attract electrostatically. However, VSEPR Theory and CFT are widely considered the simplest and historic ways to introduce molecular structureMolecular structure
The molecular structure of a substance is described by the combination of nuclei and electrons that comprise its constitute molecules. This includes the molecular geometry , the electronic properties of the...
and d-orbital splitting to students. As a result, material based on these theories is still included in most courses and most standardized tests.
The Major Models
Valence Bond Theory (VBT)Valence bond theory
In chemistry, valence bond theory is one of two basic theories, along with molecular orbital theory, that were developed to use the methods of quantum mechanics to explain chemical bonding. It focuses on how the atomic orbitals of the dissociated atoms combine to give individual chemical bonds...
an early bonding theory that has developed into Modern valence bond theory
Modern valence bond theory
Modern valence bond theory is the term used to describe applications of valence bond theory with computer programs that are competitive in accuracy and economy with programs for the Hartree-Fock method and other molecular orbital based methods. The latter methods dominated quantum chemistry from...
. VBT views bonds as weakly coupled orbitals with each atom sharing a valence electron in a manner governed by 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 (...
or 18 electron rule. Lewis structures are a representation of VBT's most basic bonding while molecular geometry is derived from orbital hybridization. Orbital hybridization is often taught with VESPR Theory despite significant differences in the underlying theory.
Valence shell electron pair repulsion (VSEPR) Theory
VSEPR theory
Valence shell electron pair repulsion theory is a model in chemistry used to predict the shape of individual molecules based upon the extent of electron-pair electrostatic repulsion. It is also named Gillespie–Nyholm theory after its two main developers...
The simplest and most primitive of the theories currently taught. Describes molecular geometry through the repulsion of electron fields which include bonds and lone pairs. It does not require any application of orbital shape.
Crystal Field Theory (CFT)
Crystal field theory
Crystal field theory is a model that describes the electronic structure of transition metal compounds, all of which can be considered coordination complexes. CFT successfully accounts for some magnetic properties, colours, hydration enthalpies, and spinel structures of transition metal complexes,...
This approximation begins with the geometries of the d orbitals derived from quantum mechanics. Ligands with their electron density are assumed to destabilize the metal d orbitals they interact with raising their energy while the remain d-orbitals drop in energy to balance the overall change in energy.
Ligand Field Theory (LFT)
Ligand field theory
Ligand field theory describes the bonding, orbital arrangement, and other characteristics of coordination complexes. It represents an application of molecular orbital theory to transition metal complexes. A transition metal ion has nine valence atomic orbitals, five d, one s, and three p orbitals...
Considered a hybrid of CFT and MO Theory or simple an approximate application of MO Theory to transition metal
Transition metal
The term transition metal has two possible meanings:*The IUPAC definition states that a transition metal is "an element whose atom has an incomplete d sub-shell, or which can give rise to cations with an incomplete d sub-shell." Group 12 elements are not transition metals in this definition.*Some...
complexes.
Molecular Orbital (MO) Theory
Molecular orbital theory
In chemistry, molecular orbital theory is a method for determining molecular structure in which electrons are not assigned to individual bonds between atoms, but are treated as moving under the influence of the nuclei in the whole molecule...
A current and often applied model of molecular bonding. MO Theory assumes that bonds are derived from a linear combination of atomic orbitals
Linear combination of atomic orbitals molecular orbital method
A linear combination of atomic orbitals or LCAO is a quantum superposition of atomic orbitals and a technique for calculating molecular orbitals in quantum chemistry. In quantum mechanics, electron configurations of atoms are described as wavefunctions...
. In this linear combination each pair of atomic orbitals involved in bonding results in a bonding and anti-bonding orbital. The destabilized of orbitals of CFT are now seen as anti-bonding component of orbitals that have overall been stabilized through bonding interactions.
Computational Chemistry
Modern computational chemistryComputational chemistry
Computational chemistry is a branch of chemistry that uses principles of computer science to assist in solving chemical problems. It uses the results of theoretical chemistry, incorporated into efficient computer programs, to calculate the structures and properties of molecules and solids...
applies components of bonding models to simulate various chemical phenomenon associated with bonding. Computational chemistry also extends beyond covalent bonding to investigate the interactions of groups of molecules and higher order structure. In these systems the chemical bond is often simplified and approximated to reduce computing time.