Most Accepted Theory For Covalent Bonding

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Alright guys...for the past month and a half my chemistry class consisted on lectures on chemical bonding.
From the days of Kossel and Lewis
To modern concepts of VSEPR,Hybridisation and Molecular orbital theory.

According to Wikipedia they say the two basic models based on quantum mechanics are Valence bond theory and Molecular orbital theory...but ofcourse they both...are...well...different ! Like really different !

Could anyone tell me which one is the more accepted theory and why we cant come up with one conclusive theory ?

I have searched quite a bit on the net but i havent come accross any conclusive answer.
Few links to get started
https://courses.lumenlearning.com/boundless-chemistry/chapter/valence-bond-theory/

https://en.m.wikipedia.org/wiki/Valence_bond_theory

https://en.m.wikipedia.org/wiki/Molecular_orbital_theory


Edit 1

I forgot to mention that one plus point of MO theory is that it can explain oxygen's paramagnetic cherecter and fractional bond order in electron deficient compounds
 
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  • #2
Charles Link
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Valence bond is best described by ionic bonds where one atom needs an electron or two to fill an electronic shell, and the other atom gives up an electron or two to empty the shell. Then, the bond is very much ioinic/electrostatic. ## \\ ## With molecular orbital theory, sometimes the bonding can be described by hybrid type orbitals that result in some cases from symmetry considerations. The molecular orbital approach is very much a model of what occurs. The solution of the Schrodinger equation for the complex systems of a couple atoms with sometimes many electrons is much too complex for any exact analysis.
 
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TeethWhitener
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Valence bond is best described by ionic bonds where one atom needs an electron or two to fill an electronic shell, and the other atom gives up an electron or two to empty the shell. Then, the bond is very much ioinic/electrostatic. ## \\ ## With molecular orbital theory, sometimes the bonding can be described by hybrid type orbitals that result in some cases from symmetry considerations. The molecular orbital approach is very much a model of what occurs. The solution of the Schrodinger equation for the complex systems of a couple atoms with sometimes many electrons is much too complex for any exact analysis.
Most of this is incorrect. The canonical example of VB theory is H2. The concept of hybridization also originated from VB theory.

At the end of the day, both theories attempt to solve the Schrodinger equation for a complicated potential.

MO theory starts by approximating the molecular wavefunction strictly as a linear combination of atomic orbitals:
$$\Psi = \sum c_i\varphi_i$$
where each ##\varphi_i## is an atomic orbital and the ##c_i## are coefficients that are optimized in a self consistent procedure.

VB theory in a sense "pre-builds" bonding/antibonding/covalent/ionic/etc. orbitals from atomic orbitals and creates a linear combination of these orbitals. For example, one of the orbitals in H2 would be:
$$\psi_{\sigma bond} =\frac{1}{\sqrt 2}( \varphi_{1sA}(1)\varphi_{1sB}(2)+\varphi_{1sB}(1)\varphi_{1sA}(2))(\alpha_1\beta_2-\alpha_2\beta_1)$$
where each ##\varphi## is a spatial 1s orbital centered on one or the other hydrogen, and the ##\alpha,\beta## are spin functions. (Edit: this "pre-built" orbital represents a sigma bond between two 1s orbitals located on each H atom in H2.) Several of these "pre-built" orbitals are then built up in a linear combination and their coefficients are optimized.

MO theory is dominant today, but VB theory has seen resurgences in fits and starts. The main drawback to simple MO theory is that it does not correctly handle simple bond breaking (in particular, a simple MO type theory, Hartree-Fock theory, incorrectly predicts that H2 dissociates into H+ and H-), but extensions to the model overcome this limitation.
 
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Most of this is incorrect. The canonical example of VB theory is H2. The concept of hybridization also originated from VB theory.

At the end of the day, both theories attempt to solve the Schrodinger equation for a complicated potential.

MO theory starts by approximating the molecular wavefunction strictly as a linear combination of atomic orbitals:
$$\Psi = \sum c_i\varphi_i$$
where each ##\varphi_i## is an atomic orbital and the ##c_i## are coefficients that are optimized in a self consistent procedure.

VB theory in a sense "pre-builds" bonding/antibonding/covalent/ionic/etc. orbitals from atomic orbitals and creates a linear combination of these orbitals. For example, one of the orbitals in H2 would be:
$$\psi_{\sigma bond} =\frac{1}{\sqrt 2}( \varphi_{1sA}(1)\varphi_{1sB}(2)+\varphi_{1sB}(1)\varphi_{1sA}(2))(\alpha_1\beta_2-\alpha_2\beta_1)$$
where each ##\varphi## is a spatial 1s orbital centered on one or the other hydrogen, and the ##\alpha,\beta## are spin functions. (Edit: this "pre-built" orbital represents a sigma bond between two 1s orbitals located on each H atom in H2.) Several of these "pre-built" orbitals are then built up in a linear combination and their coefficients are optimized.

MO theory is dominant today, but VB theory has seen resurgences in fits and starts. The main drawback to simple MO theory is that it does not correctly handle simple bond breaking (in particular, a simple MO type theory, Hartree-Fock theory, incorrectly predicts that H2 dissociates into H+ and H-), but extensions to the model overcome this limitation.
Thank you so much.This was extremely helpful !
 
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Charles Link
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Most of this is incorrect.
I should have read the "links" furnished by the OP. Most of my knowledge of valence bonds and molecular bonds is at the level of high school and first year college chemistry.
 
  • #6
DrDu
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Just wanted to mention, that both valence bond and molecular orbital theory are (different) approximations to the real bonding situation in molecules. However, both approximations can be improved systematically (this is called "configuration interaction") and when doing this, you obtain the same final result independently of whether you start from MO or VB theory.
Btw, also VB theory predicts correctly the paramagnetic ground state of oxygen. A singlet ground state for oxygen was only predicted by the pre-quantum mechanical Lewis description of bonding.
You should consider to read an introductory book on quantum chemistry.
 
  • #7
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Just wanted to mention, that both valence bond and molecular orbital theory are (different) approximations to the real bonding situation in molecules. However, both approximations can be improved systematically (this is called "configuration interaction") and when doing this, you obtain the same final result independently of whether you start from MO or VB theory.
Btw, also VB theory predicts correctly the paramagnetic ground state of oxygen. A singlet ground state for oxygen was only predicted by the pre-quantum mechanical Lewis description of bonding.
You should consider to read an introductory book on quantum chemistry.

Are you sure that VB correctly predicts correctly the paramagnetic ground state for oxygen ?

Because our chemistry professor gave us that one of the major drawbacks of VB was that it couldnt do so.

Could you please provide me with a link so that i can discuss this with him.


Anyway i would love to read a book on quantum chemistry and i shall definately when i get the time,
Right now im surrounded by a sea of textbooks,MCQ material.and notes !!!!!
 
  • #8
DrDu
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DrDu
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Thank you so much !!!
This was really nice of you !!!!
You are welcome! Anyhow, the reason for oxygen being paramagnetic is easy to understand not using the language of MO: Assume the two atoms to be aligned along the z axis, then the 2 s orbitals will be filled, the pz orbitals will form a sigma bond and hence there are 3 electrons left per atom which are located in the px and py orbitals. There are two possibilities: 4 electrons will be in the px orbitals and 2 electrons in the py orbitals ( or equivalently 4 in py and 2 in px). This alternative corresponds to the Delta singlet oxygen. The other possibility is that there are 3 electrons in both the px and py orbitals. Considering spin, we can form a singlet and a triplet state, the latter being the ground state of the oxygen molecule. But why?
In the first case, we have one two-electron bond but also a strong repulsion between the 4 electrons in the px orbitals. In the second case we have two 3 electron bonds, and, in the case of the triplet, the repulsion between the electrons is further reduced by Hund's rule. To put it alternatively, the electrons hop concertedly in the two adjacent 3 electron bonds so as to avoid each other.
 

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