Why is the MO theory for O2 different from VB theory?

In summary: Maybe, but the simple fact is that oxygen is not sp2 hybridized in VB theory, and therefore the other posters' assertions are incorrect.
  • #1
richardlhp
13
0
Hi,

just wondering about MO theory. Take O2 for example; O is sp2 hybridised according to VB theory. However, when drawing the MO diagram for O2, one considers the sigma overlap between the s orbital and p orbitals separately (according to the theory), rather than considering the overlap of sp2 orbitals and one pz pi overlap. Why is this so?

Thanks to anyone who can help!

Cheers!
 
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  • #2
Who does consider O2 as sp2 hybridized in VB theory? That's nonsense.
 
  • #3
O is sp2 hybridised according to VB theory

who told you this??:rolleyes:
 
  • #4
If you want to know what's really going on, you have to set up and solve the schrodinger equation for the system. This is difficult and sometimes unsolvable, so we have some simpler models.

One is hybridization theory, which is the mixing of atomic orbitals to make hybrid orbitals for one atom. This is useful to explain the shapes and reactivity of some organic molecules amongst other things.

Another is Molecular Orbital theory which is the mixing of atomic orbitals of similar energy for two bonded atoms. This is useful to explain things like the paramagnetism of oxygen.

Don't mix the two unless you're using quantum mechanics to make a new theory.

Furthermore, the reason the other posters are saying that oxygen isn't sp2 hybridised is because MO theory is a better model for oxygen. One could argue that sp2 hybridised orbitals bear the closest resemblance to the MO theory model orbitals (particularly the antibonding orbitals), but this is pretty much beside the point.
 
  • #5
No, pseudophonist. It doesn't even make much sense in Valence Bond theory to assume Oxygen in O2 to be sp2 hybridized.


And, btw., you can also explain pretty well in valence bond theory why oxygen is paramagnetic as first shown already in 1937 by Wheland.
 
  • #6
True enough, I didn't know about Wheland. MO does make the prediction of oxygen's magnetic properties very easy to see though, while it was thought for a long time that VB theory couldn't predict that.
 
  • #7
Well, basically it is quite easy also in VB: Two 3-electron bonds are more stable than one 2-electron bond and one 4-electron bond. Then use Pauli principle.
 
  • #8
Btw., we know that the singulet state of the oxygen, which is prediced by a naive use of valence bond theory, corresponds to an excited state of the oxygen molecule. However, singulet oxygen is paramagnetic, too, due to the orbital momentum in the Delta state. The paramagnetism of Delta O2 is about 80 per cent of singulet oxygen. In fact, when you read early articles on MO-theory of oxygen, paramagnetism didn't come as a surprise.
 

1. What is hybridisation and why is it important?

Hybridisation is a concept in chemistry that describes the mixing of atomic orbitals to form new hybrid orbitals with different properties. It is important because it allows us to explain the observed shapes and bond angles in molecules, which are crucial for understanding their chemical and physical properties.

2. How is hybridisation related to molecular orbital (MO) theory?

Hybridisation is closely related to MO theory, as both concepts involve the mixing of atomic orbitals. In MO theory, the atomic orbitals combine to form molecular orbitals, which are delocalized over the entire molecule. Hybridisation helps to explain the different types of molecular orbitals that are formed and their relative energies.

3. What is the difference between sp, sp2, and sp3 hybridisation?

These are different types of hybridisation that occur when atomic orbitals mix. In sp hybridisation, one s orbital and one p orbital combine to form two sp hybrid orbitals. In sp2 hybridisation, one s orbital and two p orbitals combine to form three sp2 hybrid orbitals. In sp3 hybridisation, one s orbital and three p orbitals combine to form four sp3 hybrid orbitals.

4. How does hybridisation affect the bond angles in a molecule?

Hybridisation plays a crucial role in determining the bond angles in a molecule. The type of hybridisation influences the angles between the bonded atoms, as the hybrid orbitals will arrange themselves in a way that minimizes repulsion between electron pairs. For example, molecules with sp hybridisation have bond angles of 180 degrees, while molecules with sp3 hybridisation have bond angles of 109.5 degrees.

5. Can hybridisation occur in all types of molecules?

Yes, hybridisation can occur in all types of molecules, although it is more common in covalent compounds. In some cases, hybridisation may not be necessary to explain the bonding in a molecule, such as in pure p orbitals or in molecules with delocalized bonds. However, hybridisation is a useful concept for understanding the bonding in most molecules.

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