The different types of molecular orbitals:
- The molecular orbital theory uses a Linear Combination of Atomic Orbitals(LCAO) to form molecular orbitals which cover the whole molecule
- These molecular orbitals are divided into bonding orbitals , anti-bonding orbitals and non-bonding orbitals
- So today we will only touch on the bonding and anti-bonding orbitals.
- If the orbital is the type which the electrons present in the orbital have a higher chance to be found between the nuclei, then this is said to be a bonding orbital as it will tend to hold the nuclei together.
- However if the orbital is the type which the electrons present in the orbital have a higher chance to be found elsewhere other than between the nuclei, then this is said to be an anti-bonding orbital and the bond will be weaker.
- In order to understand the anti-bonding orbital, we must first know what is the Linear Combination of Atomic Orbital.
- This model follows these assumptions made:
- 1)Molecular orbitals are formed by 2 overlapping atomic orbitals
- 2)Only atomic orbitals with almost the same energy level can react to a certain point
- 3)When 2 atomic orbitals they react in 2 very different ways and form both bonding and anti-bonding orbitals
The in-phase reaction
- Where the atomic orbitals overlap, the in-phase interaction leads to an increase in the intensity of the negative charge in the region where they overlap.
- This creates an increase in negative charge between the nuclei and an increase in the attraction between the electron and the nuclei for the atoms.
- The greater attraction leads to lower potential energy and because electrons in the molecular orbital are of lower potential energy than in separate atomic orbitals, energy would be required to shift the electrons back into the 1s orbitals of separate atoms.
- This keeps the atoms together in the molecule, so we call this orbital a bonding molecular orbital.
The out-of-phase reaction
- Where the atomic orbitals overlap, the out-of-phase interaction leads to a decrease in the intensity of the negative charge.
- This creates a decrease in negative charge between the nuclei and a decrease in the attraction between the electron charge and the nuclei for the atoms in the bond.
- The lesser attraction leads to higher potential energy and the electrons are more stable in the 1s atomic orbitals of separate atoms, so electrons in this type of molecular orbital destabilizes the bond between atoms.
The above example shows how the 1s orbital overlaps and thus having 2 different kinds of reactions. However for the 2p orbitals (and subsequently 3p , 3d , 4p , 4d , 4f and so on), there will be more scenarios as they can overlap differently.
- The p atomic orbitals of the two atoms can interact in two different ways, parallel or end-on. The molecular orbitals are different for each type of interaction.
- There is less overlap for the parallel atomic orbitals and when the interaction is in-phase, less overlap leads to less electron charge enhancement between the nuclei.
- This leads to less electron charge between the nuclei for the pi bonding molecular orbital than for the sigma bonding molecular orbital.
- Less electron character between the nuclei means less attraction, less stabilization, and higher potential energy for the pi bonding molecular orbital compared to the sigma bonding molecular orbital.
- When the interaction is out-of-phase, less overlap leads to less shift of electron charge from between the nuclei.
- This leads to more electron charge between the nuclei for the pi anti-bonding molecular orbital than for the sigma anti-bonding molecular orbital.
- More electron charge between the nuclei means more attraction and lower potential energy for the pi anti-bonding molecular orbital compared to the sigma anti-bonding molecular orbital.
