Molecular Orbital theory is primarily used to explain the bonding in molecules the cannot be defined by Valence shortcut Theory. These space molecules that usually involve some form of resonance. Resonance implies that a bond is neither solitary nor twin but part hybrid the the two. Valence bond theory only explains the bonding of single or dual or triple bonds. It does not provide an explanation because that resonance bonding.

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Molecular orbital concept does explain resonance.

The rules of Molecular orbit Theory:

First principle: The number of molecular orbitals developed is always equal come the number of atomic orbitals carried by the atom that have actually combined. second principle: Bonding molecule orbitals are reduced in power that the parental orbitals, and also the antibonding orbitals are higher in energy. 3rd principle: electrons of the molecule space assigned to orbitals from lowest to successively greater energy fourth principle: atomic orbitals combine to type molecular orbitals most properly when the atom orbitals are of similar energy.

OK. For this reason what execute those values actually mean???

Principle 1: example - Hydrogen ( H2 ) each hydrogen atom has a solitary valence orbital, this gift the 1s orbital. Two molecular orbitalsmay be created by the constructive and destructive overlap the these 2 atomic orbitals. For this reason if you have two 1s atom orbitals you have the right to only make two molecular orbitals native them. This is the an initial Principle.


According come MO Theory, the two molecular orbitals that kind are referred to as s (sigma = bonding) and s* (sigma star = antibonding). In the situation of H2 both the the valence electron that type the bond in between the hydrogens fill the bonding or s orbital.

Principle 2 & 3: This interaction of atom orbitals, which gives rise to the molecular orbitals, may also be stood for in the kind of an orbit (electron) power diagram which mirrors the relative energies the the orbitals. In the particular case the hydrogen each of the diverted atoms has one electron in its 1s orbital and also when the atoms incorporate to form H2 the two electrons might be accommodated (with the opposite spins) in the bonding molecular orbital, as illustrated below. The 2nd principle explains why electron would want to to fill molecular orbitals in the first place. Together you should recognize by now, stability originates from lowering energy needs. Think around it. Don"t girlfriend feel far better when her energy demand is lowered? If not, I would certainly be happy to increase your homework? Anyway...because the bonding molecular orbitals carry out a reduced energy, an ext stable state for the electrons, they fill these orbitals first. This also explains the third principle statement together well.


Principle 4: If you keep in mind in H2 we an unified two 1s orbitals to kind a single lower energy s molecule orbital. The 4th principle claims that secure molecular orbitals are most basic to type when created out of atomic orbitals of similar energies. This way that 1s orbitals should integrate with 1s orbitals and also 2p orbitals should integrate with 2p orbitals etc. To form the many stable molecule orbitals.

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Filling up the molecule Orbitals:

We start with:


We then type Molecular Orbitals:


Which fill from lowest to highest power as follows:


Bond Order

The link order because that a molecule deserve to be established as follows: bond order = ½ (bonding electron − antibonding electrons). Therefore, the H2 molecule has actually a link order that ½ (2 − 0) = 1. In other words, there is a single bond connecting the 2 H atoms in the H2 molecule. In the situation of He2, top top the other hand, the bond order is ½ (2 − 2) = 0. This way that He2 is not a steady molecule.

Let"s shot an example: N2

Each Nitrogen atom has 7 electrons and an electron construction of 1s2 2s2 2p3 . This returns a total of 14 electron to occupational with. How countless molecular orbitals deserve to form? Which molecule orbitals will form? which orbitals are filled first? What is the shortcut order for N2?


Something come notice:

1) because dinitrogen totally fills the s1s and s*1s orbitals lock cancel each other out and it becomes an ext obvious as to why that is the valence electrons the actually regulate bonding. Together you continue from duration to duration in the regular table this trend of cancellation amongst the core electrons continues.