Applications of Molecular Orbit Theory

Electronic configuration and Molecular Behavior:

• The distribution of electrons among various molecular orbitals is called the electronic configuration of the molecule.

Important information of configuration:

1. Stability of Molecules:

1. The molecule is stable if  NB is greater than NA
2. The molecule is unstable if NB is less than NA
   Where, NB → Bonding e- s NA → Anti-Bonding e- s

2. Bond order:

• Bond order is defined as one half the difference between the number of electrons present in the bonding and the anti-bonding orbitals.Bond order = ½ [NB - NA]

• The rules discussed in the above regarding the stability of the molecule can be restated in terms of Bond order as follows.
• A positive Bond order means a stable molecule while negative (or) zero bond order means an unstable molecule.
• Nature of Bond: Integral bond order values of 1, 2 (or) 3 correspond to single, double (or) triple bonds respectively.

3. Bond Length:

• The bond order between two atoms in a molecule may be taken as an approximate measure of the Bond length.
• Bond length decreases bond order increases.

4. Magnetic Nature:

• If all the molecular orbitals in a molecule are doubly occupied the substance is diagrammatic.
• If one (or) more molecular orbitals are singly occupied it is paramagnetic.
  EX: O2 Molecule

Bonding in some Homonuclear diatomic molecules

1. Hydrogen Molecule (H2):

• There are 2 electrons in hydrogen molecule which are present in σ1S molecular orbital so electronic configuration of hydrogen molecule is H2 : (σ1S)2
  The bond order of H2 molecule can be calculated as given below
  Bond order = [NB - NA] / 2
• Since there is no unpaired electron in hydrogen molecule, therefore it is diamagnetic.
  

2. Helium molecule (He2):

• In (He2) molecule there are 4 electrons. These electrons will be accommodated in σ1S and  σ*1S molecular orbital leading to electronic configuration.
  He2 : (σ1S)2 (σ*1S)2
• Bond order He2  is ½ (2-2) = 0
• He2  Molecule is therefore unstable and does not exist.
  

3. Lithium Molecule (Li2):

• The electronic configuration of lithium is 1S2,2S1. There are six electrons in Li2 the electron configuration of Li2 molecule therefore is Li2 : (σ1S)2 (σ*1S)2 (σ2S)2
• The above configuration is also written as KK (σ2S)2 where KK represents the closed K shell structure (σ1S)2 (σ*1S)2  
• There are four electrons present in bonding molecular orbitals and two electrons in anti-bonding molecular orbitals.
  Bond order ½ [4 - 1] = 1
• It has no unpaired electrons so it is diamagnetic.
  

4. Oxygen molecule (O2):

• The electronic configuration of oxygen atom is 1S2 2S2 2P4 each oxygen atom has 8 electrons hence, in O2 molecule there are 16 electrons. The electronic configuration of O2 molecule therefore
  O2 : (σ1S) (σ*1S)2 (σ2S)2 (σ*2S)2 (σ2PZ)2 (π2P2x= π2P2y) (π*2P1x = π*2P1y)
• From the electronic configuration of O2 molecule, t is clear that ten electrons are present in anti-bonding molecular orbitals and six electrons are present in anti-bonding molecular orbitals.
  Its bond order = ½ [NB - NA] = ½ [10 - 6] = 4
• It may be noted that it contains two unpaired electrons π*2Px and π*2Py molecular orbitals.
• O2 Molecule should be paramagnetic.
  

5. Nitrogen molecule (N2):

• The electronic configuration of Nitrogen is 1S2 2S2 2P3, there are 14 electrons in N2. The electronic configuration of
  N2 : σ1S2 < σ*1S2 < σ2S2 < σ*2S2 < (π2P2x=π2P2y) < σ2P2z
• Bond order of N2 = ½ [10 -4] = 3
• There is no unpaired electron in the electronic configuration of N2
• It is diamagnetic in nature