Electronic Configurations and Molecular Behaviour

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Molecular orbital energy level diagram gives very valuable information regarding the behaviour of molecules. The important information conveyed by the electronic configuration of a molecule is:

Stability of a Molecule

We have learnt that the electrons in bonding molecular orbitals contribute to attraction between the atoms and thereby help in the formation of bond. On the other hand the electrons in antibonding molecular orbitals contribute to repulsion between the atoms and, therefore oppose the formation of bond. Thus, the electrons in bonding molecular orbitals will give stability to the molecule whereas those in antibonding molecular orbitals will decrease the stability.

Thus, if N_b represents the number of electrons in bonding molecular orbitals and N_a represents the number of electrons in antibonding molecular orbitals, then the following generalization can be made:

(i) The molecule is stable if N_b > N_a

(ii) The molecule is unstable if N_b < N_a

(iii) The molecule is unstable if N_b = N_a.

According to (iii), even if the numbers of electrons in bonding MOs and the numbers of electrons in antibonding MOs are same, the atoms do not combine to form molecules. This is because of the fact that the effect of anti-bonding electrons is slightly more than that of bonding electrons.

Bond Order

The relative stability of a molecule can be determined on the basis of bond order. It is defined as the number of covalent bonds in a molecule. It is equal to one half of the difference between the number of electrons in the bonding and antibonding molecular orbitals.

The bond orders of 1, 2 or 3 correspond to single, double or triple bond. But bond order may be fractional also in some cases.

Information conveyed by bond order

The bond order conveys the following important information:

(i) If the value of bond order is positive, it indicates a stable molecule and if the value of bond order is negative or zero, it means that the molecule is unstable.

(ii) The stability of a molecule is measured by its bond dissociation energy. But the bond dissociation energy is directly proportional to the bond order. Greater the bond order, greater is the bond dissociation energy. For e.g., nitrogen molecule with bond order equal to 3 has larger bond dissociation energy (945 kJ mol^-1) than oxygen molecule (bond dissociation energy 495 kJ mol^-1) with a bond order equal to 2.

(iii) Bond order is inversely proportional to the bond length. Higher the bond order value, smaller is the bond length. For e.g., the bond length in nitrogen molecule is shorter than in oxygen molecule. The table below gives the bond order, bond dissociation energy and bond length of oxygen and nitrogen.

Molecule	Bond order	Bond Dissociation energy	Bond length
Oxygen	2	495 kJ mol - 1	121 pm
Nitrogen	3	945 kJ mol - 1	110 pm

 

Magnetic Character

If all the electrons in the molecule of a substance are paired, the substance is diamagnetic. On the other hand, if there are unpaired electrons in the molecule, the substance is paramagnetic. Magnetic moment can be calculated as follows:

where n is the no. of unpaired electrons.