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Chapter: 11th 12th std standard Class Organic Inorganic Physical Chemistry Higher secondary school College Notes

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Electronic configuration of a molecule and its correlation with molecular behaviour

The distribution of electrons among various molecular orbitals is called electronic configuration of a molecule. It can give us very important information about the molecules as explained below.

 

Electronic configuration of a molecule and its correlation with molecular behaviour

The distribution of electrons among various molecular orbitals is called  electronic configuration of a molecule. It can give us very important information about the molecules as explained below.

1. Stability of a molecule in terms of a number of electrons in bonding and antibonding molecular orbitals. From the electronic configuration it is possible to find out the number of electrons in bonding molecular orbitals(Nband number of electrons in antibonding molecular orbitals (Na).

(a) If Nb>Na, the molecule is stable : This is evident because in this case the influence of bonding electrons will be more than the influence of antibonding electrons, resulting in a net force of attractionb).

(b)  If Nb< Na, the molecule is unstable : This is again obvious because in this case he influence of antibonding electrons will be more than the influence of bonding lectrons, resulting in a net force of repulsion.

(c) If Nb = Na, the molecule is unstable : This is because in this case the influence of bonding electrons will be equal to the influence of antibonding electrons resulting in no net force of attraction.

2. Bond order and stability of a molecule or an ion. The stability of a molecule or an ion can also be determined from another parameter called bond order. Bond order may be defined as half the difference between the number of  electrons in bonding molecular orbitals (Nb) and the number of electrons in antibonding molecular orbitals (Na) i.e,

 

Bond Order = (Nb - Na )

The resulting molecule or ion will be stable if Nb> Na i.e., if bond order is positive. The resulting molecule or ion will be unstable if Nb  Na i.e, if bond order is negative or zero.

3. Relative stability of molecules or ions in terms of bond order : The stability of a molecule or an ion is directly proportional to bond order. Thus, a  molecule with bond order 3 (e.g., N2) is more stable (i.e., has a higher bond dissociation energy) than a molecule with bond order 2 (e.g., O2) or 1 (e.g., Li2).

4. Nature of bond in terms of bond order : A chemical bond can be single, double or triple but cannot be a fraction, on the otherhand bond order can be a fraction.

5. Bond length in terms of bond order : Bond length is found to be inversely proportional to bond order. Greater the bond order, shorter the bond length and vice versa.

For example, the bond length in nitrogen molecule (bond order = 3) is shorter than in oxygen molecule (bond order = 2), which in turn is shorter than in hydrogen molecule (bond order = 1).

6. Diamagnetic and paramagnetic nature of the molecule : If all the electrons in the molecule are paired then the substance is diamagnetic in nature.

On the other hand, if the molecule has unpaired electron(s) it is paramagnetic in nature.

 

Molecule      Bond order Bond dissociation energy       Bond length

Nitrogen            3                   945 kJ mol-1          110 pm

 

Oxygen            2                   495 kJ mol-1           121 pm

 

Lithium            1                 110 kJ mol-1         267 pm

 

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