Chemistry Class 11 NCERT Solutions: Chapter 4 Chemical Bonding and Molecular Structure Part 8

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Q: 34. Write the important conditions required for the linear combination of atomic orbitals to form molecular orbitals.


The given conditions should be satisfied by atomic orbitals to form molecular orbitals:

(A) The combining atomic orbitals must have the same or nearly the same energy. This means that in a homonuclear molecule, the 1s-atomic orbital of an atom can combine with the 1s-atomic orbital of another atom, and not with the 2s-orbital.

(B) The combining atomic orbitals must have proper orientations to ensure that the overlap is maximum.

(C) The extent of overlapping should be large.

Q: 35. Use molecular orbital theory to explain why the molecule does not exist.

Answer: The electronic configuration of Beryllium is

The molecular orbital electronic configuration for molecule can be written as:

Hence, the bond order for is


Number of electrons in anti-bonding orbitals

A negative or zero bond order means that the molecule is unstable. Hence, molecule does not exist.

Q: 36. Compare the relative stability of the following species and indicate their magnetic properties;

Answer: There are 16 electrons in a molecule of Dioxygen, 8 from each oxygen atom. The electronic configuration of oxygen molecule can be written as:

Since the 1s orbital of each oxygen atom is not involved in boding, the number of bonding electrons and the number of anti-bonding orbitals

Bond order

Similarly, the electronic configuration of can be written as:

Bond order of


Electronic configuration of

Electronic configuration of

Bond order of


Bond dissociation energy is directly proportional to bond order. Thus, the higher the bond order, the greater will be the stability. On this basis, the order of stability is

Q: 37. write the significance of a plus and a minus sign shown in representing the orbitals.

Answer: Molecular orbitals are represented by wave functions. A plus sign in an orbital indicates a positive wave function while a minus sign in an orbital represents a negative wave Function.

Q: 38. Describe the hybridisation in case of Why are the axial bonds longer as compared to equatorial bonds?

Answer: The ground state and excited state outer electronic configurations of phosphorus are:

Ground state:

Excited state:

Phosphorus atom is hybridized in the excited state. These orbitals are filled by the electron pairs donated by five Cl atoms as:

The five d hybrid orbitals are directed towards the five corners of the trigonal bipyramidals. Hence, the geometry of PCl_5 can be represented as:


There are five sigma bonds in . Three bonds lie in one plane and make an angle of with each other. These bonds are called equatorial bonds.

The remaining two bonds lie above and below the equatorial plane and make an angle of with the plane. These bonds are called axial bonds.

As the axial bond pairs suffer more repulsion from the equatorial bond pairs, axial bonds are slightly than equatorial bonds.