Chemistry Class 11 NCERT Solutions: Chapter 4 Chemical Bonding and Molecular Structure Part 7

Download PDF of This Page (Size: 173K)

Q: 30. which hybrid orbitals are used by carbon atoms in the following molecules?















Q: 31. What do you understand by bond pairs and lone pairs of electrons? Illustrate by giving one example of each type.


When two atoms combine by sharing their one or more valence electrons, a covalent bond is formed between them.

The shared pairs of electrons present between the bonded atoms are called bond pairs. All valence electrons may not participate in bonding. The electron pairs that do not participate in bonding are called lone pairs of electrons.

For example, in (ethane), there are seven bond pairs but no lone pair present.

In there are two bond pairs and two lone pairs on the central atom (oxygen).

Q: 32. Distinguish between a sigma and a pi bond.

Answer: The following are the differences between sigma and pi-bonds:

Sigma and Pi Bonds

Sigma ( ) bond

Pi ( ) Bond

(A) It is formed by the end to end overlap of orbitals.

Ti is formed by the lateral overlap of orbitals.

(B) The orbitals involved in the overlapping are

These bonds are formed by the overlap of orbitals only.

(C) It is a strong bond.

It is weak bond.

(D) The electron cloud is symmetrical about the line joining two nuclei.

The electron cloud is not symmetrical.

(E) It consists of one electron cloud, which is symmetrical about the internuclear axis.

There are two electron clouds lying above and below the plane of the atomic nuclei.

(F) Free rotation about bonds is possible.

Rotation is restricted in case of bonds.

Q: 33. Explain the formation of molecule on the basis of valence bond theory.


Let us assume that two hydrogen atom (A and B) with nuclei ( ) and electrons are taken to undergo a reaction to from a hydrogen molecule.

When A and B are at a large distance, there is no interaction between them. As they begin to approach each other, the attractive and repulsive forces start operating.

Attractive force arises between:

(A) Nucleus of one atom and its own electron i.e.,

(B) Nuclei of two atoms i.e.,

The force of attraction brings the two atoms together, whereas the force of repulsion tends to push them apart.

Image: 1

Image: 2

The magnitude of the attractive forces is more than that of the repulsive forces. Hence, the two atoms approach each other. As a result, the potential energy decreases. Finally, a state is reached when the attractive forces balance the repulsive forces and the system acquires minimum energy. This leads to the formation of a dihydrogen molecule.