Chemistry Class 11 NCERT Solutions: Chapter 4 Chemical Bonding and Molecular Structure Part 9

Q: 39. Define hydrogen bond. Is it weaker or stronger than the van der Waals forces?

Answer:

A hydrogen bond is defined as an attractive force acting between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule (may be of the same kind).

Due to a difference between electronegativities, the bond pair between hydrogen and the electronegative atom gets drifted far away from the hydrogen atom. As a result, a hydrogen atom becomes electropositive with respect with respect to the other atom and acquires a positive charge .

Equation

The magnitude of H-Bonding is maximum in the solid state and minimum in the gaseous state.

There are two types of H-bonds:

(i) Intermolecular H-bond e.g., HF, Equation etc

(ii) Intramolecular H-bond e.g., o-nitrophenol

Hydrogen bond are stronger than Van der Walls forces since hydrogen bond are regarded as an extreme form of dipole-dipole interaction.

Q: 40. What is meant by the bond order? Calculate the bond order of: Equation

Answer: Bond order is defined as one-half of the difference between the number of electrons present in the bonding and anti-bonding orbitals of a molecule.

If Equation is equal to the number of electrons in an anti-bonding orbital, then Equation is equal to the number of electrons in a bonding orbital.

Bond order Equation

If Equation then the molecule is said be stable. Equation However, if Equation than the molecule is considered to be unstable.

Bond order of Equation can calculated from its electronic configuration as: Equation

Number of bonding electron, Equation

Number of anti-bonding electrons, Equation

Bond order of nitrogen molecule Equation

Equation

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

Equation

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

Bond order Equation

Equation

Equation

Hence, the bond order of oxygen molecule is 2.

Similarly, the electronic configuration of Equation can be written as:

Equation

Equation

Equation

Bond order of Equation

= 2.5

Thus, the bond order of Equation is Equation .

The electronic configuration of Equation ion will be:

Equation

Equation

Equation

Bond order of Equation

Equation

Thus, the bond order of Equation

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