Chapter 4 - Chemical Bonding and Molecular Structure

Chemical Bonding and Molecular Structure explains why atoms combine, how bonds form and why molecules have particular shapes and properties. It is one of the most important chapters in Class 11 because its ideas are used throughout inorganic and organic chemistry.

Why do atoms form bonds?

Bond formation generally lowers the energy of a system. Atoms may reach more stable configurations by losing, gaining or sharing electrons.

Lewis approach and the octet rule

Lewis symbols represent valence electrons as dots. Students use them to draw structures containing single, double, triple and coordinate bonds.

The octet rule is useful but has exceptions:

• Incomplete-octet compounds

• Odd-electron molecules

• Expanded-octet compounds

Formal charge and resonance help select and interpret possible Lewis structures.

Ionic bonding

An ionic bond is the electrostatic attraction between oppositely charged ions. Formation and stability depend on ionisation enthalpy, electron-gain enthalpy, ionic size, charge and lattice enthalpy.

Bond parameters and polarity

Students learn:

• Bond length

• Bond angle

• Bond enthalpy

• Bond order

• Dipole moment

• Bond polarity

A molecule containing polar bonds may be non-polar if the bond dipoles cancel due to symmetry.

VSEPR theory

Electron pairs around a central atom arrange themselves to minimise repulsion. Lone pairs repel more strongly than bond pairs and therefore change bond angles and molecular shapes.

Students predict structures such as BeCl₂, BF₃, CH₄, NH₃, H₂O, PCl₅, SF₆ and XeF₄.

Valence bond theory and hybridisation

Valence bond theory describes covalent bonds through orbital overlap. Students compare sigma and pi bonds and learn hybridisation types including sp, sp², sp³, sp³d and sp³d².

Hybridisation, electron-pair geometry and molecular geometry must be distinguished carefully.

Molecular orbital theory

Atomic orbitals combine to form bonding and antibonding molecular orbitals. Students write molecular orbital configurations, calculate bond order and predict stability and magnetic behaviour.

Bond order is related to bond strength and bond length. Unpaired electrons make a species paramagnetic.

Hydrogen bonding

Hydrogen bonding may be intermolecular or intramolecular. It affects boiling point, solubility, viscosity and structure.

What can students study on Teachoo?

• Lewis structures and formal charge

• Resonance structures

• VSEPR shapes and bond angles

• Hybridisation questions

• Sigma and pi bond counting

• Dipole-moment comparisons

• Molecular orbital configurations

• Bond-order and magnetic-behaviour questions

• NCERT solutions, MCQs and case-based questions

Common mistakes

• Predicting shape without counting lone pairs

• Confusing electron geometry with molecular geometry

• Assigning hybridisation from shape alone without drawing the structure

• Counting a double bond as two electron domains in VSEPR

• Filling molecular orbitals in the wrong order

• Calling a molecule polar merely because it contains polar bonds

Best way to study this chapter

For every molecule: count valence electrons, draw the Lewis structure, identify electron domains, apply VSEPR, determine hybridisation and then assess polarity. Use molecular orbital theory separately where required.

Frequently asked questions

What is a chemical bond?

A chemical bond is the attractive force that holds atoms or ions together in a stable species.

What is the difference between sigma and pi bonds?

A sigma bond forms by head-on overlap along the internuclear axis. A pi bond forms through sideways overlap above and below that axis.

How is molecular shape predicted?

Draw the Lewis structure, count electron domains around the central atom and apply VSEPR theory while considering lone pairs.

What is hybridisation?

Hybridisation is the mixing of atomic orbitals of similar energy on the same atom to form equivalent, directed hybrid orbitals.

How is bond order calculated in molecular orbital theory?

Bond order is half the difference between the number of electrons in bonding and antibonding molecular orbitals.

Why is oxygen paramagnetic?

An oxygen molecule has two unpaired electrons in antibonding molecular orbitals.

Why is water bent but carbon dioxide linear?

Oxygen in water has two lone pairs that affect molecular shape. Carbon in carbon dioxide has two electron domains and no lone pairs.

Learn Chemical Bonding and Molecular Structure with Teachoo for molecular shapes, hybridisation, bond order, diagrams and complete NCERT solutions.