JEE Chemistry: Chemical Bonding and Molecular Structure Complete Guide

Chemical bonding and molecular structure form a vital topic in JEE Chemistry, providing the foundation to understand how atoms combine to form molecules and compounds with varied properties. Mastery of this subject helps in explaining physical properties, reactivity, and geometry of molecules. This guide covers essential bonding theories, molecular geometry, hybridization, molecular orbital theory, and much more.

1. Introduction to Chemical Bonding

Atoms combine by forming chemical bonds to achieve stability, typically attaining noble gas electronic configuration. Bonds arise due to interactions involving valence electrons.

1.1 Types of Chemical Bonds

• Ionic Bond: Electrostatic attraction between oppositely charged ions formed by transfer of electrons.
• Covalent Bond: Sharing of electron pairs between atoms.
• Coordinate (Dative) Bond: Covalent bond where both electrons come from the same atom.
• Metallic Bond: Attraction between positive metal ions and delocalized electrons.

2. Ionic Bonding

Ionic bonding generally occurs between metals and non-metals.

2.1 Formation of Ionic Bonds

Metals lose electrons to form cations; non-metals gain electrons to form anions. Electrostatic forces bind them.

2.2 Properties of Ionic Compounds

• High melting and boiling points.
• Brittle and hard solids.
• Conduct electricity in molten or aqueous state.
• Generally soluble in polar solvents like water.

2.3 Factors Affecting Ionic Bond Strength

• Charge magnitude of ions.
• Size of ions (smaller ions → stronger bond).
• Lattice energy plays a crucial role.

3. Covalent Bonding

Covalent bonding involves sharing of electrons, usually between non-metals.

3.1 Lewis Dot Structures

Represent valence electrons as dots around element symbols to visualize bonding.

3.2 Octet Rule

Atoms tend to complete eight electrons in their valence shell. Exceptions include hydrogen (duet), boron (6 electrons), and expanded octets (P, S, etc.).

3.3 Bond Length and Bond Energy

• Bond Length: Distance between nuclei of two bonded atoms.
• Bond Energy: Energy required to break the bond.
• Multiple bonds (double, triple) have shorter lengths and higher bond energies.

3.4 Polarity of Covalent Bonds

Differences in electronegativity cause bond polarity, creating dipoles.

4. Coordinate (Dative) Bonding

Formed when one atom donates both electrons in a bond (e.g., NH₃ + BF₃ → NH₃BF₃).

5. Metallic Bonding

Metals form a lattice of positive ions surrounded by a sea of delocalized electrons.

5.1 Properties of Metallic Bonds

• Conduct electricity and heat.
• Malleability and ductility.
• Luster due to electron interaction with light.

6. Valence Shell Electron Pair Repulsion (VSEPR) Theory

VSEPR theory predicts molecular shape based on repulsions between electron pairs in the valence shell.

6.1 Steps to Predict Shape

1. Draw Lewis structure.
2. Count bonding and lone pairs on central atom.
3. Predict geometry minimizing repulsions.

6.2 Common Shapes

• Linear (180°): BeCl₂
• Trigonal Planar (120°): BF₃
• Tetrahedral (109.5°): CH₄
• Trigonal Bipyramidal (90°, 120°): PCl₅
• Octahedral (90°): SF₆
• Bent, Trigonal Pyramidal - due to lone pairs.

7. Hybridization

Hybridization explains molecular shapes by mixing atomic orbitals to form new hybrid orbitals.

7.1 Types of Hybridization

• sp: Linear, 2 electron groups.
• sp²: Trigonal planar, 3 electron groups.
• sp³: Tetrahedral, 4 electron groups.
• sp³d: Trigonal bipyramidal, 5 electron groups.
• sp³d²: Octahedral, 6 electron groups.

7.2 Examples

• BeCl₂ - sp
• BF₃ - sp²
• CH₄ - sp³
• PCl₅ - sp³d
• SF₆ - sp³d²

8. Molecular Orbital Theory (MOT)

MOT treats electrons as wavefunctions spread over the entire molecule, forming bonding and antibonding molecular orbitals.

8.1 Bonding and Antibonding Orbitals

• Bonding orbitals: Lower energy, electron density between nuclei.
• Antibonding orbitals: Higher energy, electron density outside nuclei region.

8.2 Bond Order

Bond order = \( \frac{(\text{number of bonding electrons} - \text{number of antibonding electrons})}{2} \)

A bond order > 0 indicates a stable molecule.

8.3 Examples

• \(\mathrm{H_2}\): Bond order = 1 (stable)
• \(\mathrm{He_2}\): Bond order = 0 (unstable)

9. Resonance

Some molecules cannot be represented by a single Lewis structure. Resonance structures are multiple valid structures representing the molecule.

Actual structure is a hybrid of all resonance forms, stabilizing the molecule.

9.1 Examples

• Benzene (\(\mathrm{C_6H_6}\))
• Nitrate ion (\(\mathrm{NO_3^-}\))

10. Polarity of Molecules

Molecular polarity depends on bond polarities and molecular geometry.

• Nonpolar molecules: Symmetrical geometry cancels dipoles (e.g., \(\mathrm{CO_2}\)).
• Polar molecules: Asymmetrical geometry leads to net dipole moment (e.g., \(\mathrm{H_2O}\)).

11. Hydrogen Bonding

Special intermolecular attraction between hydrogen bonded to highly electronegative atoms (N, O, F) and lone pairs on another molecule.

Responsible for anomalous properties of water and other compounds.

12. Important Concepts & Summary

• Chemical bonds form due to energy minimization.
• Ionic bonds form between metals and non-metals by electron transfer.
• Covalent bonds form by sharing electrons; polarity depends on electronegativity difference.
• VSEPR theory predicts molecular shape based on electron pair repulsions.
• Hybridization explains bonding orbitals and molecular geometries.
• Molecular Orbital Theory describes bond formation in terms of orbitals combining over the entire molecule.
• Resonance structures represent delocalized bonding.
• Polarity influences physical properties and reactivity.
• Hydrogen bonding is a strong dipole-dipole interaction critical to biological systems.

13. Tips for JEE Preparation

• Practice drawing Lewis structures and predicting shapes using VSEPR.
• Understand hybridization through orbital diagrams.
• Memorize common molecular shapes and their bond angles.
• Solve previous years’ JEE problems on molecular orbital theory and resonance.
• Focus on conceptual clarity for polarity and intermolecular forces.

Mastery of chemical bonding and molecular structure will not only help you in JEE but build a strong foundation for organic and inorganic chemistry concepts.