Introduction to Chemical Bonding and Molecular Structure

Chemical bonding explains why atoms combine to form molecules and compounds. Understanding bonding and molecular structure is crucial in Chemistry as it affects properties like melting point, boiling point, solubility, and reactivity. For NEET aspirants, this chapter is high-yielding because questions often focus on bond types, molecular shapes, polarity, and hybridisation.

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Types of Chemical Bonds

Atoms form bonds to achieve stability, usually by attaining a full valence shell. Major types of chemical bonds include:

1. Ionic Bond (Electrovalent Bond):

• Formed between metals and non-metals

• Complete transfer of electrons

• Example: NaCl

• Properties: High melting & boiling points, conducts electricity in molten/aqueous state

2. Covalent Bond:

• Formed between non-metals

• Sharing of electron pairs

• Example: H₂O, CO₂

• Properties: Low melting & boiling points, poor conductors

3. Coordinate or Dative Covalent Bond:

• One atom contributes both electrons for bonding

• Example: NH₄⁺, H₃O⁺

4. Metallic Bond:

• Delocalised electrons between metal cations

• Explains conductivity and malleability of metals

Lewis Structures and Octet Rule

Lewis dot structures represent valence electrons in atoms and molecules.

Octet Rule:

• Atoms generally strive for 8 electrons in the valence shell

• Hydrogen follows the duet rule (2 electrons)

• Exceptions: BF₃ (incomplete octet), SF₆ (expanded octet)

Lewis structures help predict bond type, lone pairs, and molecule geometry.

Polarity of Molecules

Polarity arises from uneven distribution of electron density.

• Polar bond: Difference in electronegativity between atoms

• Non-polar bond: No significant difference in electronegativity

• Polar molecules: Result from polar bonds arranged asymmetrically

• Non-polar molecules: Polar bonds arranged symmetrically or all non-polar bonds

Example: H₂O (polar), CO₂ (non-polar)

Polarity affects solubility, boiling point, and chemical reactivity.

VSEPR Theory (Valence Shell Electron Pair Repulsion)

VSEPR theory predicts the shape of molecules based on repulsion between electron pairs.

Shapes of Molecules:

• Linear: AB₂ (CO₂)

• Trigonal Planar: AB₃ (BF₃)

• Tetrahedral: AB₄ (CH₄)

• Trigonal Bipyramidal: AB₅ (PCl₅)

• Octahedral: AB₆ (SF₆)

Effect of Lone Pairs:

• Lone pairs repel bonding pairs more strongly

• Causes deviations from ideal bond angles

• Example: NH₃ (trigonal pyramidal), H₂O (bent)

Hybridisation

Hybridisation is the mixing of atomic orbitals to form equivalent hybrid orbitals for bonding.

Common Types of Hybridisation:

• sp: Linear geometry, 180° (BeCl₂)

• sp²: Trigonal planar, 120° (BF₃)

• sp³: Tetrahedral, 109.5° (CH₄)

• sp³d: Trigonal bipyramidal, 90° & 120° (PCl₅)

• sp³d²: Octahedral, 90° (SF₆)

Hybridisation explains bond angles, shapes, and molecular orbitals.

Resonance

Resonance occurs when a molecule can be represented by two or more valid Lewis structures.

• Example: Benzene (C₆H₆), O₃

• Stabilizes molecules by delocalizing electrons

Questions on resonance are frequently asked in NEET.

Intermolecular Forces

Intermolecular forces determine physical properties of substances.

• Dipole-Dipole Interaction: Between polar molecules

• Hydrogen Bonding: H bonded to N, O, or F

• London Dispersion Forces: Between all molecules, stronger for larger molecules

Hydrogen bonding is especially important in biological molecules like DNA and proteins.

Molecular Orbital Theory

Molecular orbital theory describes electrons in molecules using molecular orbitals.

• Bonding molecular orbitals: lower energy

• Antibonding molecular orbitals: higher energy

• Bond order = (Nb – Na)/2

• Determines stability of molecule

Example: O₂ has bond order 2 and is paramagnetic.

Numerical and NEET Tips

• Draw Lewis structures before solving questions

• Memorize hybridisation of common molecules

• Practice VSEPR and polarity numericals

• Understand trends in bond length and bond energy

This approach ensures high accuracy in NEET.

Quick Summary Table

Conclusion

Chemical Bonding and Molecular Structure is a cornerstone of NEET Chemistry. Mastery of bonding types, hybridisation, VSEPR theory, molecular polarity, and resonance is essential to answer both conceptual and application-based NEET questions. Regular practice, along with understanding NCERT examples, ensures accuracy and scoring potential. StudentBro.in provides structured, exam-oriented notes to help students excel in this high-yield chapter.