Questions
5 MCQs per paper
Difficulty
Medium-Hard
Importance
Core foundation for all chemistry
Overview
Chemical Bonding and Molecular Structure explores the fundamental forces that hold atoms together to form stable molecules. Mastery of VSEPR, Hybridisation, and MOT is essential as these concepts predict molecular geometry and magnetic behavior, which are frequent targets in competitive and board exams. Understanding electron distribution is the key to unlocking reaction mechanisms and properties.
VSEPR Theory
The Valence Shell Electron Pair Repulsion (VSEPR) theory determines the geometry of a molecule based on the repulsion between electron pairs in the valence shell of the central atom. It predicts the shape by minimizing repulsion between bonding and non-bonding electron pairs.
- Lone pair-lone pair repulsion > Lone pair-bond pair > Bond pair-bond pair
- Predicts bond angles based on steric number
- Does not account for orbital overlap
- Ideal geometry vs. distorted geometry due to lone pairs
Hybridisation
Hybridisation is the concept of mixing atomic orbitals of similar energies to form a new set of equivalent hybrid orbitals. It explains the equivalent bond lengths and bond angles observed in molecules like methane and benzene.
- sp: Linear, 180 degrees
- sp2: Trigonal planar, 120 degrees
- sp3: Tetrahedral, 109.5 degrees
- sp3d: Trigonal bipyramidal
- sp3d2: Octahedral
- Steric number = sigma bonds + lone pairs
Molecular Orbital Theory (MOT)
MOT treats electrons in a molecule as occupying molecular orbitals that belong to the whole molecule rather than individual atoms. It is the most robust theory for explaining the paramagnetism of O2 and calculating bond order.
- Bond Order = 1/2(Nb - Na)
- Bond strength is directly proportional to bond order
- Paramagnetic: contains unpaired electrons
- Diamagnetic: all electrons paired
- Energy ordering: sigma 1s < sigma* 1s < sigma 2s < sigma* 2s
Formula Sheet
Steric Number = (V + M - C + A) / 2
Bond Order = (Bonding Electrons - Antibonding Electrons) / 2
Exam Tip
Always draw the Lewis structure and calculate the steric number first to identify hybridization before attempting to predict the geometry or bond angle.
Common Mistakes
- Confusing bond order calculations by ignoring the difference in orbital energy order for O2 vs B2/C2/N2
- Failing to account for lone pairs when determining the final geometry (shape vs. electron geometry)
- Assuming all hybridized orbitals must involve d-orbitals for coordination numbers above four
More Revision Notes
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