Covalent Bonding
High-Yield Summary
- VSEPR theory predicts molecular geometry: electron pairs (bonding + lone) around a central atom arrange to minimize repulsion.
- Electronic geometry counts ALL electron pairs (bonding + lone); molecular geometry counts only bonding pairs (ignores lone pairs when naming shape).
- Lone pairs repel more than bonding pairs → each additional lone pair compresses bond angles below the ideal 109.5°.
- Overall molecular polarity = vector sum of bond dipoles — symmetric geometry can cancel polar bonds to zero net dipole (CCl₄ nonpolar despite polar C–Cl bonds).
- Valence bond theory: covalent bonds form from constructive interference (bonding orbital) of overlapping atomic orbitals; destructive interference gives an antibonding orbital.
CH₄ vs. NH₃ vs. H₂O — Same Electronic Geometry, Different Molecular Geometry
| Molecule | Bonding Pairs / Lone Pairs → Molecular Geometry (Bond Angle) |
|---|---|
| CH₄ (methane) | 4 bonding, 0 lone → Tetrahedral (~109.5°) |
| NH₃ (ammonia) | 3 bonding, 1 lone → Trigonal pyramidal (~107°) |
| H₂O (water) | 2 bonding, 2 lone → Bent (~104.5°) |
Key Terms
- Electronic geometry
- Spatial arrangement of ALL electron pairs (bonding + lone) around the central atom.
- Molecular geometry
- Spatial arrangement of only the bonding pairs; lone pairs are ignored when naming the shape.
- Coordination number
- Number of atoms directly bonded to the central atom; determines molecular geometry once lone pairs are factored in.
- Bonding orbital
- Formed by constructive interference (in-phase overlap) of atomic orbitals.
- Antibonding orbital
- Formed by destructive interference (out-of-phase overlap); has a node of zero electron density.
Sigma (σ) vs. Pi (π) Bonds
| Sigma (σ) | Pi (π) |
|---|---|
| Head-to-head orbital overlap, along the internuclear axis | Side-by-side overlap, above/below the axis |
| Allows free rotation | Restricts free rotation |
| Every single bond = 1σ | Double bond = 1σ + 1π; triple bond = 1σ + 2π |
Common MCAT Trap
- CH₄, NH₃, and H₂O all share the SAME (tetrahedral) electronic geometry despite having three different molecular geometries — don't conflate the two terms.
- CCl₄ has four individually polar C–Cl bonds but is NONPOLAR overall — geometry (symmetric cancellation), not just bond polarity, determines net molecular polarity.
- Free rotation is only about σ bonds — a π bond (present in every double/triple bond) blocks rotation, which is why cis/trans isomers exist around double bonds.
Quick Recall
Why do bond angles shrink from 109.5° (CH₄) to 107° (NH₃) to 104.5° (H₂O)?
Why is CCl₄ nonpolar despite having polar bonds, while H₂O is polar?
What determines whether orbital overlap forms a bonding or antibonding orbital?