Molecular Shapes & Bonding
🔍 Why NH3 has stronger polarity than NF3:
NH3 dipole moment = \(4.90 \times 10^{-30}\) C m
NF3 dipole moment = \(0.80 \times 10^{-30}\) C m
Reason: In NH3, the lone pair’s pull adds to the N-H bonds’ polarity. In NF3, the lone pair pulls against the N-F bonds, reducing overall polarity! ⚡
⚖️ Fajans’ Rules (Covalent Character in Ionic Bonds):
- ↑ Covalent character if: Small cation + Large anion
- ↑ Covalent character if: Higher charge on cation (e.g., Al3+ > Na+)
- Transition metal cations (\((n-1)d^nns^0\)) polarize anions more than noble-gas-config cations (\(ns^2np^6\))
Why? Strong cations pull anion’s electrons → shared electron density (like covalent bonds)! 🔗
🌟 VSEPR Theory (Predicting Molecular Shapes)
Golden Rule: Electron pairs repel each other and spread out as far as possible! 💥
Key Postulates:
- Shape depends on valence electron pairs (bonding or lone pairs) around the central atom.
- Repulsion strength: Lone Pair–Lone Pair (lp–lp) > Lone Pair–Bond Pair (lp–bp) > Bond Pair–Bond Pair (bp–bp)
- Lone pairs occupy more space → stronger repulsion → squishes bond angles! 📏
- Treat double/triple bonds as one “super pair”.
🚀 Shapes Without Lone Pairs (Central Atom)
| Electron Pairs | Shape | Angle | Examples |
|---|---|---|---|
| 2 | Linear | 180° | BeCl2, HgCl2 |
| 3 | Trigonal Planar | 120° | BF3 |
| 4 | Tetrahedral | 109.5° | CH4, NH4+ |
| 5 | Trigonal Bipyramidal | 90°, 120° | PCl5 |
| 6 | Octahedral | 90° | SF6 |
🎯 Shapes With Lone Pairs (Central Atom)
| Molecule Type | Bond Pairs | Lone Pairs | Shape | Examples | Angle Change |
|---|---|---|---|---|---|
| AB2E | 2 | 1 | Bent | SO2 | 120° → 119.5° |
| AB3E | 3 | 1 | Trigonal Pyramidal | NH3 | 109.5° → 107° |
| AB2E2 | 2 | 2 | Bent | H2O | 109.5° → 104.5° |
| AB4E | 4 | 1 | See-Saw | SF4 | – |
| AB3E2 | 3 | 2 | T-Shaped | ClF3 | – |
| AB5E | 5 | 1 | Square Pyramidal | BrF5 | – |
| AB4E2 | 4 | 2 | Square Planar | XeF4 | – |
Remember: Lone pairs squish bond angles! More lone pairs = smaller angles. 😊
💎 NEET Spotlight: Must-Know Concepts
- Dipole Moments: Compare NH3 vs. NF3 (lone pair alignment matters!).
- Fajans’ Rules: Predict covalent character in ionic compounds (cation size/charge is key!).
- VSEPR Shapes:
- No lone pairs: Linear → Octahedral
- With lone pairs: Bent (H2O), Pyramidal (NH3), T-shaped (ClF3)
- Lone Pair Repulsion: lp–lp > lp–bp > bp–bp → Bond angle reduction!
- Valence Bond Theory: Explains bond formation via atomic orbital overlap (beyond Lewis/VSEPR).
Keep practicing shapes – you’ve got this! 💪
