CHM015 · Bonding & Molecular Shape · Classes 9–12
Molecular Shapes (VSEPR)
Objective: To predict molecular shapes from electron domains (VSEPR theory).
🧰 Virtual materials and apparatus
- Virtual central atom
- Bonding pairs (sticks) and lone pairs (blue lobes)
📐 Formulas and SI units
shape from AXₘEₙ (m bonding, n lone)
angles: 180°, 120°, 109.5°, 90°
💡 Theory
Electron domains around a central atom (bonds and lone pairs) repel and spread as far apart as possible. The arrangement gives the shape: 2 domains linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral. Lone pairs take space but are not counted in the shape name, and they squeeze bond angles.
🪜 Guided virtual procedure
- Set 4 bonding domains and 0 lone pairs: tetrahedral (CH₄).
- Change to 3 bonding + 1 lone pair: trigonal pyramidal (NH₃).
- Change to 2 bonding + 2 lone pairs: bent (H₂O).
- Explore 5 and 6 domains.
- Write each shape with its ideal angle.
📋 Observations Simulated values
✅ Result and conclusion
Two bonding and two lone pairs give a bent molecule like water, with a bond angle of about 104.5°.
🔬 Real lab vs simulation
Real angles differ from ideal values because lone pairs repel more strongly; VSEPR does not work for every molecule (e.g. transition-metal complexes).
⚠️ Safety
A conceptual model only. Real laboratory chemicals are handled only by teachers in a school laboratory.
❓ Quick check
1. Shape of CH₄?
2. Why is H₂O bent?
3. Angle in a linear molecule?
🏋️ Practice
EasyShape of NH₃?
Answer: Trigonal pyramidal
MediumShape of BF₃?
Answer: Trigonal planar
ChallengeShape of SF₆?
Answer: Octahedral