Use the VSEPR theory to predict the shape of each of the following molecules:
a. methane (CH4)
b. hydrogen selenide (H2S)
c. ammonia (NH3)
d. silicon tetrachloride (SiCl4)
e. water (H2O)
f. formaldehyde (H2CO)
The Correct Answer and Explanation is:
Let’s apply Valence Shell Electron Pair Repulsion (VSEPR) Theory to determine the shapes of the given molecules. VSEPR theory predicts molecular geometry based on electron pairs repelling each other to be as far apart as possible around a central atom.
a. Methane (CH₄)
Shape: Tetrahedral
Bond Angle: 109.5°
- Central atom: Carbon (C)
- Carbon has 4 valence electrons and forms 4 single bonds with hydrogen atoms.
- No lone pairs on the central atom.
- Four bonding pairs arrange themselves tetrahedrally.
b. Hydrogen Selenide (H₂Se)
Shape: Bent (or Angular)
Bond Angle: ~104.5°
- Central atom: Selenium (Se)
- Se has 6 valence electrons; 2 form bonds with H, and 4 are in two lone pairs.
- AX2E2 molecular geometry.
- Like water, the repulsion from lone pairs bends the shape.
c. Ammonia (NH₃)
Shape: Trigonal Pyramidal
Bond Angle: ~107°
- Central atom: Nitrogen (N)
- N has 5 valence electrons; 3 bond with hydrogen, and 2 are a lone pair.
- AX3E molecular geometry.
- Lone pair repels more strongly, compressing the bond angle below 109.5°.
d. Silicon Tetrachloride (SiCl₄)
Shape: Tetrahedral
Bond Angle: 109.5°
- Central atom: Silicon (Si)
- Si forms 4 single bonds with Cl atoms.
- No lone pairs on Si.
- Four bonding pairs arrange tetrahedrally.
e. Water (H₂O)
Shape: Bent (or V-shaped)
Bond Angle: ~104.5°
- Central atom: Oxygen (O)
- O has 6 valence electrons; 2 form O–H bonds, and 4 are in two lone pairs.
- AX2E2 geometry.
- Lone pairs repel more strongly than bonding pairs, forcing the H–O–H angle to contract from the ideal 109.5° to about 104.5°.
- This bent geometry gives water its polar nature and many of its unique properties.
f. Formaldehyde (H₂CO)
Shape: Trigonal Planar
Bond Angle: 120°
- Central atom: Carbon (C)
- Carbon is double bonded to oxygen and single bonded to two hydrogens.
- No lone pairs on carbon.
- AX3 geometry (3 regions of electron density around carbon).
- All regions spread out in a plane to minimize repulsion.
Summary Table:
| Molecule | Shape | Bond Angle |
|---|---|---|
| CH₄ | Tetrahedral | 109.5° |
| H₂Se | Bent | ~104.5° |
| NH₃ | Trigonal Pyramidal | ~107° |
| SiCl₄ | Tetrahedral | 109.5° |
| H₂O | Bent | ~104.5° |
| H₂CO | Trigonal Planar | 120° |
VSEPR theory helps predict the 3D geometry of molecules by focusing on electron domain repulsion, including bonding and non-bonding pairs.