Use the VSEPR model to predict the geometry and approximate bond angle of the following molecules and ions: a) AsH3, b) OF2, c) AlCl4-, d) I3-, e) C2H4 Explain each prediction.
The Correct Answer and Explanation is:
To predict the geometry and approximate bond angles of molecules and ions using the Valence Shell Electron Pair Repulsion (VSEPR) model, we count electron domains around the central atom and consider lone pairs versus bonding pairs. Here are predictions for each:
a) AsH₃ (Arsine)
- Electron Domains: 4 (3 bonding pairs with H, 1 lone pair)
- Electron Domain Geometry: Tetrahedral
- Molecular Geometry: Trigonal pyramidal
- Bond Angle: Slightly less than 109.5°, typically around 92°
Explanation: Arsenic has five valence electrons. Three form bonds with hydrogen, and one pair remains lone. The lone pair repels more strongly than bonding pairs, pushing the H-As-H bond angles slightly below the ideal tetrahedral angle.
b) OF₂ (Oxygen difluoride)
- Electron Domains: 4 (2 bonding pairs with F, 2 lone pairs)
- Electron Domain Geometry: Tetrahedral
- Molecular Geometry: Bent
- Bond Angle: Approximately 103°
Explanation: Oxygen has six valence electrons. Two are used to bond with fluorine atoms, and two pairs are lone. The two lone pairs create greater repulsion, resulting in a bent shape with a smaller bond angle than 109.5°.
c) AlCl₄⁻ (Tetrachloroaluminate ion)
- Electron Domains: 4 (4 bonding pairs, no lone pairs)
- Electron Domain Geometry: Tetrahedral
- Molecular Geometry: Tetrahedral
- Bond Angle: 109.5°
Explanation: Aluminum forms four bonds with chlorine atoms. The added electron from the negative charge completes the octet. With no lone pairs, the geometry remains ideal tetrahedral.
d) I₃⁻ (Triiodide ion)
- Electron Domains: 5 (2 bonding pairs, 3 lone pairs)
- Electron Domain Geometry: Trigonal bipyramidal
- Molecular Geometry: Linear
- Bond Angle: 180°
Explanation: The central iodine atom has five electron regions (including three lone pairs). Lone pairs occupy equatorial positions to minimize repulsion, leaving the bonded atoms in a linear arrangement.
e) C₂H₄ (Ethene)
- Electron Domains: 3 around each carbon (2 single bonds to H, 1 double bond between C)
- Electron Domain Geometry: Trigonal planar
- Molecular Geometry: Trigonal planar at each carbon
- Bond Angle: 120°
Explanation: Each carbon forms three sigma bonds and one pi bond, resulting in a trigonal planar arrangement. All atoms lie in the same plane, and bond angles are about 120°.
In each case, the geometry results from minimizing electron pair repulsion around the central atom.
