Give the electron-domain and molecular geometries for the following molecules and ions: (a) HCN, (b) SO3 2 – , (c) SF4, (d) PF6 -, (e) NH3Cl+, (f) N3 -.
Answer
Electron-Domain and Molecular Geometries of Selected Molecules and Ions
Introduction to Molecular Geometry:
The electron-domain geometry considers all regions of electron density (bonds and lone pairs), while the molecular geometry describes the shape formed by the atoms only. Both are essential for understanding molecular structure, reactivity, and polarity.
(a) HCN (Hydrogen Cyanide)
- Electron-Domain Geometry: Linear
- Molecular Geometry: Linear
- Explanation: Carbon forms a triple bond with nitrogen and a single bond with hydrogen, with no lone pairs on the central atom (C). Hence, two electron domains form a straight line.
(b) SO32− (Sulfite Ion)
- Electron-Domain Geometry: Tetrahedral
- Molecular Geometry: Trigonal Pyramidal
- Explanation: Sulfur has 3 bonding domains and 1 lone pair. The lone pair repels the bonds, resulting in a trigonal pyramidal shape.
(c) SF4 (Sulfur Tetrafluoride)
- Electron-Domain Geometry: Trigonal Bipyramidal
- Molecular Geometry: See-Saw
- Explanation: Sulfur has 4 bonded fluorines and 1 lone pair. The lone pair occupies an equatorial position, causing a see-saw shape.
(d) PF6− (Hexafluorophosphate Ion)
- Electron-Domain Geometry: Octahedral
- Molecular Geometry: Octahedral
- Explanation: Phosphorus forms six equivalent bonds to fluorine atoms with no lone pairs, resulting in a symmetric octahedral shape.
(e) NH3Cl+ (Protonated Ammonium Chloride)
- Electron-Domain Geometry: Tetrahedral
- Molecular Geometry: Trigonal Pyramidal
- Explanation: Nitrogen is bonded to 3 hydrogens and 1 chlorine atom with no lone pairs after protonation, maintaining tetrahedral electron geometry but pyramidal shape.
(f) N3− (Azide Ion)
- Electron-Domain Geometry: Linear
- Molecular Geometry: Linear
- Explanation: The ion has resonance structures with delocalized bonds, but the atoms lie on a straight line, giving a linear geometry.
Summary Table
| Molecule/Ion | Electron-Domain Geometry | Molecular Geometry |
|---|---|---|
| HCN | Linear | Linear |
| SO₃²⁻ | Tetrahedral | Trigonal Pyramidal |
| SF₄ | Trigonal Bipyramidal | See-Saw |
| PF₆⁻ | Octahedral | Octahedral |
| NH₃Cl⁺ | Tetrahedral | Trigonal Pyramidal |
| N₃⁻ | Linear | Linear |
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