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 -.
The Correct Answer and Explanation is:
To determine the electron-domain and molecular geometries for each of the given molecules and ions, we need to consider the number of bonding pairs and lone pairs of electrons around the central atom. The electron-domain geometry accounts for both bonding and lone pairs, while the molecular geometry considers only the bonding pairs.
(a) HCN (Hydrogen Cyanide)
- Electron-domain geometry: Linear
- Molecular geometry: Linear
- Explanation: HCN consists of a central carbon atom bonded to a nitrogen atom and a hydrogen atom. The carbon has two bonds (one to nitrogen and one to hydrogen), and there are no lone pairs on the carbon, resulting in a linear electron-domain geometry. Since there are no lone pairs on the carbon, the molecular geometry is also linear.
(b) SO3 2- (Sulfite Ion)
- Electron-domain geometry: Trigonal planar
- Molecular geometry: Bent
- Explanation: The sulfur atom in SO3 2- is surrounded by three oxygen atoms and has one lone pair. The electron-domain geometry is trigonal planar due to the three regions of electron density. However, since one of these is a lone pair, the molecular geometry is bent, similar to the shape of a water molecule.
(c) SF4 (Sulfur Tetrafluoride)
- Electron-domain geometry: Trigonal bipyramidal
- Molecular geometry: See-saw
- Explanation: The sulfur atom in SF4 is surrounded by four fluorine atoms and one lone pair of electrons. This gives five regions of electron density around sulfur, leading to a trigonal bipyramidal electron-domain geometry. With one lone pair, the molecular geometry is see-saw, where the lone pair occupies one of the equatorial positions.
(d) PF6- (Hexafluorophosphate Ion)
- Electron-domain geometry: Octahedral
- Molecular geometry: Octahedral
- Explanation: PF6- has six fluorine atoms bonded to the phosphorus atom, with no lone pairs on phosphorus. The electron-domain geometry is octahedral, and since all positions are occupied by bonding pairs, the molecular geometry is also octahedral.
(e) NH3Cl+ (Ammonium Chloride Ion)
- Electron-domain geometry: Tetrahedral
- Molecular geometry: Tetrahedral
- Explanation: NH3Cl+ consists of a central nitrogen atom bonded to three hydrogen atoms and one chloride ion. There are no lone pairs on the nitrogen, so the electron-domain geometry is tetrahedral. Since all positions are occupied by bonding pairs, the molecular geometry is also tetrahedral.
(f) N3- (Azide Ion)
- Electron-domain geometry: Linear
- Molecular geometry: Linear
- Explanation: In N3-, the central nitrogen is bonded to two other nitrogen atoms. There are no lone pairs on the central nitrogen, and the molecule has two bonds, resulting in a linear electron-domain geometry. The molecular geometry is also linear, as all atoms are aligned along a straight line.
These molecular shapes are determined using VSEPR theory, which assumes that electron pairs (bonding or lone) will arrange themselves to minimize repulsion.
