Determine the electron geometry, molecular geometry, and idealized bond angles for each molecule. In which cases do you expect deviations from the idealized bond angle?
a. PF3
b. SBr2
c. CHCl3
d. CS2
The correct answer and explanation is:
Let’s go through each molecule and determine their electron geometry, molecular geometry, and idealized bond angles.
a. PF3 (Phosphorus trifluoride)
- Electron Geometry: Trigonal pyramidal
- Molecular Geometry: Trigonal pyramidal
- Idealized Bond Angles: 107°
- Explanation: In PF3, phosphorus (P) is the central atom with three bonded fluorine (F) atoms and one lone pair of electrons. The electron geometry is trigonal pyramidal due to the presence of the lone pair. The ideal bond angles in this geometry are approximately 107°, but they are slightly smaller than the idealized 109.5° angle seen in a tetrahedral arrangement, due to the repulsion caused by the lone pair. There are deviations because the lone pair exerts more repulsion compared to bonding pairs.
b. SBr2 (Sulfur dibromide)
- Electron Geometry: Bent (V-shaped)
- Molecular Geometry: Bent (V-shaped)
- Idealized Bond Angles: 104.5°
- Explanation: In SBr2, sulfur (S) is the central atom with two bromine (Br) atoms and two lone pairs. The electron geometry is tetrahedral, but because of the lone pairs, the molecular geometry is bent or V-shaped. The bond angle is approximately 104.5°, which is smaller than the ideal tetrahedral angle (109.5°) due to lone pair-bonding pair repulsion. The lone pairs push the bonded atoms closer together.
c. CHCl3 (Chloroform)
- Electron Geometry: Tetrahedral
- Molecular Geometry: Tetrahedral
- Idealized Bond Angles: 109.5°
- Explanation: In CHCl3, carbon (C) is the central atom with one hydrogen (H) and three chlorine (Cl) atoms. The electron geometry and molecular geometry are both tetrahedral, with ideal bond angles of 109.5°. This is typical for a molecule with four bonding pairs of electrons and no lone pairs on the central atom. There are no significant deviations from the ideal bond angles in this case.
d. CS2 (Carbon disulfide)
- Electron Geometry: Linear
- Molecular Geometry: Linear
- Idealized Bond Angles: 180°
- Explanation: In CS2, carbon (C) is the central atom with two sulfur (S) atoms bonded to it. The molecule adopts a linear geometry, as there are two bonding pairs of electrons and no lone pairs on the central carbon. The ideal bond angle is 180°, which is characteristic of a linear molecule. There are no deviations from the ideal bond angle due to the absence of lone pairs and multiple bonds.
Summary of Deviations:
- In PF3 and SBr2, deviations from idealized bond angles occur due to the presence of lone pairs of electrons. These lone pairs create more repulsion and reduce the bond angles slightly compared to the ideal geometry.
- CHCl3 and CS2 exhibit ideal bond angles because of their simple bonding arrangements with no lone pair interference.