Arrange the six different molecular geometries in chem 115 in order of increasing bond angle, from the molecular geometry with the smallest bond angle to the molecular geometry with the largest bond angle.
The Correct Answer and Explanation is :
In Chem 115, the six common molecular geometries, based on the VSEPR (Valence Shell Electron Pair Repulsion) theory, are arranged by increasing bond angles as follows:
- Bent (AX2E2): 104.5° (e.g., H2O)
- Trigonal pyramidal (AX3E): 107° (e.g., NH3)
- Tetrahedral (AX4): 109.5° (e.g., CH4)
- Trigonal planar (AX3): 120° (e.g., BF3)
- Trigonal bipyramidal (AX5): 90°, 120°, and 180° (e.g., PCl5)
- Octahedral (AX6): 90° (e.g., SF6)
Explanation:
The VSEPR theory helps predict the 3D geometry of molecules by assuming that electron pairs around a central atom repel each other and thus arrange themselves as far apart as possible. The bond angle depends on both the number of bonding pairs (sigma bonds) and lone pairs of electrons around the central atom. The more lone pairs a molecule has, the smaller the bond angle due to increased repulsion.
- Bent (104.5°): The presence of two lone pairs in water (H2O) reduces the bond angle from the tetrahedral ideal of 109.5° to 104.5° due to lone-pair repulsion.
- Trigonal pyramidal (107°): The lone pair on nitrogen in ammonia (NH3) causes a slight compression of the bond angle from 109.5° to 107°.
- Tetrahedral (109.5°): Methane (CH4) has no lone pairs, so it retains the ideal tetrahedral bond angle of 109.5°.
- Trigonal planar (120°): Molecules like boron trifluoride (BF3) have a planar arrangement with 120° angles due to three bonding pairs and no lone pairs.
- Trigonal bipyramidal: This geometry features both 90° (between axial and equatorial bonds) and 120° angles (between equatorial bonds).
- Octahedral (90°): SF6 forms a symmetrical octahedral geometry with 90° bond angles due to six bonding pairs and no lone pairs.