Use VSEPR to predict the most stable structures of the following species:
A. SCl2F4
B. XeO2
C. SiH3-
D. BH3
The correct answer and explanation is:
Here are the VSEPR-based most stable geometries for each species:
A. SCl₂F₄
Structure: Trigonal bipyramidal electron geometry
Shape: Seesaw
- Explanation: Sulfur (S) has 6 valence electrons. In SCl₂F₄, it’s bonded to 6 atoms (2 Cl, 4 F), meaning 6 regions of electron density. However, this suggests hypervalency, possible for third-period elements like sulfur. The molecule adopts a trigonal bipyramidal geometry.
To minimize electron pair repulsion, the more electronegative fluorines occupy equatorial positions, and the larger chlorine atoms go in the axial positions. This results in a seesaw shape when lone pairs (if any) are factored in. But in this case, there are no lone pairs, just bonded atoms. The actual 3D arrangement is a distorted trigonal bipyramidal.
B. XeO₂
Structure: Bent
- Explanation: Xenon (Xe) has 8 valence electrons. In XeO₂, it’s bonded to two O atoms (2 regions) and has two lone pairs, for a total of 4 electron regions.
According to VSEPR (AX₂E₂), this gives a bent shape, similar to H₂O. The lone pairs repel more strongly than bonding pairs, pushing the O atoms closer together.
C. SiH₃⁻
Structure: Trigonal pyramidal
- Explanation: Silicon (Si) has 4 valence electrons. It forms three bonds with hydrogen and gains one extra electron due to the negative charge, giving 4 regions of electron density (3 bonding + 1 lone pair).
This corresponds to AX₃E, which results in a trigonal pyramidal geometry, similar to ammonia (NH₃).
D. BH₃
Structure: Trigonal planar
- Explanation: Boron (B) has 3 valence electrons and forms three bonds with hydrogen, for a total of 3 bonding pairs and no lone pairs.
This results in an AX₃ arrangement and a trigonal planar geometry. BH₃ is electron-deficient, which makes it reactive and prone to dimerization (forming B₂H₆).
Summary of Structures:
- SCl₂F₄ – Seesaw (trigonal bipyramidal base)
- XeO₂ – Bent
- SiH₃⁻ – Trigonal pyramidal
- BH₃ – Trigonal planar
These geometries are predicted using the Valence Shell Electron Pair Repulsion (VSEPR) model, which minimizes repulsions between electron pairs around a central atom.