Predict the electron-domain and molecular geometries of SF5+.
The correct answer and explanation is:
Correct Answer:
- Electron-Domain Geometry of SF₅⁺: Octahedral
- Molecular Geometry of SF₅⁺: Square pyramidal
Explanation (Approx. 300 words):
The molecule SF₅⁺ (sulfur pentafluoride cation) is derived from the neutral molecule SF₆, sulfur hexafluoride. To determine the geometry of SF₅⁺, we must first consider the Lewis structure and VSEPR theory (Valence Shell Electron Pair Repulsion).
Step 1: Count the Valence Electrons
- Sulfur (S) has 6 valence electrons.
- Each fluorine (F) has 7 valence electrons × 5 = 35.
- The +1 charge indicates the loss of 1 electron.
Total = 6 + 35 – 1 = 40 valence electrons
Step 2: Determine the Electron-Domain Geometry
The sulfur atom forms 5 S–F bonds. In SF₆, sulfur makes 6 bonds (forming an octahedral shape). In SF₅⁺, it forms only 5 bonds and has no lone pairs, but still occupies 6 electron domains: five bonding pairs and one empty site due to the missing electron from the positive charge.
This arrangement still corresponds to an octahedral electron-domain geometry (based on 6 regions of electron density), according to VSEPR theory.
Step 3: Determine the Molecular Geometry
With one position vacant (like a lone pair or an empty orbital), and five bonded fluorine atoms, the shape becomes square pyramidal. This is because:
- Four fluorine atoms occupy the corners of a square in the equatorial plane.
- The fifth fluorine occupies an axial position.
- The sixth site is unoccupied due to the missing electron (positive charge), creating a “lone pair-like” vacancy.
Conclusion
Therefore:
- The electron-domain geometry remains octahedral.
- The molecular geometry is square pyramidal, as one position is vacant and five atoms surround the central sulfur.