What is the molecular geometry of SF5-? (SHOW WORK, DRAW THE LEWIS DOT STRUCTURE)
seesaw
linear
trigonal planar
bent
tetrahedral
trigonal pyramidal
trigonal bipyramidal
octahedral
square planar
The Correct Answer and Explanation is:
To determine the molecular geometry of the ion SF₅⁻, we need to follow a systematic approach involving drawing the Lewis structure, determining the electron geometry, and finally identifying the molecular geometry based on VSEPR theory.
Step 1: Count Valence Electrons
- Sulfur (S) has 6 valence electrons.
- Fluorine (F) has 7 valence electrons × 5 atoms = 35 electrons.
- Extra electron due to the negative charge: +1
Total valence electrons = 6 (S) + 35 (F) + 1 (charge) = 42 valence electrons
Step 2: Draw the Lewis Structure
- Place S in the center (least electronegative).
- Surround with 5 F atoms, each forming a single bond.
- Each S–F bond uses 2 electrons × 5 = 10 electrons.
- 42 – 10 = 32 electrons remaining
- Distribute the remaining 32 electrons to complete octets for the 5 F atoms:
- Each F needs 6 more electrons (3 lone pairs) × 5 = 30 electrons used.
- 32 – 30 = 2 electrons left → place as a lone pair on S.
Step 3: Determine Electron and Molecular Geometry
- Steric number of central atom (S) = 5 bonded atoms + 1 lone pair = 6 regions of electron density
- According to VSEPR theory, 6 regions → octahedral electron geometry
- With 1 lone pair, molecular shape = square pyramidal
Final Answer:
✅ Correct Molecular Geometry: Square Pyramidal
Explanation
The molecular geometry of SF₅⁻ is square pyramidal, as determined using the Valence Shell Electron Pair Repulsion (VSEPR) model. First, we calculate the number of valence electrons: sulfur contributes 6, the five fluorine atoms contribute 35 in total, and the extra negative charge adds 1 more, yielding a total of 42 valence electrons.
We construct the Lewis dot structure by placing sulfur in the center with five fluorine atoms surrounding it. Each S–F single bond uses 2 electrons, accounting for 10 electrons. The remaining 32 electrons are distributed to satisfy the octet rule for each fluorine, leaving 2 electrons. These are placed as a lone pair on the central sulfur atom.
Now, sulfur has 6 electron domains: 5 bonding pairs and 1 lone pair. The electron geometry for 6 electron regions is octahedral. However, because one of these regions is a lone pair, the actual molecular geometry (which only considers the positions of atoms) becomes square pyramidal.
In square pyramidal geometry, four fluorine atoms occupy the corners of a square plane around the central sulfur, while the fifth fluorine occupies a position above or below this plane (axial position). The lone pair occupies the position opposite the axial fluorine, slightly distorting the symmetry. This geometry minimizes electron pair repulsion, especially between the lone pair and bonding pairs.
Thus, the molecular geometry of SF₅⁻ is square pyramidal, not trigonal bipyramidal or any of the other options. This geometry has implications for the ion’s polarity and reactivity, as the lone pair introduces asymmetry in the molecular structure.
