Determine the sf4 lewis structure for sulfur tetrafluoride (SF4), then use the electron-domain count to predict the VSEPR electron-domain geometry and molecular shape.
Step 1: Count total valence electrons
Sulfur contributes 6 valence electrons, and each fluorine contributes 7 valence electrons.
\[ N_{\text{val}} = 6 + 4 \cdot 7 = 6 + 28 = 34 \]
Step 2: Choose a skeleton and connect atoms
Fluorine is almost always terminal (forms one bond), so sulfur is placed in the center with four single bonds to fluorine.
Electrons used in four single bonds:
\[ 4 \text{ bonds} \cdot 2 \text{ e}^- / \text{bond} = 8 \text{ e}^- \]
Remaining electrons after bonding:
\[ 34 - 8 = 26 \text{ e}^- \]
Step 3: Complete octets on terminal atoms (fluorine)
Each F already has one bond (2 electrons around it from the bond) and needs 6 more electrons (three lone pairs) to reach an octet.
Electrons required to complete octets on 4 fluorines:
\[ 4 \cdot 6 = 24 \text{ e}^- \]
Remaining electrons after completing fluorine octets:
\[ 26 - 24 = 2 \text{ e}^- \]
Step 4: Place remaining electrons on the central atom
The remaining 2 electrons become one lone pair on sulfur. This gives sulfur a total of 5 electron domains (4 bonding pairs + 1 lone pair), which corresponds to an expanded octet around sulfur (10 electrons around S when counting bonding electrons).
SF4 Lewis structure summary: central S with four S–F single bonds; each F has three lone pairs; S has one lone pair.
Step 5: Check formal charges (verification)
Formal charge is computed by:
\[ \text{FC} = (\text{valence e}^-) - (\text{nonbonding e}^-) - \frac{(\text{bonding e}^-)}{2} \]
| Atom | Valence e− | Nonbonding e− | Bonding e− | Formal charge |
|---|---|---|---|---|
| S | 6 | 2 | 8 | \(6 - 2 - \frac{8}{2} = 0\) |
| F (each) | 7 | 6 | 2 | \(7 - 6 - \frac{2}{2} = 0\) |
All atoms have formal charge 0, supporting the chosen SF4 Lewis dot structure as the best representation.
Step 6: VSEPR geometry from electron domains
Sulfur has 5 electron domains, so the electron-domain geometry is trigonal bipyramidal. With one equatorial lone pair (minimizes repulsions), the molecular geometry becomes seesaw (AX4E).
Typical bond-angle expectations for SF4 (VSEPR)
In a trigonal bipyramidal arrangement, ideal angles are 90° (axial–equatorial) and 120° (equatorial–equatorial). The lone pair increases repulsions, so real angles in the seesaw shape are slightly less than 90° and slightly less than 120°.
Final statement
The sf4 lewis structure places sulfur at the center with four S–F single bonds and one lone pair (34 total valence electrons), yielding trigonal bipyramidal electron-domain geometry and a seesaw molecular shape by VSEPR.