Problem
Construct the sf6 lewis structure for sulfur hexafluoride, including all lone pairs, and verify the structure using the total valence-electron count, the octet/expanded-octet requirement, and formal charges.
Step-by-step solution
1) Count total valence electrons
Sulfur (S) is in Group 16, so it contributes 6 valence electrons. Fluorine (F) is in Group 17, so it contributes 7 valence electrons per atom.
| Element | Number of atoms | Valence electrons per atom | Total contributed |
|---|---|---|---|
| S | 1 | 6 | \(1 \cdot 6 = 6\) |
| F | 6 | 7 | \(6 \cdot 7 = 42\) |
| Total | \(6 + 42 = 48\) | ||
The sf6 lewis structure must place exactly \(48\) valence electrons.
2) Choose the central atom and connect the skeleton
Fluorine is almost always terminal (it forms one bond). Sulfur is placed in the center and connected to six fluorine atoms with single bonds to form the initial skeleton.
3) Place bonding electrons
Six S–F single bonds use:
\[ 6 \cdot 2 = 12 \text{ electrons} \]
Remaining electrons after bonding:
\[ 48 - 12 = 36 \text{ electrons} \]
4) Complete octets on the fluorine atoms (lone pairs)
Each fluorine already has one bond (2 electrons shared), so each F needs 6 more electrons (three lone pairs) to complete an octet.
Lone-pair electrons required for six fluorines:
\[ 6 \cdot 6 = 36 \text{ electrons} \]
The remaining 36 electrons are placed as three lone pairs on each F. No electrons remain for lone pairs on sulfur.
5) Check the central-atom electron count (expanded octet)
Sulfur is surrounded by six bonding pairs, which corresponds to \(12\) electrons around S. This is an expanded octet (allowed for third-period elements such as sulfur).
6) Confirm formal charges
Formal charge is computed by:
\[ \text{FC} = V - \left(N + \frac{B}{2}\right) \]
- Each fluorine: \(V = 7\), \(N = 6\) (three lone pairs), \(B = 2\) (one single bond). \[ \text{FC}_\mathrm{F} = 7 - \left(6 + \frac{2}{2}\right) = 7 - (6 + 1) = 0 \]
- Sulfur: \(V = 6\), \(N = 0\), \(B = 12\) (six single bonds). \[ \text{FC}_\mathrm{S} = 6 - \left(0 + \frac{12}{2}\right) = 6 - 6 = 0 \]
All atoms have formal charge \(0\), consistent with the preferred Lewis structure for SF6.
Visualization
Result summary
- Bonds: six single S–F bonds.
- Lone pairs: three lone pairs on each fluorine; none on sulfur.
- Electron count: \(12\) bonding electrons and \(36\) nonbonding electrons, total \(48\).
- Formal charges: \(0\) on S and on every F.
- Geometry note: VSEPR classification AX6 gives an octahedral arrangement with ideal \(90^\circ\) F–S–F angles.