Understanding the Context

A Lewis structure, developed by Gilbert N. Lewis in 1916, visually represents valence electrons around atoms in a molecule. It shows single bonds (shared electrons), double or triple bonds, and lone pairs (non-bonding electrons) using dots and lines. For SF₄, determining the correct Lewis structure requires following key principles:

• Count total valence electrons
  
• Arrange atoms based on central atom geometry
  
• Use single bonds to attach electrons
  
• Distribute remaining electrons to satisfy octet rule (except for expanded octets when applicable)
  
• Identify lone pairs on terminal atoms and the central atom

Breaking Down SF₄: Valence Electrons and Bonding

Sulfur (S) is in Group 16 of the periodic table and possesses six valence electrons. Fluorine (F), in Group 17, has seven valence electrons—each forming one sigma bond with sulfur.

Key Insights

• Total valence electrons:
  S: 6
  × 4 F atoms: 4 × 7 = 28
  Total: 34 electrons
  
• In SF₄, S forms four single S–F covalent bonds using eight electrons.
  
• Remaining electrons: 34 – 8 = 26 electrons = 13 lone pairs
  
• These 13 lone pairs are distributed among atoms to minimize energy and stabilize the molecule.

Step-by-Step Construction of the SF₄ Lewis Structure

Step 1: Identify the central atom
Sulfur (S), being less electronegative than fluorine, is the central atom.

Step 2: Arrange four single bonds
Attach four single bonds from sulfur to four fluorine atoms in a trigonal bipyramidal layout—three equatorial positions at 120° and two axial positions at 90° to equator.

Step 3: Distribute remaining electrons
Remaining 13 lone pairs go primarily to fluorine (each achieving octet), with lone pairs also occupying available spaces on sulfur.

Final Thoughts

Sulfur ends up with four bonding pairs and one lone pair, totaling five electron domains—signature of a trigonal bipyramidal geometry.

Step 4: Adjust for lone pair placement
To minimize electron-electron repulsion, the lone pair occupies an equatorial position, distorting the ideal geometry.

Final SF₄ Lewis Structure:

• Central S atom bonded via four single bonds
  
• One lone pair in an equatorial position
  
• Four F atoms arranged with 90° axial-equatorial angles and 120° equatorial angles
  
• Molecular shape classified as see-saw (distorted tetrahedral)

Molecular Geometry and Bond Angles of SF₄

Due to the lone pair arranging electron pairs geometrically, SF₄ adopts a see-saw shape—a trigonal bipyramidal framework with a lone pair in one equatorial position. This geometry causes significant asymmetry.

• Axial bond angles: ~180° (minimal distortion from linear)
  
• Equatorial bond angles: ~90° and ~120°
  
• The lone pair repels equatorial and axial bonds, compressing bond angles and generating unequal S–F bond distances:
  
  • Axial F–S bonds slightly shorter (~ bond length ≈ 1.73 Å)
    
  • Equatorial F–S bonds slightly longer (~1.77 Å)