Question
Predict the geometry, hybridisation, and bond angle of using VSEPR theory.
Solution — Step by Step
Xe has 8 valence electrons. Each F contributes 1 electron to the bond (we count Xe’s contribution to the four bond pairs).
Total around Xe = 8 (Xe valence). Of these, 4 are used in bonds, leaving electrons = 2 lone pairs.
Total electron domains around Xe = 4 bond pairs + 2 lone pairs = 6 domains.
6 domains arrange octahedrally. With 2 lone pairs, they sit on opposite (axial) positions to minimise lone-pair repulsion.
The 4 F atoms occupy the equatorial plane → square planar geometry.
Bond angle: (between adjacent F atoms in the square).
Hybridisation: 6 domains = .
Final answer: Square planar, hybridisation, bond angles
Why This Works
VSEPR’s central insight: electron pairs (bond and lone) repel each other and arrange to maximise their separation. Lone pairs occupy more space than bond pairs, so we always check whether their placement minimises lone-lone repulsion.
For 6 domains, the two lone pairs go opposite each other (180°) — this is the only arrangement that puts them at maximum distance. The remaining 4 bond pairs form a square in the equatorial plane.
Alternative Method
Use the AXE notation: is . Look up the geometry table:
- → square planar
- 6 domains →
Memorising the table speeds up routine VSEPR questions.
Common Mistake
Students confuse (square planar, 6 domains) with (tetrahedral, 4 domains). Both have 4 bonds, but the lone pairs change everything. Always count total electron domains (bonds + lone pairs), not just bonds.
Quick formula: total domains .
For : domains. Matches.