Question
(NEET 2023 PYQ) Among the following, the molecule with sp³d² hybridisation and octahedral geometry is: (a) PCl₅, (b) SF₆, (c) BrF₃, (d) XeF₂. State the hybridisation, geometry, and shape of each option, and identify the correct answer.
Solution — Step by Step
The hybridisation depends on the steric number = (number of bonded atoms) + (lone pairs on central atom).
- PCl₅: P has 5 bond pairs, 0 lone pairs. Steric number = 5. Hybridisation: sp³d. Geometry: trigonal bipyramidal.
- SF₆: S has 6 bond pairs, 0 lone pairs. Steric number = 6. Hybridisation: sp³d². Geometry: octahedral.
- BrF₃: Br has 3 bond pairs, 2 lone pairs. Steric number = 5. Hybridisation: sp³d. Shape: T-shaped (octahedral electron geometry, but bent due to lone pairs).
- XeF₂: Xe has 2 bond pairs, 3 lone pairs. Steric number = 5. Hybridisation: sp³d. Shape: linear.
The question asks for sp³d² (steric number 6) and octahedral. Only SF₆ has this combination.
In SF₆, sulfur uses 1 s orbital, 3 p orbitals, and 2 d orbitals to form 6 equivalent sp³d² hybrid orbitals. Each forms a bond with F, all at 90° to neighbours, giving a perfect octahedron with no lone pairs and no distortion.
Answer: (b) SF₆.
Why This Works
The steric number rule directly maps onto hybridisation:
- 2 → sp (linear)
- 3 → sp² (trigonal planar)
- 4 → sp³ (tetrahedral)
- 5 → sp³d (trigonal bipyramidal)
- 6 → sp³d² (octahedral)
- 7 → sp³d³ (pentagonal bipyramidal)
If lone pairs are present, the electron geometry follows the steric number, but the molecular shape is what’s left after removing lone-pair “seats”. For sp³d² with one lone pair: square pyramidal. With two lone pairs: square planar.
NEET often tests subtle differences — e.g., “octahedral” vs “square planar” vs “square pyramidal” all have sp³d² hybridisation. Always check both the hybridisation and the actual shape (lone pairs).
Alternative Method
VSEPR theory: count electron domains around the central atom. SF₆ has 6 bond domains and 0 lone-pair domains, so the geometry is octahedral. Same conclusion via a different framework.
Common Mistake
Students sometimes count -bonds in the steric number — wrong. Only -bonds and lone pairs count. Each multiple bond contributes only one to the steric number, regardless of bond order.