2.2.13 (HL)—Expanded octets

Some HL species have five or six electron domains around a central atom. Draw the Lewis formula, count the domains, and apply VSEPR to assign the corresponding geometry.

For five domains, start from trigonal-bipyramidal electron geometry; lone pairs prefer equatorial positions because that reduces 90° interactions, producing common shapes such as seesaw and T-shaped. For six domains, start from octahedral geometry; removing one or two lone-pair positions gives common square-pyramidal or square-planar molecular shapes. Name molecular shape from atom positions after the electron-domain arrangement is fixed.

Five bonding domains give trigonal-bipyramidal PCl₅ and six give octahedral SF₆. If lone pairs are present, keep the electron-domain arrangement but remove lone-pair positions when naming molecular shape, choosing positions that minimize the strongest repulsions.

Representative question

[Maximum number: 2]

Deduce the molecular geometry of $\mathrm{PCl}_{3}$ and $\mathrm{PCl}_{5}$ .
$\mathrm{PCl}_{3}:$

$\mathrm{PCl}_{5}$ :

$\mathrm{PCl}_{3}$ . trigonal pyramid $\mathrm{PCl}_{5}$ . trigonal bipyramid

Retrieve the covalent pathway: shared pairs and bond order lead to geometry, polarity and molecular polarity; structure determines network properties, IMF behaviour and chromatography; HL representations extend to resonance, formal charge, sigma/pi bonds and hybridization.

Check the representation first, then count domains, apply geometry, identify polarity or forces, and connect the structure to the requested property or HL bonding description.

2.2.13 (HL)—Expanded octets

Expanded Octets and VSEPR

Applying Expanded-Octet VSEPR

Assessment in practice

The Covalent Model Summary

Question 1