Expanded Octet
- Elements in Period 3+ can have more than 8 electrons around them due to
available d-orbitals
- These molecules exceed the octet rule: PCl₅ (10 e⁻), SF₆ (12 e⁻)
- VSEPR still applies — lone pairs occupy more space than bonding pairs
- Lone pairs on expanded octets reduce symmetry: SF₆ is octahedral, but XeF₄ is square planar due
to 2 lone pairs
- Period 2 elements (C, N, O, F) cannot expand their octet — no accessible
d-orbitals
Expanded Octet Geometries
- 5 electron domains (trigonal bipyramidal): PCl₅ (0 LP), ClF₃ (2 LP → T-shaped),
XeF₂ (3 LP → linear)
- 6 electron domains (octahedral): SF₆ (0 LP), IF₅ (1 LP → square pyramidal),
XeF₄ (2 LP → square planar)
- Lone pairs in trigonal bipyramidal always occupy equatorial positions to
minimize repulsion
- Lone pairs in octahedral go trans to each other (180° apart) if there are two
IB HL Exam Tips
- Draw VSEPR: count total electron domains, place lone pairs in positions that
maximize separation
- Bond angles deviations: lone pairs compress adjacent bond angles (e.g., ClF₃
has angles < 90°)
- Naming convention: molecular shape = shape defined by ATOMS only (not lone
pairs)
- Polarity: symmetric molecules (SF₆, XeF₄) are nonpolar; asymmetric (ClF₃, IF₅)
are polar