Electron-Pair Geometries and Molecular Geometries

SN (steric number)

The total number of electron regions (bonding + lone pairs) around the central atom.

SN = 3, 3 bonded atoms, 0 lone pairs

Electron Geometry: Trigonal planar; Molecular Geometry: Trigonal planar; Bond Angle: 120°; Example: CH₂O

SN = 3, 2 bonded atoms, 1 lone pair

Electron Geometry: Trigonal planar; Molecular Geometry: Bent (angular); Bond Angle: <120°; Example: SO₂

SN = 4, 4 bonded atoms, 0 lone pairs

Electron Geometry: Tetrahedral; Molecular Geometry: Tetrahedral; Bond Angle: 109.5°; Example: CH₂Cl₂

SN = 4, 3 bonded atoms, 1 lone pair

Electron Geometry: Tetrahedral; Molecular Geometry: Trigonal pyramidal; Bond Angle: <109.5°; Example: NH₃

SN = 4, 2 bonded atoms, 2 lone pairs

Electron Geometry: Tetrahedral; Molecular Geometry: Bent (angular); Bond Angle: <109.5°; Example: H₂O

SN = 5, 5 bonded atoms, 0 lone pairs

Electron Geometry: Trigonal bipyramidal; Molecular Geometry: Trigonal bipyramidal; Bond Angle: 90°, 120°; Example: PF₅ (or PF₄Cl)

SN = 5, 4 bonded atoms, 1 lone pair

Electron Geometry: Trigonal bipyramidal; Molecular Geometry: Seesaw; Bond Angle: <90°, <120°; Example: SCl₄

SN = 5, 3 bonded atoms, 2 lone pairs

Electron Geometry: Trigonal bipyramidal; Molecular Geometry: T-shaped; Bond Angle: <90°; Example: BrF₃

SN = 5, 2 bonded atoms, 3 lone pairs

Electron Geometry: Trigonal bipyramidal; Molecular Geometry: Linear; Bond Angle: 180°; Example: XeF₂

SN = 6, 6 bonded atoms, 0 lone pairs

Electron Geometry: Octahedral; Molecular Geometry: Octahedral; Bond Angle: 90°; Example: SF₆

SN = 6, 5 bonded atoms, 1 lone pair

Electron Geometry: Octahedral; Molecular Geometry: Square pyramidal; Bond Angle: <90°; Example: IF₅

SN = 6, 4 bonded atoms, 2 lone pairs

Electron Geometry: Octahedral; Molecular Geometry: Square planar; Bond Angle: 90°; Example: XeF₄

Effect of lone pairs on bond angles

Lone pairs repel more strongly than bonding pairs, slightly reducing bond angles.

Possible geometries for SN = 6 with 3 or 4 lone pairs

Octahedral base with 3 lone pairs (T-shaped) or 4 lone pairs (linear), though rarely encountered.