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The Shapes of Molecules: VSEPR Theory

  • The shape (geometry) of a molecule influences physical and chemical properties, including melting point, boiling point, and reactivity

  • Valence Shell Electron Pair Repulsion (VSEPR) Theory: based on Lewis structures and the principles that:

    • Valence electron pairs repel each other

    • The geometry around the central atom will minimize the repulsion

  • VSEPR Nomenclature

    • A = central atom

    • X = bonding atom

    • E = lone pair

  • If a molecule has 2 or more resonance structures, the VSEPR model can be applied to any one of them

  • If there’s more than one central atom in a molecule, consider the bonding about each atom independently

  • Steric number = number of atoms bonded to central atom + number of lone pairs

    • Used to predict geometries

Molecules Without Lone Pairs

Formula Type

SN

Geometry

Bond angle

AX₂

2

Linear

180°

AX₃

3

Trigonal planar

120°

AX₄

4

Tetrahedral

109.5°

AX₅

5

Trigonal bipyramidal

120° (equatorial)90° (axial)

AX₆

6

Octahedral

90°

Molecules With Lone Pairs

  • Electrons in bonds have less “spatial distribution that lone pairs, meaning:

    • Electrons in bonds take up less space

    • Lone-pair electrons can be anywhere

    • Lone pair electrons take up more space and therefore experience more repulsion

    • Repulsive forces for molecules with lone pairs

      • Lone pair/lone pair repulsion (most repulsion) -> lone pair/bonding pair repulsion -> bonding pair/bonding pair repulsion (least repulsion)

    • In molecules with lone-pair electrons, angles between bonded atoms tend to be smaller

    • When atomic size increases, lone pairs occupy larger spatial volumes, so angles between bonded atoms tend to be even smaller

Formula Type

SN

Geometry

Bond angle

AX₂E

3

Bent

Less than 120°

AX₃E

4

Trigonal pyramidal

Less than 109.5°

AX₂E₂

4

Bent

Less than 109.5°

AX₄E

5

See-saw

Less than 120° (equatorial)Less than 90° (axial)

AX₃E₂

5

T-shaped

Less than 90°

AX₂E₃

5

Linear

180° - not less than because there’s no other way to minimize repulsion

AX₅E

6

Square pyramidal

Less than 90°

AX₄E₂

6

Square planar

90°

AX₃E₃

6

T-shaped

Less than 90°

AX₂E₄

6

Linear

180°

The Shapes of Molecules: VSEPR Theory

  • The shape (geometry) of a molecule influences physical and chemical properties, including melting point, boiling point, and reactivity

  • Valence Shell Electron Pair Repulsion (VSEPR) Theory: based on Lewis structures and the principles that:

    • Valence electron pairs repel each other

    • The geometry around the central atom will minimize the repulsion

  • VSEPR Nomenclature

    • A = central atom

    • X = bonding atom

    • E = lone pair

  • If a molecule has 2 or more resonance structures, the VSEPR model can be applied to any one of them

  • If there’s more than one central atom in a molecule, consider the bonding about each atom independently

  • Steric number = number of atoms bonded to central atom + number of lone pairs

    • Used to predict geometries

Molecules Without Lone Pairs

Formula Type

SN

Geometry

Bond angle

AX₂

2

Linear

180°

AX₃

3

Trigonal planar

120°

AX₄

4

Tetrahedral

109.5°

AX₅

5

Trigonal bipyramidal

120° (equatorial)90° (axial)

AX₆

6

Octahedral

90°

Molecules With Lone Pairs

  • Electrons in bonds have less “spatial distribution that lone pairs, meaning:

    • Electrons in bonds take up less space

    • Lone-pair electrons can be anywhere

    • Lone pair electrons take up more space and therefore experience more repulsion

    • Repulsive forces for molecules with lone pairs

      • Lone pair/lone pair repulsion (most repulsion) -> lone pair/bonding pair repulsion -> bonding pair/bonding pair repulsion (least repulsion)

    • In molecules with lone-pair electrons, angles between bonded atoms tend to be smaller

    • When atomic size increases, lone pairs occupy larger spatial volumes, so angles between bonded atoms tend to be even smaller

Formula Type

SN

Geometry

Bond angle

AX₂E

3

Bent

Less than 120°

AX₃E

4

Trigonal pyramidal

Less than 109.5°

AX₂E₂

4

Bent

Less than 109.5°

AX₄E

5

See-saw

Less than 120° (equatorial)Less than 90° (axial)

AX₃E₂

5

T-shaped

Less than 90°

AX₂E₃

5

Linear

180° - not less than because there’s no other way to minimize repulsion

AX₅E

6

Square pyramidal

Less than 90°

AX₄E₂

6

Square planar

90°

AX₃E₃

6

T-shaped

Less than 90°

AX₂E₄

6

Linear

180°