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Chapter 9: Molecular Geometry and Bonding Theories – Study Notes

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Chapter 9: Molecular Geometry and Bonding Theories

9.1 Molecular Shapes

Molecular shape is a fundamental concept in chemistry, influencing physical and chemical properties. While Lewis structures show bonding and lone pairs, they do not directly denote the three-dimensional shape of molecules. However, they are essential for predicting molecular geometry.

  • Lewis Structures: Diagrams showing how atoms are bonded and the presence of lone pairs.

  • Common Molecular Shapes: For molecules with two or three atoms connected to a central atom, typical shapes include linear (CO2), bent (SO2), trigonal planar (SO3), trigonal pyramidal (NH3), and tetrahedral (CF4).

  • Example: CO2 is linear, NH3 is trigonal pyramidal.

9.2 The VSEPR Model

The Valence-Shell Electron-Pair Repulsion (VSEPR) Model is used to predict the shape of molecules based on the repulsion between electron pairs around a central atom.

  • Electron Pairs Repel: Both bonding and nonbonding electron pairs repel each other, influencing molecular shape.

  • Bond Angles and Lengths: These determine the size and shape of molecules.

  • Electron Domains: Regions where electrons are most likely to be found, including bonding domains (single, double, triple bonds) and nonbonding domains (lone pairs).

  • Multiple Bonds: Count as one electron domain.

  • Balloon Analogy: Electron domains arrange themselves to minimize repulsion, similar to balloons tied together.

Electron-Domain Geometries

Electron-domain geometry is determined by the number of electron domains around a central atom. Each geometry gives rise to specific bond angles and molecular shapes.

  • Count total bonding and nonbonding electron domains.

  • Multiple bonds count as one domain.

Number of Electron Domains

Arrangement

Geometry

Bond Angles

2

Linear

Linear

180°

3

Trigonal planar

Trigonal planar

120°

4

Tetrahedral

Tetrahedral

109.5°

5

Trigonal bipyramidal

Trigonal bipyramidal

120°, 90°

6

Octahedral

Octahedral

90°

Applying VSEPR to Determine Molecular Shapes

To determine molecular geometry:

  1. Draw the best Lewis structure.

  2. Determine the electron-domain geometry.

  3. Use the arrangement of bonded atoms to determine the molecular geometry.

Refer to Tables 9.2 and 9.3 for possible geometries.

Linear Electron Domain

With two electron domains, the only possible geometry is linear.

Electron Domains

Bonding Domains

Nonbonding Domains

Molecular Geometry

Example

2

2

0

Linear

CO2

Note: Molecules with only two atoms are always linear.

Trigonal Electron Domain

With three electron domains, possible geometries are:

  • Trigonal planar: All domains are bonding (e.g., BF3).

  • Bent: One domain is a lone pair (e.g., SO2).

Electron Domains

Bonding Domains

Nonbonding Domains

Molecular Geometry

Example

3

3

0

Trigonal planar

BF3

3

2

1

Bent

SO2

Tetrahedral Electron Domain

With four electron domains, possible geometries are:

  • Tetrahedral: All are bonding pairs (e.g., CH4).

  • Trigonal pyramidal: One is a lone pair (e.g., NH3).

  • Bent: Two are lone pairs (e.g., H2O).

Electron Domains

Bonding Domains

Nonbonding Domains

Molecular Geometry

Example

4

4

0

Tetrahedral

CH4

4

3

1

Trigonal pyramidal

NH3

4

2

2

Bent

H2O

Nonbonding Pairs and Bond Angles

Nonbonding (lone) pairs are physically larger than bonding pairs, leading to greater repulsion and compression of bond angles.

  • Bond Angles: Tetrahedral (109.5°), Trigonal pyramidal (107°), Bent (104.5°).

  • Example: H2O has a bond angle of 104.5° due to two lone pairs.

Multiple Bonds and Bond Angles

Double and triple bonds have larger electron domains than single bonds, exerting greater repulsive force and increasing bond angles.

  • Example: In Cl–C=O–Cl, the bond angles around the central carbon are greater than 120° due to the double bond.

Molecules with Expanded Valence Shells

Elements in periods 3 through 6 can have more than four electron domains due to available d-orbitals, breaking the octet rule.

  • Five domains: Trigonal bipyramidal geometry.

  • Six domains: Octahedral geometry.

Trigonal Bipyramidal Electron Domain

With five electron domains, four distinct molecular geometries are possible:

  • Trigonal bipyramidal

  • Seesaw

  • T-shaped

  • Linear

There are two positions: axial and equatorial. Lone pairs occupy equatorial positions.

Electron Domains

Bonding Domains

Nonbonding Domains

Molecular Geometry

Example

5

5

0

Trigonal bipyramidal

PCl5

5

4

1

Seesaw

SF4

5

3

2

T-shaped

ClF3

5

2

3

Linear

XeF2

Octahedral Electron Domain

With six electron domains, all positions are equivalent. Possible geometries:

  • Octahedral (e.g., SF6)

  • Square pyramidal (e.g., BrF5)

  • Square planar (e.g., XeF4)

Electron Domains

Bonding Domains

Nonbonding Domains

Molecular Geometry

Example

6

6

0

Octahedral

SF6

6

5

1

Square pyramidal

BrF5

6

4

2

Square planar

XeF4

Shapes of Larger Molecules

VSEPR theory can be extended to complex molecules. For larger molecules, consider the geometry about each atom individually. For example, in acetic acid (CH3COOH), different atoms have different local geometries.

9.3 Molecular Shape and Polarity

Molecular polarity depends on both the polarity of individual bonds and the overall geometry of the molecule.

  • Polar Molecules: Bond dipoles do not cancel; the molecule has a net dipole moment (e.g., H2O).

  • Nonpolar Molecules: Bond dipoles cancel due to symmetry (e.g., CO2).

  • Assigning Dipole Moment: Use vectors to sum bond dipoles.

  • Steps: Draw Lewis structure, determine electron-domain geometry, distinguish bonding/nonbonding domains.

Molecule

Polarity

Dipole Moment

CO2

Nonpolar

0 D

H2O

Polar

1.85 D

9.4 Covalent Bonding and Orbital Overlap

VSEPR does not explain why bonds exist. Valence-bond theory describes covalent bonding as the overlap of atomic orbitals from two atoms, allowing electrons of opposite spin to share space and form a bond.

  • Orbital Overlap: The shared space between two electrons forms a covalent bond.

  • Bond Strength: Determined by the minimum energy when attractive and repulsive forces balance.

  • Bond Length: The distance at which this minimum energy occurs.

9.5 Hybrid Orbitals

Hybridization is the mixing of atomic orbitals to form new, degenerate orbitals suitable for bonding.

  • sp Hybridization: Mixing one s and one p orbital yields two sp orbitals (e.g., BeF2).

  • sp2 Hybridization: Mixing one s and two p orbitals yields three sp2 orbitals (e.g., BF3).

  • sp3 Hybridization: Mixing one s and three p orbitals yields four sp3 orbitals (e.g., CH4, NH3, H2O).

  • Bond Angles: sp (180°), sp2 (120°), sp3 (109.5°).

Atomic Orbital Set

Hybrid Orbital Set

Geometry

Examples

s, p

Two sp

Linear (180°)

BeF2, HgCl2

s, p, p

Three sp2

Trigonal planar (120°)

BF3, SO3

s, p, p, p

Four sp3

Tetrahedral (109.5°)

CH4, NH3, H2O, NH4+

Hybrid Orbital Description of Water

Water (H2O) has a tetrahedral electron-domain geometry (four domains: two bonding, two nonbonding). The ideal bond angle for sp3 hybridization is 109.5°, but the measured angle is 104.9° due to repulsion from nonbonding pairs.

Hybrid Orbital Summary

  1. Draw the Lewis structure.

  2. Use VSEPR to determine electron-domain geometry.

  3. Specify the hybrid orbitals needed for the arrangement.

Additional info:

  • Hybridization helps explain observed molecular geometries and bond angles.

  • Expanded valence shells (hypervalent molecules) may involve d orbitals for elements in period 3 and beyond.

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