Chemistry Chapter 9

Chapter 9: Molecular Geometry and Bonding Theories

9.1 Molecular Shapes

The three-dimensional shape of a molecule is crucial in determining its physical and chemical properties. While Lewis structures show the connectivity of atoms and the types of bonds, they do not represent the actual spatial arrangement of atoms. Molecular geometry describes the arrangement of atoms in space, which is influenced by the number of electron domains (regions of electron density) around the central atom.

• Lewis Structures: Show the number and types of bonds but not the true geometry.

• Actual Geometry: Atoms are arranged in three dimensions to minimize electron pair repulsions.

Example: In carbon tetrachloride (CCl4), the Lewis structure suggests a flat arrangement, but the actual geometry is tetrahedral, with bond angles of 109.5°.

9.2 Valence-Shell Electron-Pair Repulsion (VSEPR) Theory

The VSEPR model predicts molecular shapes based on the repulsion between electron domains (bonding and non-bonding pairs) around a central atom. Electron domains arrange themselves as far apart as possible to minimize repulsion.

• Electron Domain: A region where electrons are likely found (bonding pairs or lone pairs).

• Bonding Pair: Shared electrons between two atoms.

• Non-bonding Pair (Lone Pair): Electrons localized on one atom.

Balloon models help visualize how electron domains repel each other, leading to specific geometries:

• 2 domains: Linear (180°)

• 3 domains: Trigonal planar (120°)

• 4 domains: Tetrahedral (109.5°)

Counting Electron Domains

Only electron domains around the central atom are counted. Double and triple bonds count as one domain each.

Electron Domain Geometries

Number of Electron Domains	Arrangement	Geometry	Bond Angles
2	Linear	Linear	180°
3	Trigonal planar	Trigonal planar	120°
4	Tetrahedral	Tetrahedral	109.5°

Expanded Electron Domains

Number of Electron Domains	Arrangement	Geometry	Bond Angles
5	Trigonal bipyramidal	Trigonal bipyramidal	120°, 90°
6	Octahedral	Octahedral	90°

Examples of Electron Domain Geometries

Molecule	# of Electron Domains	Electron Domain Geometry
BF3	3	Trigonal planar
CCl4	4	Tetrahedral
PCl5	5	Trigonal bipyramidal
XeF2	5	Trigonal bipyramidal

Predicting Molecular Geometry

The molecular geometry describes the arrangement of only the atoms (not lone pairs) in a molecule. The presence of lone pairs can alter the observed geometry from the electron domain geometry.

Common Molecular Geometries

Electron Domains	Lone Pairs	Molecular Geometry	Bond Angles
2	0	Linear	180°
3	0	Trigonal planar	120°
3	1	Bent	<120°
4	0	Tetrahedral	109.5°
4	1	Trigonal pyramidal	<109.5°
4	2	Bent	<109.5°

Stepwise Prediction Using VSEPR

1. Draw the Lewis structure and count the number of electron domains around the central atom.

2. Determine the electron domain geometry.

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

9.3 Molecular Shape and Molecular Polarity

The polarity of a molecule depends on both the polarity of its bonds and its molecular geometry. Bond dipoles are vector quantities; their direction and magnitude determine whether the molecule is polar or non-polar.

• Bond Dipole: Arrow points toward the more electronegative atom; length indicates magnitude.

• Non-polar Molecule: Bond dipoles cancel due to symmetry (e.g., CO2, CCl4).

• Polar Molecule: Bond dipoles do not cancel (e.g., H2O, NH3).

Example: CCl4 is non-polar because all four C–Cl bonds are identical and symmetrically arranged, so their dipoles cancel.

9.4 Covalent Bonding and Orbital Overlap

Valence bond theory explains covalent bonding as the overlap of atomic orbitals from different atoms. The shared electron density between nuclei forms a covalent bond.

• Sigma (σ) Bond: Electron density is concentrated along the internuclear axis (single bonds).

• Pi (π) Bond: Electron density is above and below the internuclear axis (in double and triple bonds).

9.5 Hybrid Orbitals

Hybridization is the mixing of atomic orbitals to form new, equivalent hybrid orbitals that explain observed molecular geometries. The type of hybridization depends on the number of electron domains around the central atom.

• sp Hybridization: Linear geometry (2 domains)

• sp2 Hybridization: Trigonal planar geometry (3 domains)

• sp3 Hybridization: Tetrahedral geometry (4 domains)

Example: In methane (CH4), the carbon atom undergoes sp3 hybridization to form four equivalent bonds.

9.6 Multiple Bonds

Multiple bonds consist of one sigma (σ) bond and one or more pi (π) bonds. Sigma bonds result from head-on overlap, while pi bonds result from side-by-side overlap of unhybridized p orbitals.

• Double Bond: 1 σ + 1 π bond

• Triple Bond: 1 σ + 2 π bonds

Pi bonds are generally weaker than sigma bonds due to less effective orbital overlap.

Summary Table: Electron-Domain and Molecular Geometries

Number of Electron Domains	Electron-Domain Geometry	Molecular Geometry	Bond Angles	Example
2	Linear	Linear	180°	CO2
3	Trigonal planar	Trigonal planar	120°	SO3
4	Tetrahedral	Tetrahedral	109.5°	CCl4
5	Trigonal bipyramidal	Trigonal bipyramidal	120°, 90°	PCl5
6	Octahedral	Octahedral	90°	SF6

Additional info: The VSEPR model is a powerful tool for predicting molecular shapes, but exceptions can occur, especially with transition metals and molecules with expanded octets. Hybridization theory complements VSEPR by explaining the observed bond angles and molecular geometries through orbital mixing.