Chapter 7: Molecular Geometry, Intermolecular Forces, and Bonding Theories – Study Notes

Chapter 7: Molecular Geometry, Intermolecular Forces, and Bonding Theories

Molecular Geometry (Section 7.1)

Molecular geometry describes the three-dimensional arrangement of atoms in a molecule. The Valence Shell Electron Pair Repulsion (VSEPR) model is used to predict molecular shapes based on the repulsion between electron pairs around a central atom.

• VSEPR Model: Valence Shell Electron Pair Repulsion model. Electron pairs (bonding and nonbonding) arrange themselves as far apart as possible to minimize repulsion.

• Molecular Geometry: The actual shape of the molecule, determined by the positions of the atoms.

• Electron Domain Geometry: The arrangement of all electron domains (bonding and lone pairs) around the central atom.

Counting VSEPR Electron Pairs

• Each bond (single, double, or triple) around the central atom counts as one VSEPR pair.

• Each nonbonding (lone) pair around the central atom counts as one VSEPR pair.

Common Molecular Geometries

Type	Example	Number of VSEPR Pairs	Geometry	Bond Angle
AB2	BeCl2	2	Linear	180°
AB3	BF3	3	Trigonal Planar	120°
AB4	CH4	4	Tetrahedral	109.5°
AB5	PCl5	5	Trigonal Bipyramidal	90°, 120°
AB6	SF6	6	Octahedral	90°

Examples of Geometry Prediction

• AlI3: Trigonal planar; 120°

• CCl4: Tetrahedral; 109.5°

• PBr5: Trigonal bipyramidal; 90°, 120°

• BeH2: Linear; 180°

• HCN: Linear; 180°

Variations on Geometries

Electron Domain Geometry	Molecular Geometry	Example	Bond Angle
Trigonal Planar	Bent (AB2E)	SO2	<120°
Tetrahedral	Trigonal Pyramidal (AB3E)	NH3	107.3°
Tetrahedral	Bent (AB2E2)	H2O	104.5°
Trigonal Bipyramidal	Seesaw (AB4E)	SF4	<120°, <90°
Trigonal Bipyramidal	T-shaped (AB3E2)	ClF3	<90°
Trigonal Bipyramidal	Linear (AB2E3)	I3-	180°
Octahedral	Square Pyramidal (AB5E)	BrF5	<90°
Octahedral	Square Planar (AB4E2)	XeF4	90°

Note: Lone pairs cause bond angles to decrease due to increased repulsion compared to bonding pairs.

Molecular Geometry vs. Electron Domain Geometry

• Electron Domain Geometry: Considers all electron domains (bonding and lone pairs).

• Molecular Geometry: Considers only the arrangement of atoms (ignores lone pairs).

• For most purposes, molecular geometry is the focus when describing the shape of a molecule.

Summary Table: Geometry Types and Examples

Class	Geometry	Example	Bond Angle
AB2	Linear	BeCl2	180°
AB3	Trigonal Planar	BF3	120°
AB2E	Bent	SO2	<120°
AB4	Tetrahedral	CH4	109.5°
AB3E	Trigonal Pyramidal	NH3	107.3°
AB2E2	Bent	H2O	104.5°
AB5	Trigonal Bipyramidal	PCl5	90°, 120°
AB4E	Seesaw	SF4	<120°, <90°
AB3E2	T-shaped	ClF3	<90°
AB2E3	Linear	I3-	180°
AB6	Octahedral	SF6	90°
AB5E	Square Pyramidal	BrF5	<90°
AB4E2	Square Planar	XeF4	90°

Additional info: These notes cover the foundational aspects of molecular geometry using the VSEPR model, including how to predict shapes and bond angles for a variety of molecules. The tables summarize the main geometries and their examples, which are essential for understanding molecular structure in General Chemistry.