VSEPR Theory, Molecular Geometry, Polarity, and Intermolecular Forces Study Guide

VSEPR Theory and Molecular Geometry

Introduction to VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the geometry of individual molecules based on the number of electron pairs surrounding their central atoms. The theory assumes that electron pairs (bonding and lone pairs) will arrange themselves as far apart as possible to minimize repulsion, thus determining the molecular shape.

• Electron domains include both bonding pairs and lone pairs of electrons around the central atom.

• Molecular geometry is determined by the arrangement of atoms (not lone pairs).

• Bond angles are influenced by the number of electron domains and the presence of lone pairs, which repel more strongly than bonding pairs.

Example: In methane (CH4), four bonding pairs around carbon arrange in a tetrahedral geometry with bond angles of approximately 109.5°.

Key VSEPR Geometries and Bond Angles

• Linear: 2 electron domains, bond angle = 180°

• Trigonal planar: 3 electron domains, bond angle = 120°

• Tetrahedral: 4 electron domains, bond angle = 109.5°

• Trigonal pyramidal: 4 electron domains (3 bonding, 1 lone pair), bond angle ≈ 107°

• Bent: 3 or 4 electron domains (2 bonding, 1 or 2 lone pairs), bond angle < 120° or < 109.5°

Lewis Structures and Electron Domains

Drawing Lewis structures helps identify the number of bonding and lone pairs, which is essential for predicting geometry using VSEPR theory.

• Bonding regions: Number of bonds to the central atom.

• Lone pairs: Non-bonding electron pairs on the central atom.

• Total electron regions: Sum of bonding regions and lone pairs.

Table: Common Molecules and Their Geometries

Molecule	Lewis Structure	Bonding Regions	Lone Pairs	Total Electron Regions	Electron Geometry	Molecular Geometry	Bond Angle
HCN	H–C≡N	2	0	2	linear	linear	180°
H2S	H–S–H (2 lone pairs on S)	2	2	4	tetrahedral	bent	<109.5°
CO2	O=C=O	2	0	2	linear	linear	180°
SO2	O–S=O (1 lone pair on S)	2	1	3	trigonal planar	bent	<120°
NO2-	O–N=O (1 lone pair on N)	2	1	3	trigonal planar	bent	<120°
NH3	N with 3 H and 1 lone pair	3	1	4	tetrahedral	trigonal pyramidal	107°
CH4	C with 4 H	4	0	4	tetrahedral	tetrahedral	109.5°

Polarity and Intermolecular Forces

Bond Polarity and Molecular Polarity

Bond polarity arises from differences in electronegativity between atoms. A molecule is polar if it has an uneven distribution of electron density, resulting in a dipole moment. Molecular geometry affects whether bond dipoles cancel or reinforce each other.

• Nonpolar molecules: Symmetrical geometry with identical surrounding atoms (e.g., CO2).

• Polar molecules: Asymmetrical geometry or different surrounding atoms (e.g., H2O).

Example: Water (H2O) is bent and polar; carbon dioxide (CO2) is linear and nonpolar.

Types of Intermolecular Forces

• Dispersion (London) forces: Present in all molecules, especially nonpolar ones; arise from temporary dipoles.

• Dipole-dipole forces: Occur between polar molecules due to permanent dipoles.

• Hydrogen bonding: Strong dipole-dipole interaction when H is bonded to N, O, or F.

Table: Molecular Geometry, Polarity, and Intermolecular Forces

Molecule	Molecular Geometry	Bond Angle	Molecular Polarity	Major Intermolecular Force
BF3	trigonal planar	120°	nonpolar	dispersion
CO2	linear	180°	nonpolar	dispersion
CH4	tetrahedral	109.5°	nonpolar	dispersion
H2O	bent	~109.5°	polar	hydrogen bonding
NH3	trigonal pyramidal	107°	polar	hydrogen bonding
NF3	trigonal pyramidal	107.5°	polar	dipole-dipole

Practice: Drawing Lewis Structures and Predicting Geometry

Steps for Determining Molecular Geometry

1. Draw the Lewis structure for the molecule or ion.

2. Count the number of bonding regions and lone pairs around the central atom.

3. Determine the electron geometry using VSEPR theory.

4. Identify the molecular geometry (shape) based on the positions of atoms only.

5. Estimate bond angles based on geometry and presence of lone pairs.

6. Assess molecular polarity by considering bond dipoles and overall shape.

7. Identify the major intermolecular force present.

Example: For NF3:

• Lewis structure: N with three F atoms and one lone pair.

• Electron geometry: tetrahedral.

• Molecular geometry: trigonal pyramidal.

• Bond angle: ~107.5°.

• Molecular polarity: polar.

• Major intermolecular force: dipole-dipole.

Additional Info

• Resonance structures may be present in polyatomic ions (e.g., carbonate ion, nitrate ion).

• Bond angles may deviate from ideal values due to lone pair repulsion.

• Intermolecular forces influence boiling point, melting point, and solubility.