Molecular Geometry and VSEPR Theory: Shapes and Polarity of Molecules

Molecular Geometry and VSEPR Theory

Introduction to VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory is used to predict the three-dimensional shapes of molecules. According to VSEPR, a molecule adopts the shape that minimizes repulsion between electron pairs (both bonding and lone pairs) around the central atom.

• Electron pairs (bonding and lone pairs) arrange themselves as far apart as possible to minimize repulsion.

• The resulting geometry determines the physical and chemical properties of the molecule.

Determining Molecular Geometry

Steps for Predicting Molecular Shape

1. Draw the Lewis Structure of the molecule.

2. Count the charge clouds (electron domains) around the central atom. Each lone pair, single, double, or triple bond counts as one charge cloud.

3. Arrange the charge clouds as far apart as possible to minimize repulsion.

Types of Electron Domains

• Bonding pairs: Shared electrons between atoms (single, double, or triple bonds).

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

Common Molecular Geometries

Summary Table: Electron Domains, Parent Geometry, and Molecular Geometry

Number of Electron Domains	Parent Geometry	Molecular Geometry (No Lone Pairs)	Bond Angles	Example
2	Linear	Linear	180°	CO2
3	Trigonal planar	Trigonal planar	120°	BF3
4	Tetrahedral	Tetrahedral	109.5°	CH4
5	Trigonal bipyramidal	Trigonal bipyramidal	90°, 120°, 180°	PF5
6	Octahedral	Octahedral	90°, 180°	SF6

Effect of Lone Pairs on Molecular Geometry

• Lone pairs occupy more space than bonding pairs, causing bond angles to decrease from the ideal values.

• Parent geometry describes the arrangement of all electron domains.

• Molecular geometry describes the arrangement of only the atoms (bonding pairs).

Examples of Molecular Geometries with Lone Pairs

Electron Domains	Lone Pairs	Molecular Geometry	Bond Angles	Example
3	1	Bent	<120°	SO2
4	1	Trigonal pyramidal	~107.5°	NH3
4	2	Bent	~104.5°	H2O
5	1	See-saw	90°, 120°, 180°	SF4
5	2	T-shaped	90°, 180°	ClF3
5	3	Linear	180°	XeF2
6	1	Square pyramidal	90°, 180°	BrF5
6	2	Square planar	90°, 180°	XeF4

Examples of Molecular Geometry

• CO2: Linear, O-C-O angle = 180°

• BF3: Trigonal planar, F-B-F angle = 120°

• CH4: Tetrahedral, H-C-H angle = 109.5°

• H2O: Bent, angle = 104.5°

• NH3: Trigonal pyramidal, angle = 107.5°

• PF5: Trigonal bipyramidal, angles = 90°, 120°, 180°

• SF6: Octahedral, angles = 90°, 180°

Polarity of Molecules

Definition of Polarity

• A polar molecule has a positive and a negative end (a dipole).

• Polarity depends on both the individual bond dipoles and the overall molecular geometry.

Determining Molecular Polarity

• If the bond dipoles do not cancel out due to the molecular geometry, the molecule is polar.

• Examples:

  • CO2: Nonpolar (linear geometry, dipoles cancel)

  • H2O: Polar (bent geometry, dipoles do not cancel)

  • NH3: Polar (trigonal pyramidal geometry)

  • CH4: Nonpolar (tetrahedral, dipoles cancel)

  • CH3F: Polar (tetrahedral, dipoles do not cancel)

  • Cis-difluoroethene: Polar; Trans-difluoroethene: Nonpolar

Dipole Moment

• The dipole moment ($\mu$) quantifies the polarity of a molecule.

• Measured in Debye (D).

• Greater dipole moment indicates higher polarity.

Sample Table: Dipole Moments of Selected Molecules

Molecule	Dipole Moment (D)	Geometry
HCl	1.08	Linear
H2O	1.85	Bent
NH3	1.47	Trigonal pyramidal
CO2	0	Linear
CH4	0	Tetrahedral

Practice Questions

• What is the molecular geometry and the bond angles for each of the following?

  • BH2-

  • NI3

  • ClF4-

  • SF5-

Summary

• VSEPR theory allows prediction of molecular shapes based on electron pair repulsion.

• Molecular geometry is determined by the number of bonding and lone pairs around the central atom.

• Polarity depends on both bond dipoles and molecular geometry.

• Dipole moments provide a quantitative measure of molecular polarity.