Molecular Structure and Orbitals: VSEPR Theory and Electron Pair Geometry

Molecular Structure and Theories of Bonding

Introduction to Molecular Structure

Molecular structure refers to the three-dimensional arrangement of atoms within a molecule. Understanding this structure is essential for predicting molecular properties and reactivity. Several theories have been developed to explain and predict molecular structure:

• Lewis Theory: Predicts connectivity by converting molecular formulas into Lewis structures, showing how atoms are bonded and where lone pairs reside. However, it does not predict the 3D shape of molecules or account for the role of atomic orbitals.

• Valence Shell Electron Pair Repulsion (VSEPR) Theory: Addresses the 3D arrangement of atoms by considering electron pair repulsions.

• Hybrid Orbital Theory: Explains the formation of equivalent bonding orbitals through the mixing of atomic orbitals.

• Molecular Orbital (MO) Theory: Describes bonding in terms of molecular orbitals that extend over the entire molecule.

The VSEPR Model

Basic Principles of VSEPR

The Valence Shell Electron Pair Repulsion (VSEPR) model is used to predict the geometry of molecules based on the repulsion between electron pairs (bonding and lone pairs) around a central atom. The main idea is that electron pairs arrange themselves as far apart as possible to minimize repulsion, determining the molecular shape.

• Molecular structure: The 3D arrangement of atoms in a molecule.

• Electron pair domains: Regions where electrons are likely to be found, including both bonding pairs and lone pairs.

Electron Pair Domains and Molecular Geometry

The number of electron domains (steric number) around a central atom determines the electron-pair geometry:

Electron Domains	Electron-Pair Geometry	Bond Angle	Example
2	Linear	180°	BeCl2, CO2
3	Trigonal planar	120°	BH3, CCl2
4	Tetrahedral	109.5°	CH4, NH3, H2O
5	Trigonal bipyramidal	90°, 120°	PF5
6	Octahedral	90°	SF6

Steric Number and Molecular Structure

• Steric Number 2: Linear geometry (e.g., BeCl2, CO2).

• Steric Number 3: Trigonal planar geometry (e.g., BH3); bent geometry if one domain is a lone pair (e.g., CCl2).

• Steric Number 4: Tetrahedral geometry (e.g., CH4); trigonal pyramidal (e.g., NH3) or bent (e.g., H2O) if there are lone pairs.

• Steric Number 5: Trigonal bipyramidal geometry (e.g., PF5).

• Steric Number 6: Octahedral geometry (e.g., SF6).

Lone Pair Trends and Bond Angles

• Bonding pairs are shared between two nuclei and can be close to either nucleus.

• Lone pairs are localized on one nucleus and occupy more space than bonding pairs.

• Lone pairs compress the angles between bonding pairs, resulting in smaller bond angles than the ideal geometry.

Example: In NH3 (ammonia), the bond angle is 107° (less than 109.5°), and in H2O (water), it is 104.5° due to the presence of lone pairs.

Problem Solving Strategy: Applying the VSEPR Model

1. Draw the Lewis structure for the molecule.

2. Count the electron pairs (bonding and lone pairs) around the central atom.

3. Arrange the pairs to minimize repulsion (as far apart as possible).

4. Determine the positions of the atoms based on shared electron pairs.

5. Name the molecular structure based on the positions of the atoms.

Example: For CH4, the central carbon has four bonding pairs, resulting in a tetrahedral geometry with 109.5° bond angles.

Summary Table: Electron Groups, Bonds, Lone Pairs, and Molecular Structure

Number of Groups Around Central Atom	Bonds / Lone Pairs	Electron-Pair Arrangement	Molecular Structure (What Can You See?)
2	2 bonds, 0 lone pairs	Linear	Linear (e.g., BeCl2)
3	3 bonds, 0 lone pairs	Trigonal planar	Trigonal planar (e.g., BH3)
3	2 bonds, 1 lone pair	Trigonal planar	Bent (e.g., SO2)
4	4 bonds, 0 lone pairs	Tetrahedral	Tetrahedral (e.g., CH4)
4	3 bonds, 1 lone pair	Tetrahedral	Trigonal pyramidal (e.g., NH3)
4	2 bonds, 2 lone pairs	Tetrahedral	Bent (e.g., H2O)

Additional info: The above table is inferred from standard VSEPR theory and the context of the slides.