VSEPR Theory and Molecular Geometry: General Chemistry Study Notes

Chemical Bonding II: VSEPR Theory

Introduction to VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) Theory is a model used to predict the shapes of molecules based on the repulsion between electron groups around a central atom. This theory helps explain molecular geometry by considering both bonding pairs and lone pairs of electrons.

• Electron groups include bonded atoms (single, double, or triple bonds) and lone pairs.

• Electron groups repel each other due to coulombic (electrostatic) forces.

• Lone pairs require more space than bonding pairs, leading to greater repulsion and affecting molecular shape.

• Electron groups arrange themselves as far apart as possible to minimize repulsion and lower the molecule's potential energy.

Counting Electron Groups

The geometry of a molecule is determined by the number of electron groups around the central atom (or all interior atoms, if more than one).

• Each of the following counts as a single electron group: a single bond, a double bond, a triple bond, a lone pair, or a single electron (free radical).

• Repulsion order: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair.

• The presence of lone pairs usually makes bond angles smaller than the ideal.

VSEPR Theory: Five Basic Shapes

2 Electron Groups: Linear Geometry

When there are two electron groups around the central atom, the molecule adopts a linear geometry with a bond angle of 180°.

• Examples: BeCl2, CO2

• Bond angle: 180°

3 Electron Groups: Trigonal Planar Geometry

Three electron groups around the central atom result in a trigonal planar geometry with bond angles of approximately 120°.

• Examples: BF3, CH2O

• Bond angles: 120°, but may be slightly less if lone pairs are present

4 Electron Groups: Tetrahedral Geometry

Four electron groups around the central atom lead to a tetrahedral geometry with bond angles of approximately 109.5°.

• Examples: CH4, NH4+

• Bond angles: 109.5°

5 Electron Groups: Trigonal Bipyramidal Geometry

Five electron groups around the central atom result in a trigonal bipyramidal geometry with bond angles of 90°, 120°, and 180°.

• Examples: PCl5, AsF5

• Bond angles: 90°, 120°, 180°

6 Electron Groups: Octahedral Geometry

Six electron groups around the central atom produce an octahedral geometry with bond angles of 90°.

• Examples: SF6, SeCl6

• Bond angles: 90°

Special Connection: Central vs. Terminal Atoms

Why Only Central Atom Electron Groups Matter

The geometry of a molecule is determined by how the terminal atoms are arranged around the central atom, which is influenced by how the electron groups are arranged around the central atom. The electron groups on the terminal atoms do not affect this arrangement.

• Key Point: Only electron groups on the central atom (or interior atoms) are considered when determining molecular geometry.

Summary Table: Electron Groups and Molecular Geometry

Electron Groups	Electron Geometry	Molecular Geometry	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°	PCl5
6	Octahedral	Octahedral	90°	SF6

Example Applications

• CO2: Linear geometry, 180° bond angle.

• CH4: Tetrahedral geometry, 109.5° bond angle.

• BF3: Trigonal planar geometry, 120° bond angle.

Key Terms

• Electron group: Any region of electron density around a central atom, including bonds and lone pairs.

• Lone pair: A pair of valence electrons not involved in bonding.

• Bond angle: The angle formed between three atoms across at least two bonds.

• Central atom: The atom in a molecule to which other atoms are bonded and around which geometry is determined.

Important Equations

• For VSEPR, the number of electron groups is determined by the Lewis structure.

• Bond angles for ideal geometries:

  • Linear: $180^\circ$

  • Trigonal planar: $120^\circ$

  • Tetrahedral: $109.5^\circ$

  • Trigonal bipyramidal: $90^\circ$, $120^\circ$, $180^\circ$

  • Octahedral: $90^\circ$

Summary

VSEPR theory provides a systematic way to predict molecular shapes based on the number and types of electron groups around a central atom. Understanding these shapes is essential for predicting molecular properties and reactivity.