Molecular Geometry and VSEPR Theory

Learning Objectives

Explain the basic principle of Valence Shell Electron Pair Repulsion (VSEPR) theory.

Predict the molecular geometry of simple molecules (e.g., linear, trigonal planar, tetrahedral).

Distinguish between electron geometry and molecular geometry.

Determine the polarity of a molecule based on its geometry.

The Shapes of Molecules: VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the 3D geometry of individual molecules from the number of electron pairs surrounding their central atoms.

The Core Principle

The basis of VSEPR is that electron pairs in the valence shell of a central atom will arrange themselves as far apart as possible to minimize electrostatic repulsion. Both bonding pairs (in covalent bonds) and non-bonding pairs (lone pairs) repel each other.

Electron vs. Molecular Geometry

Electron Geometry: The 3D arrangement of all electron pairs (both bonding and lone pairs) around the central atom.

Molecular Geometry: The 3D arrangement of the atoms in the molecule. Lone pairs influence the shape but are not included in the final description of the molecular geometry.

Predicting Shapes

1.Draw the Lewis structure for the molecule.

2.Count the total number of electron domains (bonding pairs + lone pairs) around the central atom.

3.Determine the electron geometry based on minimizing repulsion.

4.Determine the molecular geometry based on the positions of the atoms only.

|---|---|---|---|---|---|

| 2 | Linear | 2 | 0 | Linear | CO₂ |

| 3 | Trigonal Planar | 3 | 0 | Trigonal Planar | BF₃ |

| 3 | Trigonal Planar | 2 | 1 | Bent | SO₂ |

| 4 | Tetrahedral | 4 | 0 | Tetrahedral | CH₄ |

| 4 | Tetrahedral | 3 | 1 | Trigonal Pyramidal | NH₃ |

| 4 | Tetrahedral | 2 | 2 | Bent | H₂O |

Molecular Polarity

Even if a molecule contains polar bonds, it can be nonpolar overall if its shape is symmetrical, causing the bond dipoles to cancel out.

CO₂ (Linear): O=C=O. The two C=O bond dipoles are equal and opposite, so they cancel. CO₂ is nonpolar.

H₂O (Bent): The two O-H bond dipoles point towards the oxygen and do not cancel. H₂O is polar.

Key Terms

VSEPR Theory: Valence Shell Electron Pair Repulsion theory is a model used to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms.
Electron Geometry: The geometric arrangement of all electron pairs (bonding and non-bonding) around the central atom.
Molecular Geometry: The three-dimensional arrangement of the atoms that constitute a molecule. It is determined by the electron geometry but only describes the atom positions.
Bond Dipole: The separation of electrical charge along a bond. It occurs in polar covalent bonds.
Molecular Polarity: The net separation of charge across a whole molecule, determined by the sum of all individual bond dipoles.