6.8 Shapes and Polarity of Molecules

Introduction to Molecular Geometry

The three-dimensional structure of molecules is determined by the arrangement of atoms and electron groups around a central atom. Understanding molecular shapes is essential for predicting chemical behavior and classifying molecules as polar or nonpolar.

• Methane (CH4) is an example where the central carbon atom is bonded to four hydrogen atoms, resulting in a tetrahedral geometry with bond angles of 109°.

• Learning Goal: Predict the three-dimensional structure of a molecule and classify it as polar or nonpolar.

VSEPR Theory

Valence Shell Electron-Pair Repulsion Theory

VSEPR Theory explains the orientation of electron groups around a central atom to minimize repulsion, thereby determining the molecular shape.

• Electron groups are arranged as far apart as possible around the central atom.

• The specific shape of a molecule is determined by the number of atoms attached to the central atom.

• Core Chemistry Skill: Predicting molecular shape using VSEPR theory.

Central Atoms with Two Electron Groups

Linear Geometry

When a central atom has two electron groups, the groups are placed on opposite sides to minimize repulsion, resulting in a linear shape.

• Example: In carbon dioxide (CO2), the central carbon atom has two electron groups, leading to a linear arrangement with bond angles of 180°.

• Shape: Linear

Central Atoms with Three Electron Groups

Trigonal Planar and Bent Geometry

Three electron groups around a central atom can result in different shapes depending on the presence of lone pairs.

• Trigonal Planar: If all three groups are bonded atoms (e.g., formaldehyde, H2CO), the shape is trigonal planar with bond angles of 120°.

• Bent: If one group is a lone pair (e.g., sulfur dioxide, SO2), the shape is bent with bond angles of approximately 120°.

Central Atoms with Four Electron Groups

Tetrahedral, Trigonal Pyramidal, and Bent Geometry

Four electron groups around a central atom can result in several shapes:

• Tetrahedral: Four bonded atoms (e.g., methane, CH4) with bond angles of 109°.

• Trigonal Pyramidal: Three bonded atoms and one lone pair (e.g., ammonia, NH3).

• Bent: Two bonded atoms and two lone pairs (e.g., water, H2O).

Molecular Shapes, Electron-Groups

Table 6.16: Molecular Shapes for a Central Atom with Two, Three, and Four Bonded Atoms

Guide to Predicting Molecular Shape

Steps for Using VSEPR Theory

1. Draw the Lewis structure of the molecule.

2. Arrange the electron groups around the central atom to minimize repulsion.

3. Determine the shape by considering the atoms bonded to the central atom.

Example: Predicting Molecular Shape of H2S

Step-by-Step Application

1. Draw the Lewis structure: H–S–H, with two lone pairs on S.

2. Arrange electron groups: Four electron groups (two bonds, two lone pairs) around S, arranged tetrahedrally.

3. Determine the shape: The molecule is bent, with a bond angle of approximately 109°.

Study Check: Application of VSEPR Theory

Practice Problems

State the number of electron groups and lone pairs, and use VSEPR theory to determine the shape of the following molecules or ions:

• PF3: Four electron groups (three bonds, one lone pair) → trigonal pyramidal

• H2O: Four electron groups (two bonds, two lone pairs) → bent

• CCl4: Four electron groups (four bonds, no lone pairs) → tetrahedral

Additional info: The notes provide foundational concepts for GOB Chemistry, including molecular geometry, VSEPR theory, and practical examples for predicting molecular shapes. The tables summarize key geometries and their corresponding examples, which are essential for understanding chemical structure and reactivity.