molecular geometry

Determining Molecular Geometry using VSEPR

VSEPR (Valence Shell Electron Pair Repulsion) Theory: The basic premise of this simple theory is that electron pairs (bonding and nonbonding) repel one another; so the electron pairs will adopt a geometry about an atom that minimizes these repulsions. Use the method below to determine the molecular geometry about an atom.

  1. Write the Lewis dot structure for the molecule.

  2. Count the number of things (atoms, groups of atoms, and lone pairs of electrons) that are directly attached to the central atom (the atom of interest) to determine the overall (electronic) geometry of the molecule.

    four things three things two things
    tetrahedral trigonal planar linear

  3. Now ignore the lone pairs of electrons to get the molecular geometry of the molecule. The molecular geometry describes the arrangement of the atoms only and not the lone pairs of electrons. If there are no lone pairs in the molecule, then the overall geometry and the molecular geometry are the same. If the overall geometry is tetrahedral, then there are three possibilities for the molecular geometry; if it is trigonal planar, there are two possibilities; and if it is linear, the molecular geometry must also be linear. The diagram below illustrates the relationship between overall (electronic) and molecular geometries. To view the geometry in greater detail, simply click on that geometry in the graphic below. Although there are many, many different geometries that molecules adopt, we are only concerned with the five shown below.

The Relationship Between Overall (electronic) Geometry and Molecular Geometry

Note: click on any geometry in the graphic to view in greater detail.

ImageMap - turn on images!!!

Examples of Determining Molecular Geometry:

Example 1: tetrahedral overall geometry with no lone pairs

Example 2: tetrahedral overall geometry with one lone pair

Example 3: tetrahedral overall geometry with two lone pairs

Example 4: trigonal planar overall geometry with no lone pairs

Example 5: trigonal planar overall geometry with one lone pair