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.
 Write the Lewis dot structure for the molecule.
 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.
 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.
OVERALL (ELECTRONIC) GEOMETRY


MOLECULAR GEOMETRY

Examples of Determining Molecular Geometry: