Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110

General Chemistry

Summer 2006

Lecture Notes::Lec 12_15 June

© R. Paselk 2006


VSEPR Theory and Molecular Geometry

VSEPR (Valence Shell Electron Pair Repulsion) Theory is based on three assumptions:

VSEPR predicts geometry based on these assumptions in a few simple, sequential, steps:

  1. Draw a correct Lewis Structure.
  2. Determine the Steric Number = the number of bonded atoms + the number of lone pairs (= regions of electron density).
  3. Maximize the angles between electron pairs, placing the lone (unbonded) pairs at the extremes.

Examples of the various molecular geometries discussed below may be found in the Molecular Geometry Supplement.

For central atoms with eight outer electrons (octets) there are three possible electron pair geometries:

  1. Linear with angles of 180° ( a single pair and a triple bond, or two double bonds).
  2. Trigonal planar with angles of 120° (one double bond and two single pairs).
  3. Tetrahedral with angles of 109.5° (four single pairs). [model]

These three electron pair geometries can lead to five molecular geometries:

Small atoms:

Representative atoms with empty d-shells can also have what are sometimes referred to as expanded valence shells. In these cases the d-orbitals also participate in bonding enabling more bonds to be formed. Thus two additional electronic geometries are possible:

These two electron pair geometries can lead to six new molecular geometries in addition to another way to make a linear molecule:

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Last modified 15 June 2006