The Valence Shell Electron Repulsion (VSEPR) model

  • can predict the structure of most molecules and polyatomic ions in which the central atom is a nonmetal;
  • it also works for some structures in which the central atom is a metal.
  • pairs of electrons (in bonds and in lone pairs) repel each other.The pairs of electrons (in bonds and in lone pairs) are called “groups”.
  • Because electrons repel each other electrostatically, the most stable arrangement of electron groups (i.e., the one with the lowest energy) is the one that minimizes repulsion.
  • Groups are positioned around the central atom in a way that produces the molecular structure with the lowest energy.
  • it accurately predicts the three-dimensional structures of a large number of compounds.

 

We can use the VSEPR model to predict the geometry around the atoms in a polyatomic molecule or ion by focusing on the number of electron pairs (groups) around a central atom of interest. Groups include bonded and unbonded electrons; a single bond, a double bond, a triple bond, a lone pair of electrons, or even a single unpaired electron each count as one group. The molecule or polyatomic ion is given an AXmEn designation, where A is the central atomX is a bonded atomE is a nonbonding valence electron group (usually a lone pair of electrons), and m and n are integers. The number of groups is equal to the sum of m and n. We can describe the molecular geometry around a central atom, that is, the arrangement of the bonded atoms in a molecule or polyatomic ion.

The geometries that are predicted from VSEPR when a central atom has only bonded groups (0) are listed below

Geometries predicted using VSEPR theory (bonded groups only).

Groups around central atom

(m + n)

Geometry Name Geometry Sketch Predicted bond Angle Example
2 linear clipboard_eb9312ecdde2fa2c50e199d4916d68e1d.png 180° clipboard_efb6914731f6cd38585e16eabce2edcd0.png
3 trigonal plane clipboard_e3182b9b1c6d80b5ebdad2c66b73a2e6f.png 120° clipboard_e8cf2169220755683f691f8b2cae5242f.png
4 tetrahedron clipboard_ea695c60caef5629399acfb59769cfa0e.png 109.5° clipboard_e8da1c4a1a3b8e70de878140e4f2c2817.png
5 trigonal bipyramid clipboard_ee55b4f7ca03ee8477e1f5d6b204fd45b.png 90° and 120° clipboard_ed3acddc2a2a03fb6aa4c42074a7d5957.png
6 octahedron clipboard_ea37bd9086c0fb6b42452a9746fef9914.png 90° clipboard_e953f47ae566e30f4ea43ff7c8943aed7.png
7 pentagonal bipyramid clipboard_e35b713ebbc9ad70595fbe238e336c51e.png 90° and 72° clipboard_e6baf80619a2a2ba3cdc98bbf0a99a844.png
8 square antiprism clipboard_e7a25159fdfe4d982a4d39b00fd28fdf7.png 70.5°, 99.6° and 109.5° clipboard_ed2d62e97b03a35a92d358896ff7d3104.png

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