Valence Shell Electron Pair Repulsion (VSEPR)

VSEPR is a simple theory used to predict the shape of a molecule. It assumes that groups of electrons, either non-bonding (lone pairs) or groups of bonding electrons (this would include ALL bonding electrons to an atom, i.e. single, double, or triple bonds) will arrange themselves around the central atom to minimize the electron-electron repulsions. This results in five basic VSEPR electronic geometries; linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral, based on two, three, four, five and six groups of electrons respectively.

• There maybe more than one molecular geometry associated with each electronic geometry.
• The electronic geometry refers to ALL electrons, both bonding and non-bonding, that surround the central atom.
• The molecular geometry refers to the relative locations of only the atoms.
• VSEPR notation is a common shorthand for the electronic structure. In this notation A represents the central atom, X the atom (or group of atoms) bonded to the central atom, and E lone pairs of electrons.
• Placement of electrons?
  • The starting point is a valid Lewis Structure.
  • If there are lone pair(s) of electrons, arrange them around the central atom to maximize their distance to other lone pair electrons first, and then to other bonding groups of electrons (i.e. see the AX₄E, AX₃E₂ and AX₄E₂ geometries below, this is because the lone pair electrons are closer to the nucleus and thus repel bonding electrons stronger than other bonding electron pairs repel each other).
  • If there are multiple bonds present arrange them around the central atom to maximize their distance to other pairs of electrons. Since there are four electrons in a double bond they will repel another pair of electrons stronger than the two electrons in a single bond. Triple bonds with six electrons will repel other bonding electrons even stronger. See the figure below:

    The oxygen is doubly bonded while the F atoms are singly bonded, the higher electron density in the double bond means it requires a larger repulsion between it and any other bonds. The four equatorial bonding-pair bonding-pair contacts of 2 x 120° and 2 x 90° (left image) is more favourable than the than the three bonding-pair bonding-pair contacts of 3 x 90° (right image).

The images are displayed using Jmol. Check this link if you are not familiar with Jmol.

Note: the atom colors are:

B C N O F S Br I P Te Xe H

Electronic Geometry	Molecular Geometry     3-D Jmol images (requires Red-Cyan (or Red-Blue) anaglyph 3D stereo glasses)
Linear	Linear (AX2) CO2
Trigonal Planar	Trigonal Planar (AX3) BH3 Bent (AX2E) SO2
Tetrahedral	Tetrahedral (AX4) CH4 Trigonal Pyramidal (AX3E) NH3 Bent (AX2E2) H2O
Trigonal Bipyramidal	Trigonal Bipyramidal (AX5) PF5 See Saw (AX4E) TeF4 T-Shaped (AX3E2) BrF3 Linear (AX2E3) XeF2
Octahedral	Octahedral (AX6) SF6 Square Pyramidal (AX5E) IF5 Square Planar (AX4E2) XeF4