VSPER Theory
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VSEPR Rules
Draw the Lewis structure for the molecule or ion.
Count the total number of regions of high electron density (bonding and unshared electron pairs) around the central atom.
Double and triple bonds count as ONE REGION OF HIGH ELECTRON DENSITY. An unpaired electron counts as ONE REGION OF HIGH ELECTRON DENSITY. For molecules or ions that have resonance structures, you may use any one of the resonance structures.
Identify the most stable arrangement of the regions of high electron density as ONE of the following:
linear trigonal planar tetrahedral trigonal bipyramidal octahedral
Determine the positions of the atoms based on the types of electron pairs present (i.e., bonding pairs vs. unshared pairs). For trigonal bipyramidal and octahedral arrangements, there can sometimes be more than one possible arrangement of the bonding and unshared pairs:
Trigonal bipyramidal - place any unshared pairs in the plane of the triangle. Octahedral - if you have two unshared pairs, place them on opposite sides of the central atom. Examples
Identify the molecular structure based on the positions of the ATOMS (NOT on the regions of high electron density).
Lone-pair Lone-pair repulsion> Lone-pair bond-pair repulsion> bond-pair bond-pair repulsion
Therefore angles between lone-pair lone-pair are greatest. This can help rationalize the shape of a molecule of water. The angle betwwen the lone pairs in water is ~104.5 degrees