VSEPR stands for valence shell electron pair repulsion, and it is used to predict molecular shape. Only valence shell electrons are important for determining the shape of a molecule. Bonded pairs and lone pairs are treated approximately equally. The valence pairs repel each other; the molecular shape is given by the positions of the pairs that result in the least possible repulsion (maximum separating distance). To predict the shape of a molecule, follow these steps:
- Draw the Lewis structure of the molecule.
- Count the total number of bonding pairs () and the total number of lone pairs () around the central atom. The shape is then written AXnEm.
- Refer to the table on page 798 of the textbook to predict the shape.
To draw the molecule, use straight lines for single bonds, double and triple lines for double and triple bonds, dashed lines for single bonds going into the page, and wedged lines for single bonds coming out of the page. Include bond lines to lone pairs only for three-dimensional molecules. Remember to enclose everything in brackets with a superscript charge if it’s a polyatomic ion. Add bond dipoles if you are explaining the polarity of the molecule. Indicate the bond angles if asked—you should memorize all of the following bond angles:
- linear: 180º
- trigonal planar: 120º
- trigonal planar (with double bond): 118º, 121º
- V-shaped/bent: 104.5º
- trigonal pyramidal: 107.3º
- tetrahedral: 109.5º