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CHEM 141
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Drawing Lewis Structures

1. Count all the valence electrons, taking into account the charge if it is an ion.

2. Choose the central atom.  It will be the atom which wants the most bonds.  (for two atoms in the same column, choose the one farther down the column).

3. Arrange the electrons in such a way that every atom has eight electrons, except for hydrogen which will have two electrons.

4. If necessary, assign formal charge.  Atoms with too many bonds get a positive charge.  Atoms with not enough bonds get a negative charge.

The Make-Everybody-Happy Method

1. Choose the central atom as above.

2. Make every atom as happy as possible (while also making the overall charge work).

3. Assign formal charge as above.

The "Easy" Method (AXnn = hydrogen or halogen)

1. Count all the valence electrons for the central atom, and one electron for each substituent atom.  Take into account the total charge if necessary.

2. Divide the total by two.  The result gives you the geometry of the molecule (2 = linear, 3 = triangular planar, 4 = tetrahedral, 5 = tbp, 6 = octahedral)

Last Updated: 11/17/2014
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