Determining the molecular shape, bond angles and polarity from a Lewis dot structure
A. Draw a correct Lewis dot structure of the molecule or ion
1. Count the total number of valence electrons in the molecule or ion. Any charge on an ion will either add (if a
negative ion) or subtract (if a positive ion) the number on the charge to/from the total number of valence electrons.
2. Arrange the atoms in the molecule or ion in such a way that the basic connectivity is shown. For most simple
Lewis dot structures, there is a single central atom flanked by the other atoms.
3. Start by drawing single bonds between the central atom and all the other atoms. Recall that a single bond counts
as two electrons.
4. Continue by adding lone pairs as necessary to fulfill the octet rule on those atoms that follow the octet rule (for
instance, hydrogen follows the duet rule, so by drawing a single bond to a hydrogen, you have already fulfilled the
duet rule for that hydrogen).
5. Count the total number of valence electrons in the structure you’ve drawn. If this number matches the number in
step 1, then you are done. If this number is greater than the number in step 1, replace a single bond with a double
bond, and remove a lone pair from each of the atoms connected by the new double bond. Continue this
replacement of single bonds by double bonds until the number of valence electrons in the structure matches the
number of valence electrons found in step 1.
6. If the structure is that of an ion, put large brackets around the ion’s structure, then add the charge of the ion as a
B. Determining the molecular shape and bond angles.
1. Identify the central atom in the Lewis dot structure.