Lewis Structures
The most difficult part of drawing Lewis structures is writing the skeleton structure
of the molecules. As a general rule, the less electronegative element is at the center
of the molecule.
Example 1: the formulas of thionyl chloride (SOCl2) and sulfuryl chloride (SO2Cl2)
can be transferred into the following skeleton structures.
It is also useful to recognize that the formulas for complex molecules are often
written in a way that hints at the skeleton structure of the molecule.
Example 2: Dimethyl ether is often written as CH3OCH3, which translates to the
following skeleton structure.
Finally, it is useful to recognize that many compounds that are acids contain O-H
bonds.
Example 3: The formula for acetic acid is often written as CH3COOH, because this
molecule contains the following skeleton structure.
A step-by-step approach to drawing Lewis structures
1.
2.
3.
4.
Determine the total number of valence electrons
Write the skeleton structure of the molecule
Use two valence electrons to form each bond in the skeleton structure
Try to satisfy the octets of the terminal atoms by distributing the remaining
valence electrons as non-bonding electrons
5. Try to satisfy the octets of the central atom by distributing the remaining
electrons as non-bonding electrons.
Example 4: chlorate ion ClO3-
Molecule that contain too many or not enough electrons
Too few electrons
Occasionally we encounter a molecule that doesn’t seem to have enough valence
electrons. If we can’t get a satisfactory Lewis structure by sharing a single pair of
electrons, it may be possible to achieve this goal by sharing two or even three pairs
of electrons.
The elements that form strong double or triple bonds are C, N, O, P and S.
Example 5: Consider Formaldehyde (H2CO)
Every once and awhile, we encounter a molecule for which it is impossible to write a
satisfactory Lewis structure
Example 6: consider boron trifluoride (BF3), which contains 24 valence electrons
BF3: 3+3(7)=24
Because neither boron nor fluoride falls in this category, we have to stop with what
appears to be an unsatisfactory lewis structure.
Too many electrons
It is also possible to encounter a molecule that seems to have too many valence
electrons. When this happens, we expand the valence shell of the central atom
Example 7: Consider the Lewis structure for sulfur tetrafluoride (SF4)