CHEM 4725/8725
Lewis Dot Structures and VSEPR
Rules for Drawing Lewis Dot Structures:
1. Atoms normally form bonds by sharing enough electrons to achieve an
octet (except H  2) using the ns and 3 np orbitals.
2. Count the # of electrons each atom (and charges) contributes to
determine the total # of valence electrons available (V).
3. Count the # of electrons needed for all atoms to obey the octet rule to
determine the total # of electrons needed (A).
4. Determine the number of bonding electrons: B = A-V
5. Determine the number of bonds needed: B/2
6. Draw the skeleton of the molecule and add sigma bonds between
atoms
7. Add pi bonds if B requires it.
8. Add lone pairs to account for all valence electrons V.
9. Add formal charges: FC = (# of valence electrons an atom starts with)
– (# of bonds + # of nonbonded electrons)
Rules for Selecting More Important “Canonical” Lewis Structures:
1.
2.
3.
4.
The least electronegative atom is almost always the central atom.
Maximize the number of covalent bonds, consistent with other “rules.”
Minimize formal charges as much as possible
Distribute the formal charges in a “reasonable” way:
a. Negative formal charges on the most electronegative atom;
positive formal charges on the least electronegative
b. Place opposite formal charges as close as possible; place like
formal charges as far away as possible.
Hypervalency and “Natural Resonance” Structures:
1. “Hypervalent” atoms are typically defined as atoms that appear exceed
the octet rule. This is a bad description for a number of reasons: most
don’t actually exceed the octet rule and instead are just poorly
characterized by basic Lewis theory.
2. “Natural Resonance” theory uses ionic bonding pictures to account for
what are, in actuality, 3 center 4 electron bonds (or other bonds better
described as molecular bonds instead of local bonds).
3. Starting with the “hypervalent”, create ionic bonds until octet rule is
satisfied:
CHEM 4725/8725
Summary of VSEPR Model:
1. Electron pairs tend to arrange themselves “as if” they are minimizing
repulsions. Ideal geometries are:
a. 2  Linear
b. 3  Trigonal Planar
c. 4  Tetrahedral
d. 5  Trigonal Bipyramid
e. 6  Octahedral
f. 7  “Capped” Octahedral or Pentagonal Bipyramid
2. The relative magnitudes of electron pair repulsions follow the order:
lone pair-lone pair > lone pair-bond pair > bond pair-bond pair.
3. When lone pairs are present, the bond angles are less than predicted.
4. Lone pairs choose the “largest” site, e.g. equatorial in trig. Bipyramid.
5. If all sites are equivalent, lone pairs will choose a trans arrangement.
6. To first order approximation, the sigma framework determines VSEPR
geometry; pi bonding is typically irrelevant.