LEWIS STRUCTURES
A molecule achieves its structure by arranging its valence electrons into a particular com-
bination of bonds and lone pairs which gives the molecule the lowest energy. The Lewis proce-
dure is a method for how to dole out the valence electrons to bonds and to lone pairs. Valence
Shell Electron Pair Repulsion uses the concept of number of electron pairs to ascribe an elec-
tronic geometry to the Lewis structures. Both procedures condense the rigorous (and difficult!)
quantum mechanics to a set of simple rules.
To draw a Lewis structures make sure that you always determine the number of valence
electrons (VAL below). If there are many electrons and you run into difficulty trying to use the
rules on p. 526 of our text you may try using steps 2 - 6 below.
1. VAL (total number of valence electrons) = sum of the valence electrons of each element (sum
of the group numbers, GN) - charge (if an ion).
2. STAB (stable noble gas configuration) = sum of the electrons in the noble gas configuration
for each element (2 for H, 8 for C and beyond). Since an atom achieves its greatest stability
and lowest energy when it has a filled shell, the noble gas configuration, we assume that an
atom in a molecule or ion will also be most stable when it has its noble gas complement of
electrons around it - the ubiquitous "octet" rule.
3. BOND (total number of bonding electrons) = STAB - VAL and BP (number of bond pairs) =
BOND/2. Since electrons are shared between atoms that are bonded together the "octet"
around each atom over counts the actual number of electrons involved. The amount of over
counting is the number of bonding electrons.
4. LONE (total number of lone pair electrons) = VAL - BOND and LP (number of lone pairs) =
LONE/2. Valence electrons are of two varieties: they are either bonding electrons (BOND)
or nonbonding, lone electrons (LONE).
5. Draw the Lewis structure using your calculated BP and LP. Remember that nature likes sym-
metry, put the odd atom in the center. The central atom will generally be the less electroneg-
ative (more electropositive) element.
6. FC (formal charge on atom) = GN - LONE - BOND/2. The sum of the formal charges on all
atoms is equal to the charge on the molecule or ion.
Exceptions to the octet rule:
1. molecules or ions with an odd number of electrons
2. molecules with electron deficient atoms which are stable with less than eight electrons about
them (such as B and Al with 6 or Be with 4)
3. some molecules or ions containing an element beyond the second period - these must be
treated by a valence shell expansion
Valence Shell Expansion (VSE)
Occasionally the octet rule is violated allowing more than eight electrons to surround an
atom. The simplest case of this occurs when you are trying to attach more atoms to some central
atom than you have bonds determined via calculation with octets. The other common situation
arises when the Lewis structure drawn with octets exhibits charge separation (positive and neg-
ative FC in the same structure) and the VSE structure is more stable. In either case VSE is never
done for first or second period elements.
