LEWIS DOT STRUCTURES
1. Arrange the symbols such that the least
electronegative element is in the center and the other
elements are surrounding the central atom.
O C O
2. Give each of the elements their appropriate number of
valence electrons (dots). Remember the number of
valence electrons for a representative element is the
same as the group number.
..
..
:O
:C: O:
..
..
3. Keep track of the total numbr of valence electrons for
the compound by adding the valence electrons from each
atom. If the compound is an ion then add electrons (dots) for each
negative charge or subtract electrons (dots) for each positive
charge.
4 for C and 6 for O (twice) = 16 electrons
LEWIS DOT STRUCTURES
4. Now move the dots around so that you have 8 dots
(the octet rule) around each element (do not forget the
exceptions) while at the same time keeping the dots in
pairs. Electrons, at this point, exist as pairs (the buddy system).
5. EXCEPTIONS TO THE OCTET RULE: Group I, II, and III
need only 2, 4, and 6 electrons, respectively, around that atom.
6. If there are too few pairs to give each atom eight
electrons, change the single bonds between two atoms
to either double or triple bonds by moving the unbonded
pairs of electrons next to a bonding pair.
..
..
:O: :C: : O:
LEWIS DOT STRUCTURES
7. Once the octet rule has been satisfied for each atom
in the molecule then you may replace each pair of dots
between two atoms with a dash.
..
..
:O =C= O:
8. Now check your structure by
a) count the total number of electrons to make sure
you did not lose or gain electrons during the process.
b) Use FORMAL CHARGE (FC) calculations as a
guideline to the correct structure. A zero formal charge
is usually a good indication of a stable structure.
FC (X) = # of valence electrons - (1/2 bonding electrons + nonbonding electrons)
For our example: FC(C) = 4 - (1/2 8 + 0) = 0
FC(O) = 6 - (1/2 4 + 4) = 0
The Basics: Drawing Lewis Structures
Step 1: Calculate the total number of valence electrons in the
molecule or ion
Step 2: Determine the central atom(s) of the molecule or ion –
usually it’s the least electronegative atom.
Step 3: Draw a tentative diagram for the molecule or ion.
Rules
a) A hydrogen atom always forms one bond. Hydrogen
is always a terminal atom in a Lewis diagram – an
atom that is bonded to only one other atom.
b) A carbon atom normally forms four bonds
c) When several carbon atoms appear in the same
molecule, the are often bonded to each other. (No
cyclic compounds in Chem 60/68)
Draw the Lewis Structure for the following molecules.
1. H2O
Oxygen has 6 valence electrons & Hydrogen has 1 valence electron for a
total of 8 electrons.
..
H:O:H
..
2. CO
Oxygen has 6 valence electrons & Carbon has 4 valence electrons for a total
of 10 electrons.
:C:::O:
Draw the Lewis Structure for the following molecules.
3. BH3
Boron has 3 valence electrons & Hydrogen has 1 valence electron for a total
of 6 electrons
H:B:H
..
H
4. NH3
Nitrogen has 5 valence electrons & Hydrogen has 1 valence electron for a
total of 8 electrons
..
H:N:H
..
H
p. 362
p. 374
p. 362
p. 375
Cations
p. 363
Anions
p. 366
Resonance
p. 366
LEWIS DOT STRUCTURES
Predict the most stable structure: ONC- or OCN- or NOC..
..
..
..
..
..
:O::N::C: or :O::C::N: or :N::O::C:
1) Total electrons is:
6 e- for O + 5 e- for N + 4 e- for C + 1 e- for negative charge = 16 etotal. All structures fulfill the octet rule.
2)
FC (X) = # of valence electrons - (1/2 bonding electrons + nonbonding electrons)
structure#1:
FC(C) = 4 - (1/2 4 + 4) = -2
FC(O) = 6 - (1/2 4 + 4) = 0
FC(N) = 5 -(1/2 8 + 0) = +1
structure#2:
FC(C) = 4 - (1/2 8 + 0) = 0
FC(O) = 6 - (1/2 4 + 4) = 0
FC(N) = 5 -(1/2 4 + 4) = -1
structure #3:
FC(C) = 4 - (1/2 4 + 4) = -2
FC(O) = 6 - (1/2 8 + 0) = +2
FC(N) = 5 -(1/2 4 + 4) = -1
structure #2 has the combination with the lowest
formal charge. It also has the negative formal
charge on one of the more electronegative
atoms. Calculate the formal charge for the most
stable structure:
..
:O:C:::N:
..
(-1, 0, 0)
Practice Problem #13
ClO2.. .. ..
: O : Cl : O :
.. .. ..
SiH4
H
¨
H : Si : H
¨
H
AsH3
..
H : As : H
¨
H
Group Study Problem #13
1.
Draw the Lewis structure for the following
a) H2S
b) PH3
c) CH2O
d) NO2-
e) H2CO3
f) CBr4
g) CH2FCl
h) C2H2
I) O3
2. Calculate the formal charge for “c”,
“d”, “f”, and “I”.