1. CONSTRUCTING LEWIS DIAGRAMS: THE WHOLE STORY

2. DRAWING LEWIS DIAGRAMS
( H O H) = 8 electrons for water (H2O)
1. Count and add the number of valence electrons
contributed by each atom of the molecule or ion. : .
2. Determine the arrangement of the atoms (skeleton)
for the molecule or ion.
This you must obtain from intuition, some rules,
or information given by the instructor.
H-O-H
3. Subtract the electrons used in each of the skeletal
bonds ( 2 electrons each bond ) from the total.
(8 - 4) = 4 remaining for H2O
4. Assign the remaining electrons following the
Lewis Rules.

3. LEWIS RULES (a reminder)
1. OCTET RULE: In a completed Lewis Diagram,
Period 2 or Period 3 atoms will have a completed
octet : ( 8 electrons).
Hydrogen atoms will have a
“duet” : ( 2 electrons).
The term duet is
not formally used
2. BONDS. Bonds are made by sharing a pair of
electrons between two atoms.
Single Bonds ( 1 shared pair )
Double Bonds ( 2 shared pairs )
Triple Bonds ( 3 shared pairs )
are all allowed in constructing a Lewis Diagram.
Hydrogen, is always singly bonded.

4. N: LEWIS RULES ...... continued 3. ELECTRON PAIRS.
Electrons not involved in forming bonds
(non-bonded or unshared electrons) are arranged
in pairs.
The final structure must have the correct number
(total) of valence electrons.
4. CORRECTNESS.

5. REVIEW OF METHOD 1. Determine the total number of valence electrons
in the molecule or ion.
2. Determine the arrangement of the atoms
(the skeleton).
3. Subtract the electrons used in the skeletal
bonds ( 2 electrons each bond ) from the total.
4. Assign the remaining electrons following the
Lewis Rules (octets, duets, bonds, pairs etc.).
5. Check that the total number of electrons is
correct ( count them ! ).
Do CO2 or H(CO)Cl

6. Some Lewis Diagrams of Organic Molecules
hydrogen cyanide
acetone
ethylene
glycine
FOR MANY COMPOUNDS YOU WILL BEGIN TO SEE PATTERNS

7. COMMON BONDING PATTERNS
FOR THE ELEMENTS (NO CHARGES)

8. NORMAL COVALENT BONDING PATTERNS FOR COMMON ELEMENTS
III IV V VI VII H
monovalent
no pairs
monovalent
three pairs
trivalent
no pairs
divalent
two pairs
trivalent
one pair
tetravalent
no pairs

9. Atoms use these normal bonding patterns in
preference to others.
Try these first when building Lewis Diagrams.
However, atoms do use other bonding patterns
when forming molecules and ions whenever these
normal patterns are not possible for some reason.
When these normal patterns are used the atoms
do not carry “formal charges” (next section).

10. Total valence electrons = 26
Write the Lewis structure of nitrogen trifluoride (NF3).
Step 1 – N is less electronegative than F, put N in center
1 x N = 5
Step 2 – Count valence electrons
3 x F = 21
Total valence electrons = 26
Step 3 – Draw single bonds between N and F atoms and complete
octets on N and F atoms.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons F N
9.6

11. Total valence electrons = 24
Write the Lewis structure of the carbonate ion (CO3)2-.
Step 1 – C is less electronegative than O, put C in center
1 x C = 4
Step 2 – Count valence electrons
3 x O = 18
2- charge = 2
Total valence electrons = 24
Step 3 – Draw single bonds between C and O atoms and complete
octet on C and O atoms.
Step 4 - Check, are # of e- in structure equal to number of valence e- ?
3 single bonds (3x2) + 10 lone pairs (10x2) = 26 valence electrons
Step 5 - Too many electrons, form double bond and re-check # of e-
2 single bonds (2x2) = 4
1 double bond = 4
8 lone pairs (8x2) = 16
Total = 24 O C
9.6

12. FORMAL CHARGE

13. FORMAL CHARGE
Table 1-10
Formal Charge The charge on an atom in a molecule or ion, based on a prescribed procedure for counting which electrons may be assigned to each individual atom.
Unbonded
Bonded
Number of All One half of
= valence electrons unshared + all shared
in the neutral electrons electrons
atom
Formal Charge . .. N : . H N : .
NH2- .
5e-
6e-
( Formal Charge = = -1 )

14. Formal charge helps us determine the shape of the molecule.
Two possible skeletal structures of formaldehyde
(CH2O) H C O H C O
Calculate the Lewis structure and formal charge H C O -1 +1 H C O

15. Formal Charge and Lewis Structures
For neutral molecules, a Lewis structure in which there are no formal charges is preferable to one in which formal charges are present.
Lewis structures with large formal charges are less plausible than those with small formal charges.
Among Lewis structures having similar distributions of formal charges, the most plausible structure is the one in which negative formal charges are placed on the more electronegative atoms.
Which is the most likely Lewis structure for CH2O? H C O -1 +1 H C O
9.7

16. The sum of the formal charges of the atoms in a molecule or ion must equal the charge on the molecule or ion.

17. _ S O .. : _ SULFATE (SO4)2- 2 - +2 net ionic charge
Calculate formal charges

18. COMMON BONDING PATTERNS IN WHICH AN ATOM CARRIES A FORMAL CHARGE
CATIONS
ANIONS _ _ _ _ _
Table 1-14

19. VALENCE-BOND STRUCTURES
LEWIS DIAGRAMS ARE A TYPE OF
VALENCE-BOND STRUCTURE
BUT ARE NOT THE ONLY TYPE

20. Valence-Bond Structures :
BE SURE TO DISTINGUISH AMONG THE VARIOUS
TYPES OF VALENCE BOND STRUCTURES
Valence-Bond Structures :
1. Lewis Diagrams
2. Expanded Octet Structures
3. Incomplete Octet Structures
AND TO NOTE THAT LEWIS DIAGRAMS
ALWAYS FOLLOW THE LEWIS RULES,
BUT THE OTHER TYPES DO NOT

21. Usually occurs with B and elements of higher periods and most nonmetals. Common exceptions are: Be, B, P, S, Xe, Cl, Br, and As.

22. INCOMPLETE-OCTET STRUCTURES

23. . . . B INCOMPLETE OCTET STRUCTURES
SOME ATOMS DO NOT FOLLOW THE OCTET RULE
GROUP THREE ELEMENTS OFTEN FORM
INCOMPLETE OCTET STRUCTURES .
Boron often makes structures with
an incomplete octet. . . B
It can only form three bonds!
BF3 =
How can it do this?

24. PROMOTION OF ELECTRONS
IN THE VALENCE SHELL

25. . . . B . . . B VALENCE SHELL OF BORON [1s2]2s22p1 2s 2p “promotion”
ONE BOND . . . B
paired 2s 2p
unpaired
“promotion” .
THREE BONDS . . B 2s 2p
only three bonds
can be formed in
the normal fashion
GROUP 3A

26. . . . C . . . . C . CARBON FORMS 4 BONDS ALSO DUE TO PROMOTION
BY THE WAY ……..
CARBON FORMS 4 BONDS
ALSO DUE TO PROMOTION
TWO BONDS . .
[1s2]2s22p2 . C . 2s 2p
“promotion” .
FOUR BONDS . . C 2s 2p .
GROUP 4A

27. PROMOTION . . : : : . . . . . . . . . . VALENCE SHELL 2A 3A 4A
When forming covalent bonds,
Be B C .
all of the elements in
groups 2A to 4A
promote an electron so they
can form more bonds. . . . . . . . .
Be B C .
LEWIS DOT SYMBOLS
LEWIS DOT SYMBOLS
are always drawn showing the
promotion already accomplished.

28. EXPANDED-OCTET STRUCTURES

29. P S EXPANDED OCTET STRUCTURES GROUP 5A OR 6A ELEMENTS OFTEN FORM
Phosphorous can form up to 5 bonds P
PCl5 =
GROUP 5A
Sulfur can form up to 6 bonds S
SF6 =
3d orbitals
are available
GROUP 6A
How can it do this?

30. VALENCE SHELL OF SULFUR
Sulfur can use 3d orbitals
TWO BONDS 3s 3d 3p
promotion (x2)
SIX BONDS 3s 3d 3p
both ways of bonding are common

31. EXPANDED OCTET STRUCTURES ARE NOT
LEWIS DIAGRAMS .....
THEY DO NOT FOLLOW THE OCTET RULE !
They are specifically designated:
EXPANDED OCTET STRUCTURES
A designation often used for Phosphorous
and Sulfur structures.

32. Some structures can be drawn both ways:.. - .. : : : .. .. .. .. +2 .. .. .. .. : : : .. .. -
LEWIS DIAGRAM
EXPANDED-OCTET
note charges
note lack of
charges
Be very clear on what is asked for.

33. VALENCE-BOND STRUCTURES

34. VALENCE BOND STRUCTURES
This term is broad enough
to include Lewis Structures
and incomplete octet or
expanded octet structures.

35. THE IMPORTANCE OF
LEWIS DIAGRAMS

36. A COMMON LANGUAGE FOR CHEMISTS
Lewis Diagrams are one of the principal methods
chemists have for communicating structural
information to one another.
If you ask a chemist for the structure of a molecule,
you will likely get a drawing of a Lewis Diagram.
Handbooks use these diagrams to communicate
structural information. When you look in the CRC
handbook or the Merck Index you will find Lewis
Diagrams.
In addition, Lewis Diagrams will help you understand
the chemistry of molecules and help to identify
reactive sites. We use them to explain reactions.
Learning all that is coming soon!

37. LEWIS DIAGRAMS SHOW IT ALL !
When drawing a Lewis Diagram remember these rules.
LEWIS DIAGRAMS SHOW IT ALL !
- all atoms including hydrogens
- all bonds ( lines not dots )
- all unshared pairs ( dots )
- all formal charges
- all atoms with octets ( except H )
- the correct number of electrons ( count! )
All of these rules (except for the octet rule)
apply to all other valence-bond structures
as well. Be sure you include everything
specified here in your answers.