Lewis Structures
Valence electrons for Elements
Recall that the valence electrons for the elements can be determined
based on the elements position on the periodic table.
Lewis Dot Symbol
Valence electrons and number of bonds
Number of bonds elements prefers depending on the
number of valence electrons. In general -
→
Fa m i l y
H alo g e n s
F , B r, C l , I
C alco g e n s
O, S
N i t ro g e n
N, P
C a rb o n
C, Si
X
# C o v a l e n t B o n d s*
1 bond
O
→
→
N
→
3 bond often
C
→
4 bond
often
2 bond often
always
The above chart is a guide on the number of bonds formed by these atoms.
Lewis Structure, Octet Rule Guidelines
When compounds are formed they tend to
follow the Octet Rule.
Octet Rule: Atoms will share electrons (e-) until it is
surrounded by eight valence electrons.
4 unpaired
3unpaired
2unpaired
1unpaired up = unpaired e- 4 bonds
3 bonds
2 bonds
1 bond
O=C=O
N≡ N
O=O
F-F
Atomic Connectivity
The atomic arrangement for a molecule is usually given.
CH2ClF
Cl
H
C
H
HNO3
O
F
N
O
H
CH3COOH
O
H
H C
H
H2SO4
O
C
O
H O
S
O H
O H
O
In general when there is a single central atom in the
molecule, CH2ClF, SeCl2, O3 (CO2, NH3, PO43-), the central
atom is the first atom in the chemical formula.
Except when the first atom in the chemical formula is
Hydrogen (H) or fluorine (F). In which case the central atom
is the second atom in the chemical formula.
Find the central atom for the
following:
1) H2O
2) PCl3
3) SO3
4) CO32-
5) BeH2
6) IO3-
Setting up Bond Table
Bond table requires the chemical formula, and determining
the number of electrons around each atom.
A) Chemical Formula I.e., HNO3
B) Oe - Octet Electrons. This is always either 8 or 2 (for H)
C) Tve - Total Valence Electron.
ChemFormula
HNO3
H
N
O
Octet eOe
Tot Val eTve
1x2=2
1x1=1
1x8=8
1x5=5
3 x 8 = 24
3 x 6 = 18
34
24
Note this recipe works only if the chemical specie obeys the octet rule. For
chemical specie which violates the octet rule, this method must be modified.
Calculating the Number of Bonds and the
Remaining electrons
After setting up the bond table, calculate the number of
bonds in the chemical specie and the number of electrons.
The remaining electrons are place around the atoms in the
chemical specie such that each atom obeys the octet rule
HNO3
Oe
Bond Table
# of Bonds
34
Remaining e-
Tve
-
24
# Bonding e=
10
10/2 = 5
Tve(24) - electrons in Bond (10) = 14
Putting it Together
1) Chemical formula:
HNO3
3) Number of bonds
2) Atomic sequence:
=5
O N O
O
H
4) Remaining electrons = 14
5) Lewis Structure with 5 bonds:
O
N
O
H
O
6) Complete Lewis Structure
with 14 remaining electrons
O
N
O
H
O
Lewis Dot Structure of CO2 by
Bonds Table
A. Calculate Octet electrons (Oe-) and Total
Valence electrons to determine number of
bonds
CO2
1C
2O
Chg
Oe
1•(8)= 8
2•(8)=16
B. Calculate the number of bonds in compound
structure.
# bonds =
TVe
1•(4) = 4
2•(6)=12
24
(Oe - TVe)
2
= (24- 16) =
2
8 = 4 bonds
2
C. Calculate the remaining electrons to add to
structure to complete Lewis dot structure.
16
Remaining e- = TVe - e- used in bonding.
= 16 - 8 = 8 e Remaining
Writing Lewis Structure:
Use information from the above calculations. Total of 16e- in CO2, of which 8 electrons are
used to form 4 bonds and 8 remaining electrons are used to complete Lewis structure.
-
O
C
O
O
C
O
1,2. Write atom connectivity 3,4,5. Draw the four bonds
in the structure.
for CO2.
O
C
O
6. Place the remaining 8 electrons in the
structure to complete the Lewis Structure
Lewis Dot Structure of ClO4A. Calculate (Oe-) and (TVe)
ClO4- Oe
1 Cl 1•(8)= 8
4O
4•(8)= 32
Chg
40
B. Number of Bonds.
# bonds =
TVe
1•(7) = 7
4•(6)= 24
1
32
(40- 32) = 8 = 4 bonds
2
2
C. Remaining electrons.
Remaining e- = 32 - 8 = 24 e-Remaining
Writing Lewis Structure:
1,2. Write atom connectivity
for ClO4-.
O
O
Cl
O
O
3.4.5. Draw the four
bonds in the
structure.
O
O
O Cl
O
6. Place the remaining 24 electrons in
the structure such that each atom has an
octet to complete the Lewis Structure
O
O Cl
O
O
Summary
Lewis Structure Determination:
• Molecular Formula
• Atomic Sequence (H and F are terminal)
• Determine the # of bonds
Oe- and TVe# of Bonds = (Oe - TVe-) / 2
• Determine remaining electrons
Re = (TVe-) - (# e- in bonding)
• Make sure all atoms satisfy octet rule (Except H which
is satisfied with 2 electrons)
Rules for molecules with an overall
charge:
1.  When figuring out the number of electrons
available (ie. counting the valence electrons),
add or subtract electrons as indicated by the
charge.
2.  Create the Lewis structure the same
as always
3. Put [square] brackets around the
structure
4. Write the charge as a superscript
Try it: CO3-2
C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-
O
O
C
O
Try it: CO3-2
C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-
O
O
C
O
Try it: CO3-2
C: 4 e- , O: 6 x 3 e- , (-2): e- = total 24 e-
O
O
C
O
Try it: CO3-2
All 24 e- have been used, buy C does NOT have a
complete octet….. Need to make a double bond….Does it
matter which O it makes the double bond with?
O
O
C
O
Try it: CO3-2
The bond can go here but it could also go between the C
and either of the other Os
O
O
C
O
Finished Product J
-2
C
C
C
Resonance Structures
•  The Lewis structure for CO3 shows two different types of
bonds, single and double.
•  Double bonds also have a shorter bond length, the distance
between the nuclei of the two atoms in the bond, than
single bonds do. Thus, if the above Lewis structure for
carbonate were correct, the carbonate ion would have one
bond that is shorter and stronger than the other two.
•  This is not the case. All three of the bonds in the carbonate
ion are the same strength and the same length.
•  Interestingly, the behaviour of the bonds suggests they are
longer than double bonds and shorter than single bonds.
•  They are also stronger than single bonds but not as strong
as double bonds.
Resonance Structures (2)
•  So the carbonate ion would actually 3 bonds, with the bond
lengths between the characteristic single and double bond
lengths
•  The important points to remember about resonance forms
are:
àThe molecule is not rapidly oscillating between
different discrete forms
àThere is only one form of the each molecule, we
need to draw two (or more) Lewis structures because a
single structure is insufficient to describe the real structure
Drawing them:
•  Expressing resonance when drawing Lewis
structures is done by drawing each of the
possible resonance forms and placing
double-headed arrows between them
Do you remember polyatomic ions?
Formula
Name
Formula
Name
NH4+/NH3
Ammonium/Ammonia
CrO4-2
Chromate
C2H3O2-
Acetate
Cr2O7-2
Dichromate
CH3COO-
Acetate
MnO4-
Permanganate
CN-
Cyanide
MnO4-2
Manganate
CO3-2
Carbonate
NO2-
Nitrite
HCO3-
Bicarbonate
NO3-
Nitrate
C2O4-2
Oxalate
OH-
Hydroxide
ClO-
Hypochlorite
PO4-3
Phosphate
ClO2-
Chlorite
SCN-
Thiocyanate
ClO3-
Chlorate
SO3-2
Sulfite
ClO4-
Perchlorate
SO4-2
Sulfate
S2O3-2
Thiosulfate
HSO4-
Hydrogen sulfate
BrO-
Hypobromite
IO3-
Iodate
AsO2-3
Arsenite
SeO4-2
Selenate
BrO3-
Bromate
HSO3-
Hydrogen sulfite
Example
Lewis Structures: Examples
a) CH2ClF
Cl
H
C
H
F
b) SO2
O
S O
c)
SO42-
O
O S O
O
d) H3PO4
O
H O P O H
O
H
Co-ordinate Covalent Bond
•  A covalent bond is formed by two atoms sharing a
pair of electrons. The atoms are held together
because the electron pair is attracted by both of the
nuclei.
•  In a simple covalent bond, each atom supplies one
electron to the bond - but that doesn't have to be
the case.
•  A co-ordinate bond is a covalent bond (a shared
pair of electrons) in which both electrons come
from the same atom.
Example: NH4+
Carbonate ion:
-2
C
C
C
Try these:
1.
2.
3.
4.
5.
O3
NO3-1
HCO2-1
CN-1
SO32-
6. BrO-1
7. PO438. IO3-1
9. CHO2 -1
10. NO2+1