Drawing Lewis Structures
Writing Lewis Dot Structures
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Lewis Structure Tutorial
10.7.00 6:16 PM
Step #1
Know how to determine the valence electron for all elements.
Definition:
Valence Electrons
An electron in an outer shell of an atom that can
participate in forming chemical bonds with
other atoms.
Example:
H has 1 valence electron
F has 8 valence electrons
Carbon has 4 valence electrons
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Lewis Structure Tutorial
10.7.00 6:16 PM
Drawing Lewis Diagrams for Molecules
Step 1 Draw each element separately, showing the valence
electrons. (Use different symbols for different atoms)
Step 2 Choose the least occurring atom as the central atom
(or the least electronegative). Except hydrogens or
Oxygens.
Step 3 Add next frequently occurring atoms around the
central atom keeping symmetry in mind.
•You cannot put two of one atom next to each other
Ie/ OOAl must be O Al O
• Step 4 Add all the electrons and then put Hydrogen on
LAST, oxygen atoms on 2nd to last.
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Lewis Structure Tutorial
10.7.00 6:16 PM
Drawing Lewis Diagrams for Molecules
Step 5 Never leave an unpaired electron
Step 6 Circle your 8’s starting with the central atom
Group 1 = 2e- H;
Group 2 = 2e- or 4e- :Mg ;Mg;
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Lewis Structure Tutorial
10.7.00 6:16 PM
Step #2
(Connectivity) From the Chemical Formula, determine
the atom connectivity for the structure.
Find the central atom.
Examples:
i. Given a chemical formula, ABn, A is the central
atom and B flanks the A atom.
i.e., NH3, NCl3, NO2. In these examples, N is central
H
in the structure.
H
N
H
ii. H and F are never central atoms.
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Lewis Structure Tutorial
10.7.00 6:16 PM
Valence electrons and number of bonds
Step #3 (# of Bond) Determine the number of bonds in the
compound
Recall the number of bonds at atom prefers depending
on the number of valence electrons

Fam ily
H alogen s
F, B r, C l, I
C alcogen s
O, S
N itro gen
N, P
C arb on
C , Si
# C ovalent Bonds*
1 bond
O


N

3 bond often
C

4 bond
X
often
2 bond often
always
In general, these are the number of bonds formed by these atoms.
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Lewis Structure Tutorial
10.7.00 6:16 PM
Lewis Structure, Octet Rule Guidelines
When compounds are formed they tend to follow
the Octet Rule.
Octet Rule: Atoms will share e- until it is surrounded by
eight valence electrons.
Rules of the gamei) O.R. works mostly for second period elements.
Many exceptions especially with 3rd period elements (d-orbitals)
ii) H prefers 2 e- (electron deficient)
.
. N:
.
.
:O:
.
..
:F:
.
iii) :C:
4 u.p
3u.p
2u.p.
1u.p.
up = unpaired e4 bonds
3 bonds
2 bonds
1 bond
O=C=O
NN
O=O
F-F
iv) H & F are terminal in the structural formula (Never central)
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Lewis Structure Tutorial
10.7.00 6:16 PM
Lewis Structure, Octet Rule Guidelines
When compounds are formed they tend to follow
the Octet Rule.
Octet Rule: An atom will gain or loose e-(s) until it is
surrounded by eight valence electrons. (Seek a full octet)
Rules of the gamei) H prefers 2 e- (electron
deficient)
.
.
..
.
i) :C:
N:
:O:
4 u.p
3u.p
2u.p.
..
.
:F:
1u.p.
up = unpaired e-
4 bonds
3 bonds
2 bonds
1 bond
O=C=O
NN
O=O
F-F
iii) H & F are terminal in the structural formula (Never central)
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Lewis Structure Tutorial
10.7.00 6:16 PM
Atomic Connectivity (Bond Capabilities)
The atomic arrangement for a molecule is usually given.
CH2ClF
Cl
H
C
HNO3
N
O
F
O
O
H
CH3COOH
H
H C
H
H
O
H2Se
H
Se
C
H2SO4
O3
O
H
H O
S
O
O
O
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:
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1) H2O
a) H
b) O
2) PCl3
a) P
b) Cl
3) SO3
a) S
b) O
4) CO32-
a) C
b) O
5) BeH2
a) Be
b) H
6) IO3-
a) I
b) O
Lewis Structure Tutorial
10.7.00 6:16 PM
Lewis Dot Structure of CO2 by Bonds Table
A. Calculate Octet electrons and Total
Valence electrons to determine number of
bonds
CO2
# of e in
its octet
B. Calculate the number of bonds in compound
structure.
= (24- 16) = 8 = 4 bonds
2
2
# of
valence
e-
1C
1•(8)= 8
1•(4) = 4
2O
2•(8)=16
2•(6)=12
24
16
C. Calculate the remaining electrons to add to
structure to complete Lewis dot structure.
16 - 8 = 8 e-Remaining
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
#1 Write atom
connectivity for CO2.
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O
C
O
O
#2. Draw the four bonds
in the structure.
C
O
#3. Place the remaining 8 electrons in the
structure to complete the Lewis Structure
Lewis Structure Tutorial
10.7.00 6:16 PM
Lewis Dot Structure of ClO4- by Bonds Table
A. Calculate
ClO4-
# of e in
its octet
# of
valence
e-
B. Number of Bonds.
1 Cl
1•(8)= 8
1•(7) = 7
C. Remaining electrons.
4 O
4•(8)=32
4•(6)=24
Charge
# bonds =
32 - 8 = 24
32
Writing Lewis Structure:
#1. Write atom
connectivity for ClO4-.
O
Cl
O
#2. Draw the four
bonds in the
structure.
#3. Place the remaining 24 electrons
in the structure such that each atom
has an octet to complete the Lewis
Structure
O
O
O
O Cl
O
O Cl
O
O
O
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e-Remaining
1
40
O
(40- 32) = 8 = 4 bonds
2
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Lewis Structure Tutorial
10.7.00 6:16 PM
Lewis Structures: Examples
Example
a) CH2ClF
b)SO2
Cl
H
C
O S O
F
H
c) SO42-
d) H3PO4
O
O S
H O P O H
O
H
O
O
12
O
Lewis Structure Tutorial
10.7.00 6:16 PM
Boot elements and the Lewis Dot Structures
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Lewis Structure Tutorial
10.7.00 6:16 PM