1. Drawing Lewis Structures Writing Lewis Dot Structures

2. 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

3. 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.

4. 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;

5. 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 in the structure.
ii. H and F are never central atoms.

6.   1 bond often 2 bond often 3 bond often 4 bond always
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  X F a m i l y # C o v e n t B d s* H g s , r I c O S N P b 
1 bond often
2 bond often
3 bond often
4 bond always
In general, these are the number of bonds formed by these atoms.

7. 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 game-
i) O.R. works mostly for second period elements.
Many exceptions especially with 3rd period elements (d-orbitals)
ii) H prefers 2 e- (electron deficient)
iii) :C: N: :O: :F:
4 u.p 3u.p 2u.p. 1u.p. up = unpaired e-
4 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) .

8. 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 game-
i) H prefers 2 e- (electron deficient)
i) :C: N: :O: :F:
4 u.p 3u.p 2u.p. 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) .

9. Atomic Connectivity (Bond Capabilities)
The atomic arrangement for a molecule is usually given.
CH2ClF HNO3 CH3COOH H2Se H2SO4 O3 H C F Cl H N O
H C C
O H H O
H O S
O H O H Se 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 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

10. Lewis Dot Structure of CO2 by Bonds Table
A. Calculate Octet electrons and Total Valence electrons to determine number of bonds
B. Calculate the number of bonds in compound structure.
= (24- 16) = 8 = 4 bonds
2 2
C. Calculate the remaining electrons to add to structure to complete Lewis dot structure.
= 8 e-Remaining
CO2
# of e in its octet
# of valence e-
1 C
1•(8)= 8
1•(4) = 4
2 O
2•(8)=16
2•(6)=12 24 16
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 C O C
#1 Write atom connectivity for CO2.
#2. Draw the four bonds in the structure.
#3. Place the remaining 8 electrons in the structure to complete the Lewis Structure

11. Lewis Dot Structure of ClO4- by Bonds Table
A. Calculate
ClO4-
# of e in its octet
# of valence e-
1 Cl
1•(8)= 8
1•(7) = 7
4 O
4•(8)=32
4•(6)=24
Charge 1 40 32
B. Number of Bonds.
# bonds = (40- 32) = 8 = 4 bonds
2 2
C. Remaining electrons.
= e-Remaining
Writing Lewis Structure:
#3. Place the remaining 24 electrons in the structure such that each atom has an octet to complete the Lewis Structure
#1. Write atom connectivity for ClO4-.
#2. Draw the four bonds in the structure.

12. Lewis Structures: Examples
a) CH2ClF b) SO2
c) SO42- d) H3PO4 H P O