Lewis Dot Structures
What are they?
• Lewis dot structures of individual atoms depict
the number of valence electrons an atom has.
– For example:
• They are used to represent the bonds
between atoms and the remaining lone pairs
of valence electrons.
– For example:
(water)
(carbon dioxide)
STEP 1
Determine the number of valence electrons
(outer shell electrons) each atom has.
a) Water (H2O)
b) Carbon dioxide (CO2)
STEP 2
Find the TOTAL number of valence electrons in
the compound.
a) Water (H2O)
6 +
b) Carbon dioxide (CO2)
4 +
6
+
6
1
+
1
= 8
total
valence
electrons
= 16 total valence electrons
STEP 3
Arrange the atoms. If there is more than one
atom type in the molecule, put the least
electronegative atom in the center.
a. Recall that electronegativity decreases as atom
moves further away from fluorine on the
periodic chart.
b. EXCEPTION: Hydrogen is always placed on the
outside of the compound
STEP 3 (cont’d)
Arrange the atoms:
a) Water (H2O)
b) Carbon dioxide (CO2)
STEP 4
Arrange the electrons so that each atom
contributes one electron to a single bond
between each atom.
a) Water (H2O)
b) Carbon dioxide (CO2)
H
H
STEP 5
Count the electrons around each atom – are the
octets complete (for hydrogen – is the duet
complete)? If so, your Lewis dot structure is
complete.
H
a) Water (H2O)
8 electrons around
oxygen = complete
octet!
H
2 electrons around
hydrogen =
complete duet!
STEP 5 (cont’d)
b) Carbon dioxide (CO2)
7 electrons around
oxygen = not
complete octet 
6 electrons around
carbon = not
complete octet 
STEP 6
If the octets are incomplete, and more electrons
remain to be shared, move one electron per
bond per atom to make another bond.
b) Carbon dioxide (CO2)
STEP 7
Any electrons that are being shared by two
atoms represent covalent bonds. Redraw the
electron dots as lines for those bonds.
a) Water (H2O)
b) Carbon dioxide (CO2)
Try some on your own
On your worksheet try problems 1 and 2. Follow
the steps 1-7 to draw the Lewis dot structures
for these molecular compounds:
1. PBr3
2. N2H2
Answers
1. PBr3
2. N2H2
Problem #3
3. CH3OH
– Remember to put the least electronegative atom
in the center (C) and hydrogens belong on the
outside of the structure (always!).
Problem #3
3. CH3OH
Problem #4
When we need to represent a polyatomic ion,
the charge indicates that there is an extra
electron involved.
4. NO2-1
–
–
–
–
Nitrogen = 5 valence electrons
Oxygen = 6 valence electrons
Oxygen = 6 valence electrons
-1 charge = 1 valence electron
TOTAL = 18 valence electrons
Problem #4
4. NO2-1
– Since we have an extra electron, brackets are
used around the entire structure and the -1
charge is indicated on the outside
Problem #5
Try building dicarbon dihydride (C2H2) using the
Molecular Modeling Kit.
5. C2H2
– Read the instructions page in the kit to help
identify which color balls represent which
element.
Problem #5
5. C2H2
– It should look something like this:
Complete the packet
Use the Modeling Kits to aid you in drawing the
Lewis structures for the rest of the molecules in
your packet.