Guidelines for Constructing LEWIS STRUCTURES:
For Covalent Compounds
1. Determine the # of each kind of atom from the molecular formula.
2. Determine the TOTAL # of valence electrons available to use by
counting the number of valence electrons for each atom in the compound
and then adding all the total of all the electrons
Special NOTE for polyatomic ions:
Add one electron for each negative charge
Subtract one electron for each positive charge.
3. Determine the # of valence electrons needed to satisfy the octet rule
for all atoms EXCEPT hydrogen, H. The maximum number of valence
electrons required for hydrogen is 2. The maximum number of valence
electrons required to satisfy the octet rule for all other atoms is 8.
4. Determine the # of bonds by subtracting the # of available electrons
from the # of needed electrons.
5. Determine the # of bonds by dividing the # of bonding e-s by 2 because
there are 2 electrons in a single bond.
6. Place the chemical symbol of the element present in the smallest amount
in the center of the molecule and arrange all other atoms around it using
the number of bonds calculated in #5 above. Represent each bond by a
single line between the bonded atoms. Then add the nonbonding
electrons so that every atom (EXCEPT H) has access to 8 electrons.
Hydrogen should have access to 2 electrons.
Special NOTE for polyatomic ions:
Place square brackets around the [structure] and put the charge outside the
brackets in the upper right-hand corner.
Lewis Structure Practice
You are to complete the following Lewis structures showing work to support your structure.
Refer to the guidelines. Silicon tetrachloride and the phosphate ion have been done for you
as examples. This is an important fundamental aspect of this course upon which several
other concepts are built.
Instructions: Draw the Lewis structure for each of the following molecules or ions.
Show your work, meaning calculations of:
Available (valence) electrons
Needed electrons  REMEMBER: hydrogen only needs two
Bonding electrons
# of bonds
Be sure to show the Nonbonding electrons also!!
1. silicon tetrachloride, SiCl4
Atoms present:
Si
Cl
Available (valence) electrons
4
7
Needed electrons
8
8
Bonding electrons
40e - 32e = 8e
# of bonds
8e-/2e- per bond = 4 bonds
Cl
7
8
2. nitrogen triiodide, NI3
Atoms present:
Available electrons
Needed electrons
Bonding electrons
# of bonds
N
I
I
H
H
S
3. hydrogen sulfide, H2S
Atoms present:
Available electrons
Needed electrons
Bonding electrons
# of bonds
4. carbon dioxide, CO2
I
Cl
7
8
Cl
7
8
Total e32
40
Atoms present:
Available electrons
Needed electrons
Bonding electrons
# of bonds
C
O
O
5. acetylene, C2H2
Atoms present:
Available electrons
Needed electrons
Bonding electrons
# of bonds
6. sulfur dioxide, SO2
Atoms present:
Available electrons
Needed electrons
Bonding electrons
# of bonds
7. phosphate ion, PO4-3
Atoms present:
P
O
Available electrons
5
6
Needed electrons
8
8
Bonding electrons
40e - 32e = 8e# of bonds
8e /2e per bond = 4 bonds
8. carbonate ion, CO3-2
Atoms present:
Available electrons
Needed electrons
Bonding electrons
# of bonds
Extra
O
6
8
O
6
8
O
6
8
e3
Total e32
40