Drawing Lewis
Structures of Molecules
Chapter 4 Section 4
Step 1
• Use chemical symbols for the various
elements to draw the skeleton structure
of the compound.
– Place least electronegative atom in the
center.
– Hydrogen, fluorine, and other halogens
occupy end positions.
– Carbon forms chains.
Step 2
• Determine the number of valence electrons
associated with each atom and combine
them to determine the total number of
valence electrons.
– Example 1: F2
2 F atoms x 7 electrons = 14
– Example 2: CH3Cl
1 C atom x 4 e- = 4 e3 H atoms x 1 e- = 3 e1 Cl atom x 7 e- = 7 e14 e-
Step 2 (cont.)
• If the molecule has a charge, add or
subtract electrons accordingly.
(add electrons for a - charge, subtract them
for a + charge)
SO4-2 (sulfate)
1 S = 6 e4 O = 24 echarge = +2 e32 e-
NH4+1 (ammonium)
1N= 5e
4H= 4e
charge = -1 e
8e
Step 3
• Connect the central atom to each of the
surrounding atoms using electron pairs.
– All atoms want 8 electrons except H which
wants 2.
– Add electron pairs around the end atoms
first. If any are left over, then put them
around the central atom.
– Check to make sure that the number of
electrons drawn matches the number of total
valence electrons calculated in step 2.
Step 4
• Check the OCTET RULE !
• If the octet rule is not satisfied for any
atom, move one or more electron pairs
from the surrounding atoms to create
double or triple bonds.
• Halogens NEVER multiple bond.
Step 5
• After you are satisfied with the Lewis
structure you have constructed, perform a
final electron count verifying the total
number of electrons AND that they all
have 8 (or 2 if it’s hydrogen).
Example:
CH4
Step 1: Draw a skeleton structure.
H is never the central atom.
H
H C H
H
Example:
CH4
• Step 2: determine the number of TOTAL
valence electrons
H
• H C H
H
1 C = 4 v.e.’s
4 H = 4 v.e.’s
8 total v.e.’s
Example:
CH4
• Step 3: Connect central atom with
surrounding atoms by electron pairs
QuickTi me™ and a
T IFF (Uncom pressed) decom pressor
are needed to see t his pict ure.
Make sure the number of electrons drawn equals the total number of electrons
calculated in step #2
Example:
CH4
• Step 4: Check the OCTET RULE
QuickTi me™ and a
T IFF (Uncom pressed) decom pressor
are needed to see t his pict ure.
Do all Hydrogens have 2 electrons
and all other atoms have 8?
Example:
CH4
• Step 5: Verify that the total number of
electrons is correct
QuickTi me™ and a
T IFF (Uncom pressed) decom pressor
are needed to see t his pict ure.
8 total electrons
Now, with a partner, try these!
• NH3
• CF4
• CH3Cl
• C3H8
HOMEWORK
Draw Lewis Structures for the following molecules:
• Water
H2O
• Ethane C2H4
• Nitrogen N2
• Carbon monoxide CO