How To Draw Lewis Structures for Covalent Compounds
Using CO2 as an Example
• Calculate the number of valence electrons.– What is the electron configuration for each
atom?
C 2 - 4
O 2 – 6
1 atom of Carbon x 4 valence electrons each
2 atoms of Oxygen x 6 valence electrons each
1(4 e-) + 2(6 e-) =16 e-
Red indicates valence electrons
Draw a skeletal structure
• Identify a central atom– Often the element that appears only once in
the formula– Often carbon
C OO
Add electrons so that each atom has eight electrons
• Exception H, He, Li, Be, B
• Always do this step despite the number of valence electrons you calculated in step 1.
CO O
So now each atom has 8 electrons or dots
CO O
Central Carbon has 8 electrons
So now each atom has 8 electrons or dots
CO O
Left Oxygen has 8 electrons
So now each atom has 8 electrons or dots
CO O
Right Oxygen has 8 electrons
BUT There are a total of 20 dots or electrons. We need 16 dots or
electrons.
CO O
We must add at least one double bond.
To share an additional pair of electrons and form a double bond, two pairs must be removed from two
adjacent atoms and a pair must be shared between those atoms.
CO O
To share an additional pair of electrons and form a double bond, two pairs must be removed from two
adjacent atoms and a pair must be shared between those atoms.
CO O
Now there are 18 electrons or dots. To get 16, add another double bond (share another pair of electrons).
To share an additional pair of electrons and form a double bond, two pairs must be removed from two
adjacent atoms and a pair must be shared between those atoms.
CO O
Now there are 16 electrons. This is our final structure.
We often rearrange the electrons to show that they separate as much as
possible from each other.
CO O
Now let’s check that all atoms still have 8 electrons each.
O OC
Final Structure
O OC
8 Electrons Around Central Carbon
O OC
8 Electrons Around Left Oxygen
O OC
8 Electrons Around Right Oxygen
O OC
Final Structure
2 atoms More than 2 atoms
Bonding domains
domain
Domains
Now we can predict molecular shape and polarity for CO2
• How many bonding domains does the central atom have? 2
• How many nonbonding pairs or lone pairs or nonbonding domains does the central atom have? 0
• What is the shape from the table? Linear• Bond Angle 180 degrees
O OC
Polarity
• Draw an arrow from the more electronegative atom(s) to the less electronegative atom(s).
• If the arrows point in one general direction, the molecule is polar.
• If the arrows “cancel” out the molecule is nonpolar.
• Be certain that the structure takes into account the shape of the molecule.