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.