# Calculate the number of valence electrons.

## Presentation on theme: "Calculate the number of valence electrons."— Presentation transcript:

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 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 O C O

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. C O

So now each atom has 8 electrons or dots
Central Carbon has 8 electrons

So now each atom has 8 electrons or dots
Left Oxygen has 8 electrons

So now each atom has 8 electrons or dots
Right Oxygen has 8 electrons

BUT There are a total of 20 dots or electrons
BUT There are a total of 20 dots or electrons. We need 16 dots or electrons. O O C 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. O O C

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. O C O Now there are 18 electrons or dots. To get 16, add another double bond (share another pair of electrons).

O C O Now there are 16 electrons. This is our final structure.
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. O C 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.
Now let’s check that all atoms still have 8 electrons each.

O C O Final Structure

O C O 8 Electrons Around Central Carbon

O C O 8 Electrons Around Left Oxygen

O C O 8 Electrons Around Right Oxygen

O C O 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 degrees O C O

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.