1. Lewis Structures
SCH3U/4C
March 2016

2. Lewis Structures
We have seen Lewis diagrams for single elements, where we represent only the valence electrons for a particular element.

3. Lewis Structures
A Lewis structure consists of the electron distribution in a compound and the formal charge on each atom.

4. How to Draw Lewis Structures Example: CO2
Determine whether the compound is ionic or covalent using electronegativity differences.
C = 2.5 O = 3.0 EN = 0.5 (covalent)
Determine the total number of valence electrons by adding the valence electrons for each atom involved:
C = 4, O = 6x2 Total = 16

5. Divide the total number of valence electrons by two, to determine the number of electron pairs.
Organize the atoms so there is a central atom (usually the least electronegative) surrounded by outer atoms
O C O

6. 8 EP – 2 EP = 6 EP : : : : 6 EP – 6 EP = 6 EP : :
Distribute the electron pairs, keeping track of how many electron pairs (EP) remain:
Place one pair between the central atom and each outer atom
O : C : O
Three more pairs on each outer atom
: O : C : O :
Redistribute the electrons to ensure each atom has a satisfied octet.
: O :: C :: O :
8 EP – 2 EP = 6 EP : : : :
6 EP – 6 EP = 6 EP : :

7. Replace any bonding pairs with the appropriate bond (single or double)
: O = C = O :
Note that each oxygen molecule has 2 pairs of electrons that are not involved in bonding.
These non-bonding electron pairs are known as lone pairs : :