1. Chapter 6.2-6.3 – Fundamentals of Chemical Bonding
CHM1111 Section 04
Instructor: Dr. Jules Carlson
Class Time: M/W/F 1:30-2:20
Friday, October 21st

2. Counting Electrons
When counting electrons, count all lone pairs and each chemical bond counts as 2 electrons.
Make sure your number of electrons in your calculation and in your structure match.
Note: Lone electrons can only be placed in pairs.
Remember to add electrons to your calculation if you have an anion equal to the negative charge and subtract electrons from your calculation if you have a cation equal to the positive charge.

3. Determining Formal Charge
Formal Charge = # of valence electrons – electrons in Lewis Diagram
For bonds, count one electron for formal charge (bonds have two electrons but one is assigned to each atom)
If you have to have negative formal charge, place it on the more electronegative atom.

4. Octet Rule Conservation
The Octet rule can only be broken for elements deeper in the periodic table.
For n = 1 or n = 2, the octet rule must always be followed.
Once you reach n = 3 you have d electrons and now you can break the octet rule.
Remember PCl5. You cannot have 5 Cl bound to N even though it has 5 valence electrons.

5. I Clicker Question
After drawing the Lewis Structure for HCN, pick the incorrect statement of the following:
The C-N bond is a double bond
There are no lone pairs on C
The C-H bond is a single bond
There is a lone pair of electrons on N
There are no lone pairs on H.

6. Resonance Structures Draw the Lewis Structure for the nitrate ion.
Assign the formal charges:
In NO3-, there is an overall formal charge of -1.
However, there is a 1+ charge on N and a -1 charge on two oxygens (the ones without the double bonds)
This is supported because Oxygen is more electronegative than Nitrogen so desires electrons. -1 -1 +1 +1 +1 -1 -1 -1 -1

7. Other Examples With Resonance
Resonance occurs when there are several ways to get the same formal charge or the formal charges can be distributed between the atoms equally.
Draw the Lewis structures for:
Benzene (C6H6, cyclic)
N2O O3

8. Hints for Lewis Structures
Octet rule always applies for elements with n < 3, may apply elsewhere too.
Carbon forms 4 bonds.
Hydrogen typically forms one bond to other atoms.
When multiple bonds are forming, they are usually between C, N, O or S.
Nonmetals can form single, double, and triple bonds, but not quadruple bonds.
Always account for single bonds and lone pairs before forming multiple bonds.
Try to get all formal charges = 0, if impossible, place negative charges on electronegative atoms.
Look for resonance structures.