1. AP Chem Take out HW to be checked Today: Molecular Shapes Cont; IMFs
Lab Write-Up due Fri 10/5
Unit 2 Test Mon 10/8, Tues 10/9

3. Resonance
Resonance structures are 2 or more forms of a molecule where the chemical connectivity is the same but the electrons are distributed differently around the structure. Resonance occurs when electrons can flow through neighboring pi systems
In these structures, covalent bonding cannot be represented by one Lewis structure. The actual molecule is the “average” of all the resonance structures

4. Resonance Practice

5. Formal Charge # 𝒗𝒂𝒍 𝒆 − 𝒐𝒇 𝑭𝒓𝒆𝒆 𝑬𝒍𝒆𝒎𝒆𝒏𝒕 − # 𝒍𝒐𝒏𝒆 𝒆 − −# 𝒃𝒐𝒏𝒅𝒔
Assigned integer charges to each atom in a molecule
Allows for the comparison of electrons "owned" by an atom in a Lewis structure versus the number of electrons possessed by the same atom in its unbound, free atomic state.
𝑭𝑪= # 𝑣𝑎𝑙 𝑒 − 𝑜𝑓 𝐹𝑟𝑒𝑒 𝐸𝑙𝑒𝑚𝑒𝑛𝑡 − # 𝑢𝑛𝑏𝑜𝑛𝑑𝑒𝑑 𝑒 − – ½ # 𝑏𝑜𝑛𝑑𝑒𝑑 𝑒 −
# 𝒗𝒂𝒍 𝒆 − 𝒐𝒇 𝑭𝒓𝒆𝒆 𝑬𝒍𝒆𝒎𝒆𝒏𝒕
− # 𝒍𝒐𝒏𝒆 𝒆 − −# 𝒃𝒐𝒏𝒅𝒔

6. Evaluation of Lewis structure
Atoms in molecules want a formal charge of 0
Sum of all formal charges must equal to the overall charge of the molecule or ion
The best Lewis structure or resonance contributing structure has the least number of atoms with formal charge.
It is preferred to have a negative formal charges on the more electronegative atoms

7. Formal Charge & Best Lewis Structure Practice:

8. Polarity of Molecules (Compound)
Comparing electronegativity difference between 2 atoms in a bond, for all bonds in the compound
If there is an electronegativity difference between bonded atoms, and the molecule is asymmetrical, then the compound is polar

9. Polarity of Molecules
If there is an electronegativity difference between bonded atoms, and the molecule is completely symmetrical then the compound is nonpolar

10. Polarity of Molecules
If there is no electronegativity difference between bonded atoms, and the molecule is completely symmetrical then the compound is nonpolar

11. tetrahedral
yes
nonpolar
pyramidal no
polar
linear
yes
nonpolar
bent no
polar
Square pyramidal no
polar

12. “Like dissolves like”
Polar substances dissolve/mix in polar substances
Nonpolar substances dissolve/mix in nonpolar substances
Polar substances do NOT dissolve/mix in nonpolar substances

13. Intermolecular Forces (sometimes referred to as “Van der Waals forces”)
Attractive forces between molecules
are NOT BONDS, which is an attraction between 2 atoms
Lots of H2O molecules

14. Types of IMFs: London Dispersion Forces
The weakest intermolecular force
Is the main attractive force between nonpolar molecules
In general, the larger the molecules, the greater the dispersion forces

15. Types of IMFs Dipole-Dipole Forces Stronger than dispersion forces
The main attractive force between polar molecules

16. Types of IMFs
Hydrogen Bonding: is an intermolecular force; it is NOT actually a bond!
The strongest intermolecular force
Only occur in molecules containing Hydrogen AND one of the following: Nitrogen, Oxygen, or Fluorine (NOF)

17. Practice nonpolar no polar no Dipole-dipole nonpolar no
London dispersion forces
polar no
Dipole-dipole
nonpolar no
London dispersion
polar
Hydrogen bonding
yes
polar no
Dipole-dipole