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

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

Resonance Practice

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

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

Formal Charge & Best Lewis Structure Practice:

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

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

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

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

“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

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

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

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

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)

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