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Chem Take out HW to be checked Today: Molecular Shapes Cont; IMFs
Unit 1 Test Th 10/12
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Molecular Shapes Summary:
# of sets around central atom determines hybridization # of lone pairs on central atom determines molecular shape
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Drawing Lewis Structures
Determine the total number of valence electrons that need to be represented in the structure Set up your Lewis Structure by placing the atom that desires the most bonds in the middle Satisfy the HONC Rule and Octet Rule first If there are valence electrons that are not accounted for, put them on the central atom.
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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 βaverageβ of all the resonance structures
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Resonance Practice
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Formal Charge # πππ π β ππ ππππ π¬ππππππ β # ππππ π β β# ππππ
π
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. ππͺ= # π£ππ π β ππ πΉπππ πΈππππππ‘ β # π’πππππππ π β β Β½ # ππππππ π β # πππ π β ππ ππππ π¬ππππππ β # ππππ π β β# ππππ
π
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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
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Formal Charge & Best Lewis Structure Practice:
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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
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Polarity of Molecules If there is an electronegativity difference between bonded atoms, and the molecule is completely symmetrical then the compound is nonpolar
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Polarity of Molecules If there is no electronegativity difference between bonded atoms, and the molecule is completely symmetrical then the compound is nonpolar
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tetrahedral yes nonpolar pyramidal no polar linear yes nonpolar bent no polar Square pyramidal no polar
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βLike dissolves likeβ Polar substances dissolve/mix in polar substances Nonpolar substances dissolve/mix in nonpolar substances Polar substances do NOT dissolve/mix in nonpolar substances
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Intermolecular Forces (sometimes referred to as βVan der Waals forcesβ)
Attractive forces between molecules are NOT BONDS, which is an attraction between 2 atoms Lots of H2O molecules
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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
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Types of IMFs Dipole-Dipole Forces Stronger than dispersion forces
The main attractive force between polar molecules
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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)
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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
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