Chapter 6.5 : Molecular Geometry
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Chapter 6.5 : Molecular Geometry

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Chapter 6.5 : Molecular Geometry Chapter 6.5 : Molecular Geometry Presentation Transcript

  • Molecular geometry
    Chapter 6.5
  • Objectives
    Explain VSEPR theory
    Predict the shapes of molecules or polyatomic ions using VSEPR theory
    Explain how the shapes of molecules are accounted for by the hybridization theory
    Describe dipole-dipole forces, hydrogen bonding, induced dipoles, and London dispersion forces
    Explain what determines molecular polarity
  • Molecular Polarity
    Uneven distribution of molecular charge
    • Determined by:
    Polarity of each bond
    As well as, molecular geometry
    Review: Polarity of each bond
    Find the difference in electronegativity
    Use 0 – 0.3 nonpolar-covalent
    0.3 – 1.7 polar-covalent
    1.7 – 3.3 Ionic
    H – O
    2.1 3.5
    4. Difference is 1.4, so its polar-covalent
    _
    H – O
    H
    +
  • VSEPR Theory
    Valence-Shell Electron-Pair Repulsion
    States that repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible
    • Molecules with no unshared pairs
    • Only two atoms, always linear shape
    • BeF2, Beryllium does NOT follow octet rule
    Hydrogen, H2
    Hydrogen Chloride, HCl
    180
    • No lone pairs
    • No repulsion
    • Linear
    AB2
    F Be F
  • BF3, Boron also does NOT follow the octet rule
    F
    F B F
    • No lone pairs
    • No repulsion
    • Trigonal Planar
    120
    AB3
    • CF4, Each fluorine is exactly the same distance from each other
    F
    F C F
    F
    109.5
    • No lone pairs
    • Equal distance
    • Tetrahedral
    AB4
    • Molecules with unshared pairs of electrons
    • NH3, Unshared pair pushes down on the three Hydrogens
    H N H
    H
    AB3E
    • ONE lone pairs
    • Repulsion
    • Trigonal Pyramidal
    107
    • H2O, the unshared pairs push down on the two Hydrogens
    O H
    H
    105
    • TWO lone pairs
    • Repulsion
    • Bent 0r angular
    AB2E2
    • Others:
    • SF6, Octahedral AB6
    • PCl5, Trigonal bipyramidal AB5
  • Hybridization
    The mixing of two or more atomic orbitals of similar energies on the same atom to produce new orbitals of equal energies.
    VSEPR doesn’t show bonding orbitals
    C __ __ __ __ __
    1s 2s 2p
    C __ __ __ __ __
    1s 2s 2p
    Form hybrid orbitals
    sp3
    • Hybrid orbitals
    • Orbitals of equal energy produced by the combination of two or more orbitals on the same atom.
    • Other hybrids:
    • sp2 : trigonal planar
    • sp : linear
  • Sp3 hybrid
    C __ __ __ __ __
    1s 2s 2p
    sp3movie
    N __ __ __ __ __
    1s 2s 2p
    O __ __ __ __ __
    1s 2s 2p
    OR
  • sp2 Hybrid
    B __ __ __ __ __
    1s 2s 2p
  • spHybrid
    Be __ __ __ __ __
    1s 2s 2p
  • Intermolecular Forces
    Forces of attraction between molecules
    Boiling point (BP) is a good measure of IMF (intermolecular forces)
    Higher the BP , the stronger the IMF
    • Boiling points and bond types
    • Non polar-covalent H2 -253⁰C
    O2 -183 ⁰C
    Cl2 -34 ⁰C
    CH4 -164 ⁰C
    • Polar-covalent NH3 -88⁰C
    H2O 100⁰C
    HCl -85 ⁰C
    • Ionic NaCl 1413⁰C
    MgF2 2239⁰C
    • Metallic Cu 2567⁰C
    Fe 2750⁰C
  • Molecular Polarity and Dipole-Dipole Forces
    Dipole
    Created by equal but opposite charges that are separated by a short distance
    _
    +
    H – Cl
    • Dipole-dipole forces
    • Forces of attraction between polar molecules
  • Molecular Polarity and Dipole-Dipole Forces
    Polar
    Water, H2O
    Ammonia, NH3
    _
    H 2.1
    O 3.5
    +
    _
    H 2.1
    N 3.0
    +
    • Nonpolar
    • Carbon tetrachloride, CCl4
    • Carbon Dioxide, CO2
  • Hydrogen bonding
    Intermolecular force in which a hydrogen atom that is bonded to a highly electronegative atom is attracted to an unshared pair of electrons of an electronegative atom in a nearby molecule.
    • Creates an unusually higher boiling point.
  • London Dispersion forces
    Intermolecular attractions resulting from the constant motion of electrons and the creation of instantaneous dipoles.
    Very weak intermolecular forces
    Only intermolecular forces for noble gas atoms and nonpolar molecules
    He
    Br2