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Lecture 8.2- Lewis Dot Structures for Molecules



Section 8.2 Lecture for Honors & Prep Chemistry

Section 8.2 Lecture for Honors & Prep Chemistry



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Lecture 8.2- Lewis Dot Structures for Molecules Lecture 8.2- Lewis Dot Structures for Molecules Presentation Transcript

  • Bellwork- Ionic or covalent?
    • Are the following compounds ionic or covalent?
    • NaCl
    • H 2 O
    • MgO
    • AlCl 3
    • SO 2
    • C 12 H 22 O 12
    • CO 2
      • Lecture 8.2- Lewis Dot Structures for molecules
      • In covalent bonds, electron sharing usually occurs so that atoms attain the electron configurations of noble gases.
        • Two atoms held together by sharing a pair of electrons are joined by a single covalent bond .
        • An electron dot structure such as H:H represents the covalent bond with two dots.
          • A structural formula such as H-H shows covalent bonds with dashes.
    • Draw dot structures for each atom
    • Connect atoms so that H gets 2 and all others get 8.
    Cl H 2 dots 8 dots
        • The halogens form single covalent bonds in their diatomic molecules. Fluorine is one example.
        • A pair of valence electrons that is not shared between atoms is called an unshared pair , also known as a lone pair or a nonbonding pair.
        • H–F
    3 lone pairs 1 bonding pair
        • In water the hydrogen and oxygen atoms attain noble-gas configurations by sharing electrons.
        • The ammonia molecule has one unshared pair of electrons.
        • Methane has no unshared pairs of electrons.
  • Practice
      • Atoms form double or triple covalent bonds by sharing two pairs or three pairs of electrons.
        • A bond that involves two shared pairs of electrons is a double covalent bond .
        • A bond formed by sharing three pairs of electrons is a triple covalent bond .
        • Each oxygen atom has two “single” electrons that can bond.
        • Each oxygen atom can make two bonds.
    A single bond still leaves two unpaired electrons. O 2 has a double bond . O O
        • Carbon dioxide gas is soluble in water and is used to carbonate many beverages. A carbon dioxide molecule has two carbon-oxygen double bonds.
        • Carbon dioxide is an example of a triatomic molecule.
        • Carbon can make four bonds Oxygen makes 2 bonds.
        • Usually, the unique atom is in the middle.
    O C O
      • Simulation 6 Simulate the covalent bonding within molecules
        • A coordinate covalent bond is a covalent bond in which one atom contributes both bonding electrons.
        • In a structural formula, you can show coordinate covalent bonds as arrows that point from the atom donating the pair of electrons to the atom receiving them.
      • In a coordinate covalent bond , both of the shared electrons came from one atom.
    ammonia + H +  ammonium ion NH 3 + H +  NH 4 + Once formed, there are no identifiable differences between coordinate covalent bonds and covalent bonds .
        • A polyatomic ion , such as NH 4 + , is a covalently bound group of atoms that has a positive or negative charge and behaves as a unit.
        • Most plants need nitrogen that is already combined in a compound to grow.
  • Subtract an e - for each positive charge. Add an e - for each negative charge.
        • The energy required to break a bond is called the bond dissociation energy .
          • A large bond dissociation energy corresponds to a strong covalent bond.
        • Ozone in the upper atmosphere blocks harmful ultraviolet radiation from the sun. At lower elevations, it contributes to smog.
        • A resonance structure is needed when two or more valid electron dot structures exist.
    The structure of ozone is halfway in-between the two dot structures
      • The actual bonding of oxygen atoms in ozone is a hybrid, or mixture, of the extremes represented by the resonance forms.
      • 1. In covalent bonding, atoms attain the configuration of noble gases by
        • losing electrons.
        • gaining electrons.
        • transferring electrons.
        • sharing electrons.
      • 2. Electron dot diagrams are superior to molecular formulas in that they
        • show which electrons are shared.
        • indicate the number of each kind of atom in the molecule.
        • show the arrangement of atoms in the molecule.
        • are easier to write or draw.
      • 3. Which of the following molecules would contain a bond formed when atoms share three pairs of electrons?
        • Se 2
        • As 2
        • Br 2
        • Te 2