CHEM 205 lecture 9

• Lewis dot diagrams


  Shows bond formation
• Lewis dot diagrams


  • Amount of dots = valence electrons
  • Share them to satisfy octet rule

  
• Steps for drawing Lewis dot diagrams

  1. Count total # valence electrons from all atoms in formula

  2. Identify central atom (usually with lower electronegativity, never H)

  3. Connect rest of atoms to central atom via single bonds

  4. Use remaining electrons: add lone pair to achieve full octets (for H: duet)

  5. If central atom has <8 electrons now: convert a lone pair to a bonding pair

  6. Assign formal charges to atoms in molecules

  
• Formal charge


  = (# valence electrons on free atom) – (# nonbonding electrons + # bonds)
• + charge = lots of bonds, electron poor
• • charge: low bonds, electrons rich
• If more than 1 equivalent structure is possible for molecule = RESONANCE, draw them all

  
• Bond order

  # of pairs of electrons shared by 2 atoms in a molecule
• Bond length


  Distance between two atoms
• Bond length


  • The more bonds connecting two atoms, the stronger the bond
  • The shorter the bond, the stronger it is
  • Bond length is based on size of atoms
• Bond energy


  Energy required to break the bond (break 1 mole of the type of bond)
• Isoelectronic and isostructural species

  Two or more molecules, atoms, or ions, that have the same number of electrons and electronic structure
• Isoelectronic and isostructural species have the same chemical properties
• Exceptions to the octet rule

  • 2nd row elements: C, N, O, F: always follow the octet rule
  • 2nd row elements B, Be often have less than 8 electrons
  • Molecules with odd numbers of electrons, sometimes C,N,O,F cant obey octet rule since there isn't enough electrons available
  • 3rd row and heavier: often exceed octet rule, only row that can
• Summary strategy for drawing Lewis structures

  1. Count available valence electrons

  2. Decide on central atoms: Usually one with weakest pull on electrons, never H (it can only have 1 bond)

  3. Connect terminal atoms to central atom with single bonds

  4. Fill valence shell of each atom if possible

  5. Assign and minimize formal charges

  6. Draw resonance structures: Atoms don't move, lone pairs vs multiple bonds

  
• Valence shell electron pair repulsion model

  Structure around a given atom is determine principally by minimizing repulsions between electron pairs in the valence shell

  
• Determining bond angles from Lewis structures

  • Linear (2 pairs)
  • Trigonal planar (3 pairs)
  • Tetrahedral (4 pairs)
  • Trigonal bipyramidal (5 pairs)
  • Octahedral (6 pairs)