Lewis Structures and Molecular Geometry

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Lewis Structures and Molecular Geometry
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Drawing Lewis Structures

• A recommended procedure
• Count the total number of valence electrons (VSE)
  for the atoms (based on the column of the atom in
  the periodic table) and charge.
• (add one electron for each negative charge
  subtract one electron for each positive charge).
• HCN
• 1 4 5 -0 10 VSE 5 VSE pairs
• C2H6O
• 2(4) 6(1) 6(1) -0 20 VSE 10 VSE pairs
• CO32-
• 4 3(6) 2 24 VSE 12 VSE pairs

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Drawing Lewis Structures

• Draw the framework with single bonds. Some
  knowledge of the way the atoms are connected may
  be required.
• H C N
• Complete octets around all the atoms.
• H C N
• Exceptions H only has one pair of electrons,
  Be only has two and B has only three pairs.

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Drawing Lewis Structures

• Count the number of pairs of electrons depicted.
  If this number is less than the total, add
  electrons to the central atom until the total
  number of electrons is correct.
• H C N
• There are 7 pairs in the drawing and there should
  only be five.

Unshared pairs are non-bonding electrons
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Drawing Lewis Structures

• If there are extra pairs of electron in the
  drawing use lone-pairs (non- bonding pairs) to
  make double or triple bonds and reduce the total
  number of electron pairs.
• H C N ? H C N ?
• H C N ? H C N

• Two unshared pairs one bonded pair

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Drawing Lewis Structures

• Draw the Lewis structure for PCl5
• VSE P (5) and Cl (7x5) 40 e-, or 20 pairs of
  electrons.
• Place P in the center because it is a singleton,
  and the chlorines around it.
• Give each Cl an octet and count the pairs.
• P has more than four pairs called an expanded
  octet.

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Phosphorus Pentachloride
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Review

• Lewis structures are a 2 dimensional picture of
  the arrangement of atoms in a molecule.
• The total pairs of electrons must be correct in
  your final drawing.
• Each atom must have four pairs (an octet) of
  electrons, except H, Be and B.
• Some atoms may have more than four pairs, like
  the second row p orbitals.

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Lewis Structures Practice

• Draw Lewis Structure for the following compounds.
• CO2 7. NO3-
• H2O 8. SO2
• CH4 9. NH3
• BeCl2 10. NH4
• PO43-
• PCl6-