Lewis Dot Structure Rules:

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Lewis Dot Structure Rules

• Treat ions separately (e.g. NH4Cl)
• Count only valence electrons
• Assemble bonding framework
• Fill up non-bonding electrons on
• outer atoms
• Fill up non-bonding electrons on
• inner atoms
• Calculate Formal Charge
• Minimize Formal Charge

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To do Lewis Structures

• Must be able to recognize
• polyatomic
  ions
• Must be able to identify
• valence
  electrons
• Must be able to construct
• Bond framework

Periodic Table Column numbers!
More complex H outside High c outside
Formula hints Acidic Hs bond to O atoms
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Hints on Lewis Dot Structures

• Octet rule is the most useful guideline.
• Carbon forms 4 bonds.
• Hydrogen typically forms one bond to other atoms.
• When multiple bonds are forming, they are usually
  between C, N, O or S.
• Nonmetals can form single, double, and triple
  bonds, but not quadruple bonds.
• Always account for single bonds and lone pairs
  before forming multiple bonds.
• Look for resonance structures.

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PCl3
5(37)26 e-
Bonding Pairs
Lone Pairs (a.k.a. nonbonding electrons)
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Try Some Examples

• CH3CH2NH2
• Cl2CO
• Ozone (O3)
• NO2 vs. N2O

SPENT LOTS O TIME PRACTICING
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Formal Charge

• Difference between the of valence electrons in
  the free atom and the of electrons assigned to
  that atom in the Lewis structure.
• FC formal charge G.N. Group Number
• BE bonding electrons LPE lone pair
  electrons
• If Step 4 leads to a positive formal charge on an
  inner atom beyond the second row, shift electrons
  to make double or triple bonds to minimize formal
  charge, even if this gives an inner atom with
  more than an octet of electrons.

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Covalent Bonding

• Multiple Bonds
• It is possible for more than one pair of
  electrons to be shared between two atoms
  (multiple bonds)
• One shared pair of electrons single bond (e.g.
  H2)
• Two shared pairs of electrons double bond (e.g.
  O2)
• Three shared pairs of electrons triple bond
  (e.g. N2).
• Generally, bond distances shorten with multiple
  bonding.

Octet in each case
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Odd Number of Electrons
NO
Number of valence electrons 11
Resonance occurs when more than one valid Lewis
structure can be written for a particular
molecule (i.e. rearrange electrons)
NO2
Number of valence electrons 17
Molecules and atoms which are neutral (contain no
formal charge) and with an unpaired electron are
called Radicals
O2
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Beyond the Octet

• Elements in the 3rd period or higher can have
  more than an octet if needed.
• Atoms of these elements have valence d orbitals,
  which allow them to accommodate more than eight
  electrons.

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More than an Octet
Elements from the 3rd period and beyond, have ns,
np and unfilled nd orbitals which can be used in
bonding
P (Ne) 3s2 3p3 3d0 Number of valence electrons
5 (5 x 7) 40
PCl5
S (Ne) 3s2 3p4 3d0 Number of valence electrons
6 (4 x 7) 34
SF4
The Larger the central atom, the more atoms you
can bond to it usually small atoms such as F,
Cl and O allow central atoms such as P and S to
expand their valency.
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Less than an Octet
BCl3
Group 3A atom only has six electrons around it
However, Lewis acids accept a pair of electrons
readily from Lewis bases to establish a stable
octet
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VSEPR Definitions

• Electron group set of electrons that occupies a
  particular region around an atom.
• Ligand an atom or a group of atoms bonded to an
  inner atom
• Steric number the sum of the number of ligands
  plus the number of lone pairs in other words,
  the total number of groups associated with that
  atom.

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KNOW THESE!
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Lone Pairs Take up a Bit More Space

• Experiments show that sulfur tetrafluoride has
  bond angles of 86.9 and 101.5 . Give an
  interpretation of these bond angles

NOTE Sizes and electronegativities of exterior
atoms also affect bond angles!!!