CHEM 107, Fall 2015
Lewis Structures
Class #20
Lewis Structures &
Orbital Overlap
CHEM 107
L.S. Brown
Texas A&M University
Writing Lewis Structures
•  Our text uses a fairly systematic method
•  If you already know a reliable way to do
this, feel free to use it. But be sure that
your method works for all our examples!
•  Simple way of representing valence
electrons in a molecule
•  Easy, but very useful
•  One electron = one dot
•  One pair of shared electrons = one line
•  Two pairs = two lines, etc.
Lewis Structures
•  In class I will illustrate our book’s
approach for drawing Lewis structures
by doing a few examples that involve
the various rules.
•  The examples I’ll be doing are:
CH4, CO2, SF6, and (NH4)2CO3.
Lewis Structures
Lewis Structures
•  Treat ions separately
•  Count the valence e–’s
•  Set up bonding framework, using
2 e–’s per bond
•  3 pairs of nonbonding e–’s on each
outer atom, except H (assuming enough
e–’s)
•  Remaining e–’s go to inner atoms
•  Optimize e– distribution:
•  (a) 2nd row atom:
8 e–’s à “octet rule”
•  (b) other atoms à can sometimes have
more than 8 e–’s
•  Check for equivalent structures
(resonance)
© 2015, L.S. Brown
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CHEM 107, Fall 2015
Formal Charges
•  An “accounting device,” not real charge
on the atoms
valence e—’s
in free atom
—
valence e—’s
in structure
Resonance
•
•
•
•
Multiple Lewis structures for a molecule.
Usually “equivalent”
This is a “patch” to the Lewis model
Real molecule does NOT “bounce”
between the different structures!
•  Sum of FC’s = zero for a molecule, or
charge on an ion
Orbital Overlap & Bonding
•  Lewis structures: Bonds as shared
electrons
•  Electrons in atoms occupy orbitals.
•  Picture bonding as overlap of valence
orbitals from adjacent atoms?
Bonding in H2: Lewis Picture
•  H2 is the simplest molecule (2 electrons)
•  Lewis structures easily show single
bond.
.
H. + H à H:H
H H
Bonding in H2:
Orbital Picture
•  Interaction between valence atomic
orbitals
•  H-atom has 1s1 electron configuration
•  Picture the bond between 2 H-atoms as
interaction between the 1s orbitals.
H2 Molecule
•  Bond strength maximized at optimum
distance
–  “equilibrium bond length.”
•  Further apart:
–  weaker electron-nucleus interactions
•  Closer together:
–  strong nucleus-nucleus repulsion
© 2015, L.S. Brown
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CHEM 107, Fall 2015
H2 Potential Curve
HF - Lewis Picture
•  Try using overlap of valence orbitals,
like in H2
•  HF
E (J)
–  H: 1s1, F: 2s2, 2p5
r (m)
HF - Orbital Overlap?
•  H: 1s1, F: 2s2, 2p5
•  Bond formed from overlap of 1s orbital
on hydrogen and a 2p orbital on F.
(say 2pz )
•  Lewis structure:
..
..
.F.. à H-..F..
H. + ..
HF - Orbital Overlap?
1s from H
2p from F
Problems With Model
•  Predicts molecules with p valence
electrons should have bond angles
of 90°
•  Experiments show other bond angles
are much more common
•  What about a molecule
like methane (CH4)?
© 2015, L.S. Brown
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