Honors Chemistry
Lewis Structures and VSEPR Theory
Lewis Structures
• A Lewis structure shows the distribution of
outer (valence) electrons in an atom,
molecule, or polyatomic ion.
• Unshared electrons are shown as dots
• Bonds are shown as straight lines
• In H2O and HF, as in most molecules and
polyatomic ions, nonmetal atoms except H
are surrounded by eight electrons, an octet.
• In this sense, each atom has a noble gas
structure
• Lewis structures are written following a
stepwise procedure
A) Rules for writing Lewis Structures
(single bonds)
1. Count valence electrons available. (use
group number for nonmetals!)
2. Add electrons to take into account negative
charge
• OCl- ion:
6 + 7 + 1 = 14 valence e-
• CH3OH molecule:
4 + 4(1) + 6 = 14 valence e• SO3-2 ion:
6 + 3(6) + 2 = 26 valence e-
Draw skeleton structure, using single bonds
• Note that carbon almost always forms four
bonds. Central atom is written first in
formula, terminal atoms are most often H, O,
or a halogen
3.
Deduct two electrons for each single bond in
the skeleton
OCl- ion:
14 – 2 = 12 valence e- left
CH3OH molecule:
14 – 10 = 4 valence e- left
SO3-2 ion:
26 – 6 = 20 valence e- left
4.
• Distribute these electrons to give each atom
a noble gas structure, if possible
Examples of central atom
• POCl3 P is central atom
• SO4-2 S is central atom
• SO3-2 S is central atom
• PO4-3 P is central atom
• SCl2
S is central atom
Draw Lewis Dot Structure
Ethane C2H6
Check answer on page 167
B. Too few electrons; form multiple bonds
• Structure of NO3- ion?
• Number of valence electrons = ?
• 5 + 18 + 1 = 24
• Go to Elmo #4
Exceptions to the octet
• When we must exceed the octet, extra
electrons go on central atom. (expanded
octet)
•
•
•
•
ClF3
XeO3
ICl4BeCl2
• Consider XeF4, 36 valence electrons. Octet
structure uses 32 electrons. Put extra earound Xe
• ELMO 5
• In a few molecules, there are less than eight
electrons around the central atom
Resonance
• Sometimes there is more than one valid
structure for an molecule or ion.
• NO3
• Transparency 6
• True structure is a hybrid of those three
forms
• Use double arrows to indicate it is the “average”
of the structures.
• It doesn’t switch between them.
Note that…..
• Resonance forms are obtained by moving
electrons, not atoms.
• Resonance can be expected when is possible
to draw more that one structure that follows
the octet rule
VSEPR
• Lewis structures tell us how the atoms are
connected to each other.
• They don’t tell us anything about shape.
• The shape of a molecule can greatly affect its
properties.
• Valence Shell Electron Pair Repulsion Theory
allows us to predict geometry
VSEPR
• Molecules take a shape that puts electron
pairs as far away from each other as
possible.
• Have to draw the Lewis structure to
determine electron pairs.
• bonding
• nonbonding lone pair
• Lone pair take more space.
• Multiple bonds count as one pair.
Linear
Octahedral