Polarity, Lewis Structures,
and Resonance
Sections 8.4-8.6
Bond Polarity
• Ionic and covalent bonding is not black and white
• Sharing is not usually equal
– It’s an electron tug of war
• Must look at electronegativity to determine how
equally electrons are shared
Electronegativity
• The ability of an atom in a
molecule to attract
electrons to itself
•
• Developed by Linus Pauling
– Gave values for all
elements based on
thermochemical data
Linus Carl Pauling
(1901-1994)
Electronegativity:
• On the periodic
chart,
electronegativity
increases as you go…
– …from left to right
across a row.
– …from the bottom
to the top of a
column.
Electronegativities of the Elements
Cs (EN = 0.7) is least
electronegative
element
Au is at the peak of
an island of electronegativity,
and is most electronegative metal
F with EN = 4.0 is
most electronegative
element
Electronegativity and Bond Polarity
Electronegativity tells us what kind of bonding we have, i.e.
whether it is ionic or covalent. The greater the difference in
EN between the two elements forming the bond, the more
ionic is the bond. Typical ranges for EN differences are:
EN difference
difference
range
bonding type
Example
EN
________________________________________________________________________
__
> 2.0
0.5-2.0
<0.5
Ionic
polar covalent
nonpolar covalent
LiF
HF
F-F
4.0-1.0 = 3.0
4.0-2.1 = 1.9
4.0-4.0 = 0.0
Polar Covalent Bonds
• When two atoms share
electrons unequally, a bond
dipole results.
• The dipole moment, ,
produced by two equal but
opposite charges separated
by a distance, r, is
calculated:
 = Qr
• It is measured in debyes
(D).
Polar Covalent Bonds
The greater the
difference in
electronegativity,
the more polar is the
bond.
Bond Types and
Nomenclature
Naming ionic and covalent compounds
Naming Ionic Compounds
1. Name cation first, then anion
2. Cation = name of the element
Ca2+ = calcium
3. Anion
=
root + -ide
Cl = chloride
CaCl2 = calcium chloride
Naming Ionic Compounds
(continued)
If the metal is a transition metal:
 Transition metals may form more
than one cation
 Use Roman numeral in name
PbCl2
Pb2+ is cation
PbCl2 = lead (II) chloride
Naming Ionic Compounds
(continued)
• If a polyatomic ion is present just use
its name
• Ex: NaCN = sodium cyanide
Naming Molecular Compounds
• 1st element in the formula is named first
• 2nd element named as if it were an anion
• Greek prefixes denote how many atoms
of element are present
• Do not use mono- for the first element
P2O5 = diphosphorus pentoxide
14
Greek prefixes cont.
11
12
13
14
15
hendecadodecatriskaidekatetradecapentadeca-
16
17
18
19
20
hexadecaheptadecaoctadecaenneadecaicosa-
Lewis Structures
Representations of molecules showing all
electrons, bonding and nonbonding.
Writing Lewis Structures
1. Find the sum of
valence electrons of
all atoms
- For anions, add 1
electron for each
negative charge.
– For cations,
5 + 3(7) = 26
subtract 1
electron for each
positive charge.
PCl3
Writing Lewis Structures
2. The central atom is
the least
electronegative
that isn’t hydrogen.
Connect the outer
atoms to it by
single bonds.
Keep track of the electrons:
26  6 = 20
Writing Lewis Structures
3. Fill the octets of
the outer atoms.
Keep track of the electrons:
26  6 = 20  18 = 2
Writing Lewis Structures
4. Fill the octet of
the central atom.
Keep track of the electrons:
26  6 = 20  18 = 2  2 = 0
Writing Lewis Structures
5. If you run out of
electrons before the
central atom has an
octet…
…form multiple bonds
until it does.
Formal Charge
• Then assign formal charges.
– For each atom, count the electrons in lone pairs
and half the electrons it shares with other
atoms.
– Subtract that from the number of valence
electrons for that atom: The difference is its
formal charge.
Writing Lewis Structures
• The best Lewis structure…
– …is the one with the fewest
charges.
– …puts a negative charge on the most
electronegative atom.
Resonance
This is the Lewis
structure we
would draw for
ozone, O3.
+
-
Resonance
• But this is at odds
with the true,
observed structure
of ozone, in which…
– …both O—O
bonds are the
same length.
– …both outer
oxygens have a
charge of 1/2.
Resonance
• One Lewis structure
cannot accurately
depict a molecule
such as ozone.
• We use multiple
structures,
resonance
structures, to
describe the
molecule.
Resonance
Just as green is a
synthesis of blue and
yellow…
…ozone is a synthesis of
these two resonance
structures.
Resonance
• In truth, the electrons that form the second C—O
bond in the double bonds below do not always sit
between that C and that O, but rather can move
among the two oxygens and the carbon.
• They are not localized, but rather are delocalized.
Resonance
• The organic compound
benzene, C6H6, has
two resonance
structures.
• It is commonly
depicted as a
hexagon with a circle
inside to signify the
delocalized electrons
in the ring.