Lesson 2.7
Lewis Structures of
Molecules
Lewis Structures
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How to draw Lewis structures for molecules:
OCl- ion (bleach)
1) Count the valence electrons for each atom.
Add e- for anions (-).
Subtract e- for cations (+).
OCl
O
Cl
(14
+ 1 e-
e)
Lewis Structures
OCl- (14 e-)
•  2) Draw a skeleton
structure with lines for shared e-.
•  3) Add e- around atoms until all (14) are used
and each has 8 e- surrounding.
8 e-
O Cl
8 e-
Note that each atom is surrounded by 8 e(Each atom has an “octet”)
Lewis Structures
•  Three possibilities for central atom:
•  1) Just enough e-. Each atom gets 8, and H
gets 2.
H2O (8 e-)
H O H
Lewis Structures
•  2) Not enough e-. Move lone pairs (dots) to
make double or triple bonds until all atoms have
8 e-.
8 e-
CO2 (16
e)
O C O
8 e-
8 eNote: All atoms now have octets.
Lewis Structures
•  3) Too many e-. Atom may have “expanded
octet” (more than 8). Put extra e- around
central atom.
XeF4 (36
F
F
Xe
F
F
e)
We still
need 4
more e-!!!
Lewis Structures
•  Helpful tips:
•  C is always a central atom w/ 4 bonds.
C
C
•  O usually makes 2 bonds.
O
O
•  H and the halogens are terminal atoms that
make one bond.
H
F
•  The first atom in the formula is usually the
central atom.
Practice
•  Draw the Lewis structure for the sulfite ion,
SO322-
SO3 (26 e )
O
OSO
Practice
•  Draw the structure for methanol, CH3OH
CH3OH (14
H
H C O H
H
e)
Practice
•  Draw the Lewis structure for the nitrate ion,
NO3-
NO3 (24
-
O
O N O
e)
But wait! We
have 24 e- but N
only has 6 esurrounding it.
Whatever shall
we do?