Honors Chemistry
Name ___________________________________
Period _____
Date ____________________________________
Lewis Structure Diagrams
Lewis structure diagrams enable us to visualize the bonding structure in a molecule and to predict the polarity
(and eventually, the structural geometry)
Part I. Ionic Bonds (although Lewis dot diagrams are not as useful for ionic bonds)
1. Make certain it’s ionic, metal + non-metal
2. Determine the charges on the ions from the group in the periodic table.
3. Make sure that the sum of the charges of all the ions adds up to zero
4. Number of dots lost and gained must be the same. Then write a structure according to the format below
(anions may have all 8 dots or none).
Formula
Lewis dot structure
KCl
MgO
CaCl2
Na2O
Rb3P
Practice: Draw Lewis dot structures for the following compounds:
MgS
Ca3P2
NaCl
Fr2Se
Al2O3
Na3N
Li2O
AlBr3
CaI2
Part II. Covalent Bonds.
To construct a Lewis diagram for simple covalent compounds (with single bonds), follow these steps:
1. Look at the atoms that will be bonded. Choose the atom with the lowest electronegativity number to be
the "central" atom (except that hydrogen cannot be a central atom).
2. Count up all of the valence electrons contributed by all of the atoms in the molecule. Divide by 2. This
is the number of electron pairs available.
3. Place the remaining atoms around the central atom and put one pair of electrons (or a dash) between
each of the surrounding atoms and the central atom. This is now the "skeletal" diagram.
4. Now use the remaining electron pairs to fulfill the octet for each atom and a duet for the hydrogen
atoms. Electrons pairs not involved in bonding are "unshared pairs" and may contribute to polarity. If
you do not have enough pairs to satisfy the octet, you may need to double up or triple up, creating
double and triple bonds.
Example: NH3
1.
2.
3.
N is the central atom (H cannot be central)
N has 5 valence electrons
5 valence electrons
H has 1 valence electrons each =
3 valence electrons
8 valence electrons
= 4 electron pairs
H–N–H
‫׀‬
H
4. 3 electron pairs were used in the skeletal diagram. One unshared pair is left to fultill the octet for
nitrogen. NH3 is polar due to the one pair of unshared electrons.
‥
H–N–H
‫׀‬
H
For each of the following, indicate the number of valence electrons and draw the Lewis electron dot
structure.
HF
NH3
CH4
NF3
Valence
Electrons______
SiF4
Valence
Electrons______
Valence
Electrons______
F2
Valence
Electrons______
Valence
Electrons______
Cl2
Valence
Electrons______
Valence
Electrons______
H2 S
Valence
Electrons______
C2H6
Valence
Electrons______
CCl4
Valence
Electrons______
PF3
Valence
Electrons______
C2F6
Valence
Electrons______
Covalent Molecules with Multiple bonds
If the molecule is covalent but the number of electrons does not allow all atoms to follow the octet rule
(except hydrogen, of course), then the molecule probably has double and/or triple bonds. In this case,
put pairs of electrons between atoms to make the multiple bonds. Usually you want to make sure the
central atom has an octet first. Coordinate covalent bonds form when one atom donates a pair of
electrons to share with another.
Examples:
CH2O
Valence electrons: 12
N2
Valence
Electrons______
HCN
SO3
Valence electrons: 10
O2
Valence
Electrons______
Valence electrons: 24
CO2
Valence
Electrons______
SO2
Valence
Electrons______
O3
C2H4
Valence
Electrons______
Valence
Electrons______
CH3CO2H
Valence Electrons______
NO2
SiO2
Valence
Electrons______
C2H2
Valence Electrons______
Valence
Electrons______
ONF
SCO
Valence Electrons______
(put the N in between the
O and F)
Valence Electrons______
(put the C in between the
S and O)
Bonding in Polyatomic Ions
Large ions in which atoms are covalently bonded together but often bond ionically with other atoms are
called polyatomic ions. As a general rule, they do not make oxygen – oxygen bonds but radiate the
oxygen atoms around the central atom, making coordinate covalent bonds if needed. Count the number
of valence electrons for the atoms, then add or subtract negative charges as needed to give the correct
final charge on the ion.
Examples:
NO3-
NH4+
Valence electrons: 23 + 1 = 24
OH-
Valence electrons: 9 – 1 = 8
Valence electrons: 8
NO2-
ClO3-
CO32-
NO3-
Valence Electrons______
Valence Electrons______
Valence Electrons______
Valence Electrons______
CN-
PO3-3
SO42-
PO43-
Valence Electrons______
Valence Electrons______
Valence Electrons______
Valence Electrons______
For the following, give the name, draw the Lewis Structure, draw a three-dimensional representation using the
principals of VSEPR, and show the polarity using the delta notation for partial positive and negative charges.
SeCl2
Name:
Lewis Structure
NF3
Name:
Lewis Structure
ICl4Lewis Structure
Name:
PBr3
Name:
Lewis Structure
H3O+
Name:
Lewis Structure
AsCl5
Name:
Lewis Structure
BrF5
Lewis Structure
Name:
OF2
Name:
Lewis Structure
ClO3-
Name:
Lewis Structure
TeF4
Lewis Structure
Name: