Summary Chapter 3-4
General, Organic, & Biological Chemistry
Janice Gorzynski Smith
CHAPTER 3-4: Concepts to Know
 The difference between ionic and covalent bonds
 Define cations and anions
 Predict cation/anion charge using the octet rule or group number
 Familiar with metals with multiple potential charges (do not
need to memorize)
 Determine ionic compound formulas from the name of a compound
or from the elements that compose it.
 Criss-cross rule
 Naming ionic compounds and covalent molecules
 Familiar with polyatomic ions (do not need to memorize but must
be able to recognize)
 Draw lewis dot structures
 Determine molecular geometry
 Identify polar bonds
 Determine dipole moment of molecules
Need to Memorize
Ionic vs Covalent Bonding
Ionic Bonds result from electrostatic attraction
between a cation and anion: metal-nonmetal (with the
exception of NH4+ and H3O+ cations).
Covalent bonds result from the sharing of electrons
between two atoms: nonmetal-nonmetal.
Li
F
Ionic Bonds
Covalent Bonds
Naming
HOW TO Name an Ionic Compound
Step [1]
Determine the charge on
the cation.
Step [2]
Name the cation and the
anion
 If the cation could be
multiple charges indicate
the charge with roman
numerals or with a –ous /
-ic suffix.
Step [3]
Write the name of the
cation first then the name
of the anion
HOW TO Name a Covalent Molecule
Step [1]
Step [2]
Name the first nonmetal by
its element name and the
second using the suffix
“-ide.”
Add prefixes to show the
number of atoms of each
element.
Predicting Cations & Anions
the cation charge = the group number
the anion charge = 8 – group number
Octet Rule
The octet rule: a main group element is especially stable
when it possesses an octet of e− in its outer shell.
octet = 8 valence e−
Exceptions (need to memorize):
F
F
B F
HO
P OH
OH
only 6 e− on B
O
O
10 e− on P
HO
S OH
O
12 e− on S
Ionic Compound Formulas
HOW TO Write a Formula for an Ionic Compound
Step [1]
Identify which element is the cation
and which is the anion.
Step [2]
Determine how many of each ion type is
needed for an overall charge of zero.
 When the cation and anion have different
charges, use the ion charges to determine
the number of ions of each needed.
Step [3]
To write the formula, place the cation
first and then the anion, and omit charges.
“Criss-cross” rule
 Make magnitude of charge on one
ion into subscript for other
 When doing this, make sure that
subscripts are reduced to lowest
whole number.
Al3+ O2–
Al2O3
Lewis Dot Structures
NH3
Step [1] Arrange the atoms next to each
other that you think are bonded
together. Place H and halogens on
the periphery, since they can only
form one bond.
Step [2] Count the valence electrons. The
−
sum gives the total number of e
that must be used in the Lewis
structure. For each atom the
number of bonds = 8 – valence
electrons.
N
H
H
H
Nitrogen has 5 valence electrons, so
it will have 8 – 5 = 3 bonds.
Hydrogen will have 2-1 = 1 bond.
There are 8 total valance electrons
H
N
H
Step [3] Arrange the electrons around the
atoms. Place one bond (two e−)
between every two atoms. Use all
remaining electrons to fill octets
with lone pairs, beginning with
atoms on the periphery.
H
1 lone pair: 2
3 bonds:
6
Total e8
= total valence e-
Resonance Structures
Resonance structures exist when there are multiple lewis dot structures
with different electron arrangements with the same connectivity between
atoms. Resonance structures help us understand delocalization
(spreading) of charge within a molecule that stabilizes the anion or cation.
Negative charge not localized
on 2 of 3 oxygen, but instead
delocalized and spread over
entire molecule.
Other Examples: CO32- and O3
NO
YES
Molecular Shape
Periodic Trend: Electronegativity
Electronegativity
INCREASING
Polarity
1. Assess the relative electronegativity of atoms bonded
together, if there is a difference it is a polar bond.
2. Indicate polar bonds with δ+ / δ - or
3. If polarity of bonds does not cancel draw the overall
dipole moment of the molecule using
Electron density is disproportionately
distributed over the molecule. Above red
indicates partial negative charge, or greater
electron density, and blue indicates partial
positive charge.
Effectively oxygen is hogging the electrons
Practice Problems: Lewis Dot Structures
For each molecule draw a lewis dot structure, some have several correct answers,
try to figure them all out. Watch out some have multiple bonds. And the last
molecule is an anion, find all resonance structures.
CH4S
C5H10
C3H8O
C2H3N
C2H3O2-
Solutions will be posted separately, in the solutions folder on my facweb site.