Unit 3
How do we predict properties?
The central goal of this unit is to help you develop
ways of thinking that can be used to
predict the physical properties of chemical
compounds based on their submicroscopic structure.
Chemistry XXI
M1. Analyzing Molecular Structure
Predicting properties based
on molecular structure .
M2. Considering Conformations
Predicting properties based
on spatial conformations.
M3. Characterizing Ionic Networks
Predicting properties based
on ion charge and size.
M4. Exploring Electronic Structure
Predicting properties based on
electron-configurations.
Unit 3
How do we predict
properties?
Chemistry XXI
Module 3:
Characterizing
Ionic Networks
Central goal:
To explain and
predict the physical
properties of ionic
compounds based on
the charge and size
of the ions present in
the system.
The Challenge
Modeling
How do I predict it?
Chemistry XXI
Metals can chemically react with nonmetals to form
new chemical compounds. In most cases, these
compounds are not comprised of molecules, but of
ions arranged in solid crystalline networks.
How can we make predictions
about the composition of these
ionic compounds?
How can we predict what
properties they may have?
Important Differences
Nonmetals
Chemistry XXI
Metals
In general, atoms of nonmetals have higher
ionizations potentials and electronegativities (c)
than those of metals. Thus, nonmetals have a
higher affinity for electrons than metals.
From Polar to Ionic
Chemistry XXI
The larger the difference in electronegativity (Dc)
between bonding atoms the more polar the bond.
For Dc > ~1.7, the bonding electrons spend most of
their time close to the nonmetal atom.
Ionic Compounds
Chemistry XXI
The properties of the compounds that results from the
reaction of metals with nonmetals are better described
assuming that independent ions are formed.
Metal  + ion (cation)
Nonmetal  - ion (anion)
Ionic Networks
+ and - ions interact
with all other ions in
the system, and they do
not form molecules.
Chemistry XXI
They arrange into a
crystalline network.
NaCl
For these compounds, the
chemical formula only
indicates the ratio of + to
– ions in the lattice (1:1).
Crystal Structures
Different ionic compounds may
have different crystal structures.
BCC
FCC
Chemistry XXI
CsCl
NaCl
Every type of crystal lattice
can be built by repeating a
characteristic UNIT CELL.
Let’s Think
Chemistry XXI
Ionic compounds do not conduct
electricity as solids, but they do it
when melted. Solid ionic
compounds shatter when struck.
How would you
explain these
properties based
on their
submicroscopic
structure?
A Powerful Model
Chemistry XXI
In ionic compounds, electrons are “localized” on
ions and cannot flow. In molten state, the ions can
move freely and establish an electric current.
Charge and Size
In general, many properties of ionic compounds are
determined by the charge and the size of the ions in
the network.
r
Chemistry XXI
Interactions between
ions are determined by
Coulomb’s law.
q1q2
F 2
r
+
Thus, it is of central importance to be able to
predict the charge and size of the ions formed
whenever a metal reacts with a nonmetal.
Let’s Think
The charge that atoms of metals and nonmetals
acquire when they react with each other is
determined by their electron configurations.
Chemistry XXI
s1 s2
p1 p2 p3 p4 p5 p6
What charges would the atoms in the
highlighted families will acquire when they
react with each other? Justify your reasoning.
Ion Charges
+1 +2
-2 -1
Based on the octet rule, we
can predict that the ions
formed will have full shell
electron configurations.
+1
-1
Chemistry XXI
+2
+3 -4 -3 -2
Transition Metals
Ion charge (+)
Chemistry XXI
The prediction of ion charges for transition metals
is more difficult, because electrons in the
d-subshell may also be transferred to the nonmetal
atoms. So, they may form a variety of ions.
Atomic Number
Common
Less Common
Naming Examples: Cu(I), Cu(II), Nickel(I)
Charge Neutrality
No matter what ions are formed, the ionic network
that is formed has no net charge. These implies that
cations (+) and anions (-) combine in ratios that
ensure charge neutrality.
Consider this reaction:
Na(s) + Cl2(g) 
Chemistry XXI
Na  Na+
Cl  ClStable Ions
Sodium chloride
Thus, the ionic product is NaCl
2 Na(s)
+
Cl2(g)
What happens in this case?
 2 NaCl (s)
4 Al(s) + 3 O2(g)  2 Al2O3
Al  Al3+
O  O2Stable Ions
Thus, the ionic product is Al2O3 Aluminum oxide
Let’s Think
Predict the ratios in which the ions resulting from
these reactions will combine:
K(s) + F2(g) 
Mg(s) + O2(g) 
Chemistry XXI
Na(s) + S8 (s) 
Ca(s) + N2(g) 
Write the chemical formulas for the resulting ionic
compounds and balance each equation.
Predictions
Reaction
mixture
K(s) + F2(g)
Ions
Chemical
Formula
K+ , F-
KF
2K(s)+F2(g)2KF(s)
Potassium fluoride
Mg(s) + O2(g) Mg2+ , O2-
Chemistry XXI
Balanced Chemical
Equation
Na(s) + S8 (s)
Na+ , S2-
Ca(s) + N2(g)
Ca2+ , N3-
MgO
2Mg(s)+O2(g)2MgO
Magnesium oxide
Na2S
16Na(s)+S8(s)8Na2S(s)
Ca3N2
3Ca(s)+N2(g)Ca3N2(s)
Sodium sulfide
Calcium nitride
In general, given Am+ and Bn-, the chemical
formula that ensures neutrality will be:
AnBm
Let’s Think
During the formation of ionic compounds, atoms of
metals lose electrons while those of nonmetals
gain them:
Chemistry XXI
A – m e-  Am+
B + n e-  BnHow would you expect the
size of each type of atom
to change when becoming
ions?
How would you expect the
periodic trend for ion
sizes to be?
Smaller
cations
Ion Size
Larger
anions
Chemistry XXI
S2184
Periodic
Behavior
R increases
Properties
Chemistry XXI
The properties of an ionic
compound are determined by
the electrostatic forces among
its ions, and between these
ions any surrounding particle
(atom, ion, or molecule).
Coulomb’s Law
r
q1q2
F 2
r
+
Based on Coulomb’s Law, one may expect these
forces to be stronger:
 the larger the charge of the ions (larger q1, q2);
 the smaller the size of the ions (smaller r);
Phase Transitions
Ionic compound tend to
have high melting and
boiling points because
electrostatic forces are
pretty strong.
Chemistry XXI
Let’s Think
Interactions
Energy (KJ/mol)
Ionic
400 - 4000
Covalent
150 - 1100
Metallic
75 - 1000
H-bonding
10 - 40
Dipole-Dipole
5 - 25
Dispersion
0.05 - 40
Arrange each of these sets of
ionic compounds, in order of
increasing melting point:
NaBr
NaCl
NaF
755 oC
801 oC
993 oC
 NaCl, NaBr, NaF
NaCl
CaO
MgO
 CaO, NaCl, MgO
801 oC
2572 oC 2800 oC
Solubility
Chemistry XXI
Some ionic compounds are very soluble in water,
while others are almost completely insoluble.
Ionic compounds that
are insoluble in water
tend to be comprised
of ions with strong
interactions among
each other
(large charge,
small size).
Electrolytes
Chemistry XXI
Strong
electrolyte
Weak
electrolyte
Solutions of soluble
ionic compounds
conduct electricity.
This implies that
ions can freely move
across the system.
Solutions of slightly
soluble ionic
compounds barely
conduct electricity.
Only a small fraction of
the ions is in solution.
Let’s Think
Consider the following examples of soluble and
insoluble ionic compounds:
Chemistry XXI
Soluble
NaCl
MgBr2
K2S
CaI2
NaF
AlCl3
Insoluble
MgO
ZnS
Al2O3
FeS
CrS
Ni2O3
Use this data to derive a simple rule to make
predictions about the solubility of ionic
compounds based on ion charges.
Justify your ideas using Coulomb’s Law.
Solubility Trends
The following rule is useful to make predictions
about the solubility of ionic compounds in water:
Given the ions Am+ Bn-,
Chemistry XXI
if m x n < 4  soluble, otherwise  insoluble
Given Coulomb’s law, we may also
expect that the larger the ions, the more
soluble the compound will be (weaker
interactions among ions in the lattice).
q1q2
F 2
r
Although this rule works well in many cases, there
are always exceptions.
For example, AgCl, PbCl2, MgF2, CaF2, are insoluble.
Polyatomic Ions
There are ionic compounds in which the ions in the
lattice are not necessarily atomic (one single atom),
but molecular (several atoms covalently bonded).
OH-
Chemistry XXI
Hydroxide ion
SO42Sulfate ion
NO3Nitrate ion
PO43Phosphate ion
CO32Carbonate ion
NH4+
Ammonium ion
Comparisons
No matter the type of ion,
we can still make
predictions about the
structure and properties of
these ionic compounds
based on similar ideas.
NH4+
Cl-
Chemistry XXI
Let’s Think
Ammonium
Chloride
NH4Cl
Make predictions about the solubility of these ionic
compounds:
Barium Sulfate Magnesium Nitrate Calcium Phosphate
Ammonium Nitrate
Calcium Carbonate
Applications
Insoluble
Ca3(PO4)2
CaCO3
Soluble
NH4NO3
Mg(NO3)2
Structural
components in
living things.
Chemistry XXI
Important
fertilizers.
Insoluble
BaSO4
Facilitates
X-ray
analysis
New Applications
Chemistry XXI
Materials based on ionic compounds have a wide
variety of applications. Most ceramics are ionic
solids that are very hard and highly resistant to
corrosion and wear.
Zeolites, for example,
are ionic compounds
of Al, Si, and O, that
have a porous
structure. They are
widely used as filters
and catalysts.
Surprising Properties
Ions of transition metals have unpaired electrons,
and thus they exhibit magnetic properties.
High-T
Superconductor
Ceramics
Chemistry XXI
Solid oxides of
metals such as Fe,
Mn, and Cu exhibit
surprising electric
and magnetic
properties.
Magnetite
Fe3O4
3+
2+
1+,2+
2-
YBa2Cu3O7
Chemistry XXI
Let′s apply!
Assess what you know
Let′s apply!
Some Melting Points
All common ionic
compounds have
melting points above
100 oC and thus are
solid a room
temperature
Cl-
K+
776 oC
NH4+
338 oC
NO3-
334 oC
170 oC
CH3COO- 292 oC
112 oC
Chemistry XXI
Based on what you have learned:
 Hypothesize on the structural characteristics of
ionic compounds that could be liquid at room
temperature.
 Discuss potential applications of these types of
fluids.
“Green” Ionic Liquids
In fact, room temperature ionic liquids (RTILs) have
been synthesized. They tend to be comprised of
single charged (1+ ,1-) bulky asymmetric polyatomic
cations and large anions.
C8H15N2+
Chemistry XXI
BF4
-
PF6-
These liquid are starting to be used as solvents in
many applications. They have low volatility, which
reduces polluting effects (Green Chemistry).
Let′s apply!
Soaps are also ionic compounds in
which the anions are pretty large
hydrocarbon chains.
Explain:
Chemistry XXI
 Why are soaps
soluble in water?
 How would you
expect soap ions to
arrange in solution?
 Why do soaps help
in removing grease?
Monolayer
Micelle
Chemistry XXI
Grease
Chemistry XXI
Come up with a question about the
content of this module that you
desperately want to be answered.
Characterizing Ionic Networks
Summary
Chemistry XXI
Metals can chemically react with nonmetals to form
new chemical compounds. In most cases, these
compounds are not comprised of molecules, but of
ions arranged in solid crystalline networks.
No matter what ions are formed,
the ionic network that is formed
has no net charge. These
implies that cations (+) and
anions (-) combine in ratios that
ensure charge neutrality.
Na+ Cl-
Characterizing Ionic Networks
Summary
Chemistry XXI
The charge that atoms acquire when they react
with each other is determined by their electron
configurations. The ions that are formed tend to
be those with a full shell electron configuration.
Many properties of ionic
compounds are determined
by the charge and the size of
the ions in the network. These
forces are stronger the larger
the charge of the ions and the
smaller their size.
Coulomb’s Law
r
+
q1q2
F 2
r
Chemistry XXI
For next class,
When metals are combine they form alloys.
Investigate the basic characteristics of these
materials.
Why is that metals conduct electricity while
ionic and molecular compounds do not?