Chapter 8:
Ionic Compounds
Forming Chemical Bonds
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The atoms in compounds are held together by a
force called a chemical bond.
These bonds form because of an attraction
between oppositely charged atoms, ions, or
between electrons and the nuclei.
The valence electrons are the ones mainly
involved in bonding.
Remember that elements in the same group have
the same number of valence electrons.
Ions
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Elements react so that they can achieve the stable
electron configuration of a noble gas, usually an
octet of electrons.
Remember, a cation is a positive ion formed
when electrons are lost.
Anions are negative ions formed when electrons
are gained.
The periodic table can be used to predict charges.
Practice
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1.
2.
3.
4.
For each of the following atoms, write the econfiguration. Then write the formula of the ion
most likely to form and identify as a cation or
anion. Finally, write the e- configuration of the
ion.
Bromine, element 35
Gallium, element 31
Sulfur, element 16
Rubidium, element 37
Formation and Nature of Ionic Bonds
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To bond ionically, atoms must transfer
valence e-.
An atom that loses one or more e-, becomes a
positive ion.
An atom the gains one or more e-, becomes a
negative ion.
An ionic bond is an electrostatic force
holding oppositely charged ions together.
Problems
Write the e- configuration, in abbreviated form
for the atoms in each pair. Then determine
the ratio of the atoms in the ionic compound
formed in each case.
5. Aluminum and fluorine
6. Lithium and oxygen
7. Beryllium and selenium
8. Gallium and sulfur
Properties of Ionic Compounds
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Ionic compounds are always nonconductors
of electricity when solid, but good conductors
when melted.
They also act as electrolytes, substances that
conduct electric current when dissolved in
water.
These characteristics are ways that ionic
compounds can be identified, although each
one individually is not reliable.
Lattice Energy
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In a solid ionic compound, the positive ions are
surrounded by negative ions and the negative
ions by positive ions.
The resulting structure is called a crystal lattice
and contains a regular, repeating 3-D
arrangement of ion.
This arrangement involves strong attraction
between oppositely charged ions, and produces
certain properties, such as mp, bp and brittleness.
Lattice Energy
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The amount of energy required to separate one
mole of ions of an ionic compound is called the
lattice energy.
This value is expressed as a negative number.
The more negative the number, the stronger the
force of attraction between the ions.
Lattice energy is greater for more highly charged
ions and small for ions of lower charge or size.
Problem
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On the basis of the properties of the following
“unknowns”, classify each as either ionic or
not ionic.
a.
b.
c.
d.
Conducts electricity when solid
Conducts electricity when liquid and has a low
mp
Has a high bp and shatters when hammered
High mp and conducts electricity when dissolved
in water
Problem
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For each of the following pairs of ionic
compounds, state which would be expected to
have the higher (more negative) lattice
energy.
a. LiF or KBr
b. NaCl or MgS
c. MgO or RbI
Names and Formulas for
Ionic Compounds
Section 8.3
Why Formal Names
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Remember that scientists come from all over the
world.
The chemicals they refer to in conversation must be
easily understood.
Therefore a set of rules is used in the naming of
compounds.
This allows everyone to write a chemical formula
when given the name and the name the compound
when given the formula.
Formulas for Ionic Compounds
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Remember that ionic compounds form crystal
structures by the way the ions arrange in the
3-D structure.
The smallest ratio of the ions represented in
an ionic compound is called the formula unit.
Because the total # of e- gained by nonmetals
is equal to the # lost by metals, the overall
charge of a formula unit is zero.
Determining Charge
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Binary ionic compounds are composed of a
positively charged monatomic ions and
negatively charged monatomic ions of a
nonmetal.
A monatomic ion is a one-atom ion, such as
Mg2+
The charge of a monatomic ion is its
oxidation number.
Oxidation Numbers
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Transition metals are named that because many of
them can have more than one oxidation number.
The oxidation state of an element in an ionic
compound is the number of e- transferred from an
element to form the ion.
The oxidation # is used to determine the formulas of
the ionic compounds they form.
Writing Formulas
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When writing the formula of a compound
remember that the oppositely charged ions
must have a sum of zero.
When writing formulas, the cation symbol
comes first, followed by the anion symbol.
Subscripts are used to represent the number of
ions of each element in an ionic compound. If
no subscript, it is assumed to be one.
Example
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2.
3.
4.
5.
Let’s write the correct formula for the ionic
compounds composed of the following
elements.
Potassium and iodine
Magnesium and chlorine
Aluminum and bromine
Cesium and nitrogen
Barium and sulfur
Polyatomic Ions
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Some ionic compounds contain polyatomic
ions.
Polyatomic ions are ions made up of more
than one atom.
The charge given to a polyatomic ion applies
to the entire group of atoms.
Since polyatomic ions exist as a unit, do not
change the subscripts of the atoms in the ions.
Instead use parentheses.
Common Polyatomic Ions
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Learn the list of polyatomic ions and their
charges as listed on page 224 in table 8-6.
Naming Ions and Ionic Compounds
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Most polyatomic ions are oxyanions.
An oxyanion is a polyatomic ion composed of an
element bonded to one or more oxygen atoms.
Rules.
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The ion with more oxygen atoms is named using the root
of the nonmetal plus the suffix –ate.
The ion with fewer oxygen atoms is named using the root
of the nonmetal plus the suffix –ite.
For example:
NO3- NO2-
SO42-
SO32-
Other Oxyanions
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Some elements form more than two
oxyanions.
These are named according to the number of
oxygen atoms present.
Examples,
ClO4ClO3ClO2ClO-
Rules for Naming Oxyanions
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2.
3.
4.
The rules for naming oxyanions are as follows:
The oxyanion with the greatest number of oxygen atoms is
named using the prefix per-, the root of the nonmetal and
the suffix –ate.
The oxyanion with one less oxygen atom is named with the
root of the nonmetal and the suffix –ate.
The oxyanion with two fewer oxygen atoms is named using
the root of the nonmetal plus the suffix –ite.
The oxyanion with three fewer oxygen atoms is name using
the prefix hypo-, the root of the nonmetal, and the suffix –
ite.
Naming Ionic Compounds
1.
2.
3.
4.
5.
Name the cation first and the anion second.
Monatomic cations use the element name.
Monatomic anions take their name from the root of
the element name plus the suffix –ide.
If an element has more than one oxidation number,
the oxidation # of the element in that compound
must be indicated with a Roman numeral in
parentheses following the cation.
If the compound contains a polyatomic ion, simply
name the ion.
Examples
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1.
2.
3.
4.
5.
Name the following compounds.
NaBr
CaCl2
KOH
Cu(NO3)2
Ag2CrO4
Metallic Bonds and
Properties of Metals
Section 8.4
Metallic Bonds
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Although they are not ionic, they share
several properties.
Metallic bonds form lattices in the solid state.
In these lattices, the valence e- of the metals
overlap, to create the electron sea model.
The e- are not held by any specific atom but
rather move easily from one atom to the next.
They are often called delocalized electrons.
Metallic Bonds
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The moving of the electrons causes the
formation of the metallic cation.
Each cation is bonded to the next and they are
surrounded in a sea of e-.
A metallic bond is the attraction of a metallic
cation for delocalized electrons.
Properties of Metals
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The properties of metals can be explained by
metallic bonding.
Melting/Boiling points–
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Maleability–
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In general, moderately high melting and boiling
points
They can be hammered into sheets
Ductile
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They can be drawn into wire
Properties (cont’d)
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Durability–
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Conductivity—
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The cations are not removed from the metal
easily.
Sea of electrons allows them to conduct
electricity.
Hardness/Strength—
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Increased with an increase in the # of delocalized
e-
Alloys
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An alloy is a mixture of elements that has
metallic properties.
Alloys most commonly form when elements
have similar size or one is significantly
smaller than the other.
Two basic types, substitutional and
interstitial.
Examples of Alloys
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Examples of alloys are:
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Brass
Bronze
Cast Iron
Gold, 10 carat
Pewter
Stainless steel
Sterling silver