Section 8.1 Ionic compounds
8.1 Ionic compounds
Properties of
ionic
substances
Unlike molecular compounds, ionic substances tend to have very similar properties.
They are typically:
Ionic properties
and atomic
level structures
The properties we observe can be traced back to the underlying properties and atomic
level structure of the ionic substances. An ionic substance is formed when a positively
charged ion (due to loss of one or more electrons) is attracted to a negatively charged ion
(due to gaining one or more electrons). Ions will attract to each other, forming ionic
bonds with all neighboring atoms. Let’s take a look at sodium chloride more closely:
Ion attraction
Every sodium ion is surrounded by, and attracted to, six nearby chloride ions, and every
chloride ions is surrounded by and attracted to six nearby sodium ions. The ions attract to
each other in very large numbers. In one typical grain of table salt there are about 1018
atoms (a billion billion atoms). Since sodium ions have a charge of +1, and chloride ions
have a charge of -1, half of the atoms are sodium ions and half are chloride ions.
Positive and
negative
charges cancel
out
In any ionic crystal, the ratio of positive ions to negative ions must allow for all of the
positive charge to cancel out all of the negative charge. Otherwise, the ions in the crystal
would not be able to hold together. Below are some other ionic substances:
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• hard and brittle
• solid at room temperature
• have very high melting points
• conduct electricity if heated to a liquid state
• conduct electricity if dissolved in water or some other solvent
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Connecting ionic structure to properties
Bonding
patterns of
ionic crystals
Now let’s consider how ionic structure brings about their properties. Three of the
properties are all related: hardness, melting point and state of matter.
Ionic
substances are
brittle
Ionic substances are brittle because
putting pressure along one edge can
cause the ions to shift place so that
the positive and negative ions from
one layer are not properly aligned
with the oppositely charged ions in
the next layer. If this happens there
will be a repulsion between the
layers and the crystal will break.
Electric current
An electric current is the movement of many electric charges in a particular direction.
When melted or dissolved, the ions are free to move around, making it is possible to
conduct electricity.
Ionic substances are hard, have very high melting
points, and so they are solid at room temperature.
All of this can be explained by the bonding patterns
found in ionic crystals. Each ion is attracted to and
bonded with all of its neighbors, so the ions have an
interconnected network of bonds holding the entire
crystal together. This gives the overall crystal
strength, making it hard. This also makes it difficult
to melt. To be in a liquid state, the ions have to have
enough kinetic energy (have a high enough
temperature) to continually break free of the
attractions allowing them to flow by each other.
brittle - hard, but with the possibility of breaking or shattering relatively easily.
electric current - the directed movement of electric charges, either ions or
electrons.
A NATURAL APPROACH TO CHEMISTRY
231
Section 8.1 Ionic compounds
Polyatomic ions
An ion can also
be a small
molecule with
a charge
Until now all of our examples of ionic compounds have been made from ions that are
single atoms with either a positive or negative charge. However, an ion can also be a
small molecule with a charge. Remember, something becomes charged when there is an
unequal amount of protons and electrons, so for a molecule to become charged you need
the total number of protons and electrons in the entire molecule to be unequal. One
example of a polyatomic ion (an ion made from more than one atom), is carbonate
with the formula CO32-. One carbon is bonded to three oxygens and the entire molecule
has a -2 charge.
Bonding
patterns are
similar in ionic
substances
with
polyatomic
ions
Ionic crystals with polyatomic ions are very
similar to the ones we have seen with single
atom ions. They pack together in a regular
pattern with each positive ion attracting to all of
its negative neighbors, and each negative ion
attracting to all of its positive neighbors. Most
polyatomic ions are negative, but there are a few
positive ones. To the right is an example of
ammonium sulfate which is made from two
polyatomic ions: ammonium (NH41+) and
sulfate (SO42-). It has the formula (NH4)2SO4.
polyatomic ion - a small molecule with an overall positive or negative charge.
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Writing formulas for ionic compounds
Ionic crystals
come in many
sizes
A grain of table salt is a small piece of an ionic substance, but
as we learned earlier, that grain of salt is made from
approximately 1 billion billion ions. Not all grains of salt are
the same size. If you carefully grow a salt crystal under the
right conditions, there is no limit to how big the “grain” could
be. In an underground cavern in Germany there are single
“grains” that measure over one meter in length on a side. The
image to the right shows a large single crystal of table salt from
the Fersman Mineralogical Museum in Moscow.
Ionic formulas
tell you the
ratio of ions in
the compound.
The formula for table salt is NaCl, but what does that mean if you can have a single
crystal that has enormous variations in size? For ionic compounds, the chemical formula
tells us the ratio of positive to negative ions in the entire substance. The ratio must be just
right so that the total positive charge from all of the positive ions will equal the total
negative charge from all of the negative ions.
Write the correct formula for the compounds which will form using the following
ion pairs (Mg2+ and Cl1-; Na1+ and S2-; Al3+ and O2-).
Asked:
What is the correct ionic formula for three different ionic substances?
Given:
The charges on all of the ion pairs; see above.
Relationships: The positive ions will combine with the negative ions in a ratio so
that the positive and negative charges from the ions will be equal.
Solve:
For each Mg2+ we need two Cl1- (+2 -1-1=0) and the formula is MgCl2
For each S2-we need two Na1+ (-2 +1+1=0) and the formula is Na2S
To balance charges between Al3+ and O2- we need to multiply Al by two
and O by three (2(+3)+3(-2)=0). The formula is Al2O3
A NATURAL APPROACH TO CHEMISTRY
233
Section 8.1 Ionic compounds
Writing formulas with polyatomic ions
formulas with
polyatomic
ions
Writing ionic formulas using polyatomic ions is
basically the same as writing them with simple single
atom ions. The strategy is the same. You need to write a
formula that has the correct ratio of positive to negative
ions. The only difference is how you show that you
want more than one polyatomic ion. Since an ion like
nitrite (NO21-) is a single unit – a small molecule with a
negative charge – we treat that ion like it is a single
atom. To write a formula that says you need two or
three NO21- ions you need to put parentheses around
the ion and write it like this: three nitrites = (NO21-)3.
Write the correct formula for the compounds which will form using the following
ion pairs; (Mg2+ and SO42-; Ca2+ and PO43-; NH41+ and S2-).
Asked:
What is the correct ionic formula for three different ionic substances?
Given:
The charges on all of the ion pairs. See above.
Relationships: The positive ions will combine with the negative ions in a ratio so
that the positive and negative charge from the ions will be equal.
Solve:
For each Mg2+, need one SO42- (+2 -2=0) and the formula is MgSO4
To balance charges between Ca2+ and PO43- , multiply Ca by three and
PO4 by two: (3(+2)+2(-3)=0). The formula is Ca3(PO4)2
For each S2-, need two NH41+ (-2 +1+1=0) and the formula is (NH4)2S
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Writing names for ionic compounds
Naming binary
ionic
compounds
When naming an ionic compound, you just write the names of the ions as they appear in
the formula. For simple binary ionic compounds (ionic formulas from two single atom
ions), like NaCl, the first part of the name is just the name of the element, and the second
part of the name is a modified version of the name of the element. Typically, you drop the
standard ending and add -ide.
Names for
polyatomic
ions are found
on the
common ion
table
For polyatomic ions you will need to get the name from the table below. Otherwise, it is
done the same way as it is for binary ionic compounds. Write the name of the ions in the
order that they appear in the formula.
TABLE 8.1. Common Ions
positive ions
Aluminum Al
Lead(II) Pb2+
Ammonium NH41+
Lead(IV) Pb4+
Barium Ba2+
Magnesium Mg2+
Copper(I) Cu1+
Mercury(I) Hg21+
Copper(II) Cu2+
Mercury(II) Hg2+
Calcium Ca2+
Potassium K1+
Chromium(II) Cr2+
Silver Ag1+
Chromium(III) Cr3+
Sodium Na1+
Hydrogen* H1+
Tin(II) Sn2+
Iron(II) Fe2+
Tin(IV) Sn4+
Iron(III) Fe3+
Zinc Zn2+
3+
negative ions
Acetate C2H3O2
Hydrogen sulfate
1Chloride Cl
(bisulfate) HSO41Carbonate CO32Hydroxide OH1Chromate CrO42Nitrate NO31Chlorate ClO31Nitrite NO21Chlorite ClO21Oxide O2Cyanide CN1Peroxide O22Dichromate Cr2O72Phosphate PO43Fluoride F1Sulfate SO42Hydrogen carbonate
Sulfite SO32(bicarbonate) HCO31Sulfide S21-
*Hydrogen ions rarely exist by themselves. Often they combine with water to form hydronium ions: H3O1+.
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235
Section 8.1 Ionic compounds
Ionic naming with transition metals
Roman
numerals in the
ion name
On the previous table you will see several names for
positive ions that are followed by a Roman numeral.
This number describes the charge on that ion. Most
transition metals form multiple charges, so to know
which charge the ion should have when naming the
compound, you need to specify it with a Roman numeral.
The common
ion table
Since the transition metals and those metals in
groups 14 and 15 can form varying charges, you
need to check the common ion table before you
write the name of an ionic compound that
includes one of those elements. Most transition
metals do, but there are two common exceptions:
silver and zinc. Silver almost always forms a +1
ion and zinc a +2 ion, so you don’t need Roman
numerals in names of silver or zinc compounds.
Write the name for each of the following formulas: CaCl2, Zn(NO3)2, Fe3(PO4)2:
Asked:
Name the formulas above.
Given:
Three formulas, and a common ion table.
Relationships: The name of the formula is constructed from the name of the ions. If
the negative ion is a single atom, modify the name of the element to
end in “-ide”. If the positive ion might form multiple charges, make
sure to use a Roman numeral in the name.
Solve:
CaCl2 = calcium chloride
Zn(NO3)2 = zinc nitrate
Fe3(PO4)2 = iron(II) phosphate
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