Transcript Writing Formulas for Ionic Compounds

Writing Formulas for
Ionic Compounds
Anatomy of a
Chemical Formula
 Chemical formulas express which
elements have bonded to form a
compound. The subscripts express the
elements' numerical relationships to one
another.
Formula Unit for
an Ionic Compound
 Since ionic compounds do not exist as
single units, the formula of an ionic
compounds represents the simplest ratio of
ions. It is called a FORMULA UNIT.
 Example: MgCl2 is a formula unit of
magnesium chloride.
 Cations are written first in the formula unit;
anions second.
Formula Unit for
an Ionic Compound
 The charges of the ions that make up an
ionic compound must cancel out - they must
add up to zero!
 Example: MgCl2 is made up of one Mg+2
ion and 2 Cl- ions. The charges cancel (add
uo to zero): +2 + 2(-1) = 0
An easy way to write a chemical
formula in which the charges of
the cations and anions cancel
each other out is to use the crisscross method.
Criss-cross method
 1. Determine the ions (their symbols and
their charges) that are present in the
compound
 2. Take the NUMBER ONLY of the charge
for the cation and write it as a subscript
after the anion
 3. Take the NUMBER ONLY of the charge
for the anion and write it as a subscript
after the cation
Example: Magnesium chloride
Mg+2
Cl-
1 becomes the
2 becomes the
subscript of Mg
subscript of Cl
MgCl2
Exceptions
 Ca2+ and O2- would form Ca2O2 if you used
the criss-cross method
 However, in reality, Ca2+ and O2- form CaO.
 For ionic compounds, you must simplify
(reduce) the subscripts when necessary.
Practice Problems
 Write the formula for the compounds listed
below.
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potassium oxide
lithium nitride
calcium phosphide
magnesium sulfide
aluminum oxide
aluminum iodide
Remember: the name of the anion is
the root of the element’s name with the
suffix –ide!
Writing Formulas for Compounds
with Transition Metals
 The charge of transition metal ions and Group 4A
metal ions can vary.
Example: Iron can form a +2 or +3 ion (Fe+2 or
Fe+3); Lead can form a +2 or +4 ion (Pb+2 or Pb+4)
 In most cases, a roman numeral written in
parentheses after the metal in the compound’s
name will tell you the charge of the metal.
 When this information is NOT given to you, it
means the transition metal has only one possible
charge. To find out what that is, you must look on
your “ion sheet”.
Writing Formulas for Compounds
with Transition Metals
 Examples
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iron (II) oxide contains Fe+2 and the
compound’s formula is FeO
iron (III) oxide contains Fe+3 and the
compound’s formula is Fe2O3
Zinc oxide contains Zn+2 and the
compound’s formula is ZnO.
Practice Problems
 Write the formulas for the following
compounds:
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cobalt (III) sulfide
mercury (I) chloride
chromium (VI) oxide
tin (IV) nitride
cobalt (II) sulfide
Writing the Formula Unit
for an Ionic Compound
 Binary ionic compounds are made up of 2 kinds of
elements.
 Polyatomic ionic compounds are made up of more
than 2 kinds of elements.
(Ex. NaOH)

One or both of the ions in such a compound must be a
polyatomic ion (an ion made up of more than 1 kind of
atom). In our example, Na+ is the cation and OH- is the
polyatomic anion. OH- contains O and H!
Polyatomic Ions
 The charge of the polyatomic ion
applies to the entire ion (all of the
atoms as a group)
 A polyatomic ion acts as if it were
an ion with one kind of atom. The
different atoms act together, as a
group.
Polyatomic ions
 Example: NH4+ is a polyatomic cation made
up of one N atom and 4 H atoms bonded
together. The overall charge on this bonded
group is +1.
 Example: OH- is a polyatomic anion made
up of 1 O atom and 1 H atom bonded
together. The bonded atoms have an overall
charge of -1.
Writing Formulas for Compounds
with Polyatomic Ions
 Determine the ions (and their charges)
that are present in the compound. If one
is a polyatomic ion, use your “ion sheet”
to find the formula of the ion and its
charge.
Writing Formulas for Compounds
with Polyatomic Ions
 Follow the criss-cross rules with 1
exception:
 If more than one polyatomic ion is
required in the compound’s formula, the
formula of the polyatomic ion MUST BE
set off by parentheses.
 The subscript will be written to the right
of the parentheses to tell how many units
of the polyatomic ions are present.
Writing Formulas for Ionic
Compounds
 Example: magnesium nitrate
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Mg2+ is the cation; NO3- is the anion.
NEVER
EVER
change the subscripts of the
atoms within a polyatomic ion.
Magnesium chlorate
 Mg2+is the cation; ClO3- is the anion;
Mg(ClO3)2 is the formula of the compound.
NOT
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Mg2ClO3
MgCl2O6
Mg3ClO2
Mg2ClO3
Practice Problems
 Write the formulas for the following
compounds:
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calcium hydroxide
tin (IV) sulfate
aluminum carbonate
nickel (II) nitrate
ammonium phosphate
Naming Ionic Compounds
Rules for Naming Ionic
Compounds
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Binary ionic compounds
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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.
Li3P
lithium phosphide
Rules for Naming Ionic
Compounds
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Ionic compounds with transition metals
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The charge of the transition metal ion must be
determined.
You must work back from the formula to
determine the charge of the transition metal
ion.
Once it is determined, use parentheses after
the metal ion name to indicate this charge.
The subscript on
Co2S3
the S indicates
the charge of Co
is 3+
3+
Co
The subscript on the Co
indicates the charge of S
is 2-
2S
The name is
cobalt (III) sulfide
Rules for Naming Ionic
Compounds
 If the compound contains any polyatomic
ion, name the ion to name the compound.
NaHCO3
Sodium hydrogen carbonate
or
sodium bicarbonate
Practice Problems
1.
2.
3.
4.
5.
6.
CaSO4
CuO
AlBr3
Mg(OH)2
FePO4
NH4NO3