What are Polyatomic Ions?
How do they form ionic compounds?
Section 6.3b 6.4b
Objective
I
6.3b-4b
know the difference of a
polyatomic ion vs. a
monatomic ion
I can write formulas and
names of polyatomic ionic
compounds
Monatomic Ions QUICK REVIEW….



Mono means “one”
Monatomic ions are charged atoms.
Monatomic cations are atoms that lose electrons to
become positively charged:



Monatomic anions are atoms that gain electrons to
become negatively charged, their name ends in -ide



Lithium ion  Li+
Berllium ion  Be2+
Oxide ion  O2Bromide ion  Br-
They form ionic compounds like Li2O and BeBr2
Polyatomic Ions



Polyatomic ions are charged molecules.
The atoms within a polyatomic ion are usually very
tightly bound together, and behave as a unit
so the ion retains its identity within ionic compounds
and over the course of many chemical reactions.
N
O
O
Nitrite = (NO2)-
-
O
O
N
O
Nitrate = (NO3)-
Many polyatomic anions occur in





–ite/-ate pairs.
-ite / -ate pairs…….Do you see a pattern?
-ite
-ate
sulfite, (SO3)2-
sulfate, (SO4)2-
nitrite, (NO2)-
nitrate, (NO3)-
chlorite, (ClO2)-
chlorate, (ClO3)-
The charge of each polyatomic ion –ite/-ate pair is the same.
The –ite ending indicates less oxygen than the –ate ending.
However, the ending does not tell you the actual number of
oxygen atoms in the ion
Most polyatomic ions are anions
Polyatomic anion Naming Example
monatomic
anions end in
-ide
as you well
know!
Two less oxygens
hypo-…-ite
One less
oxygen
end in
-ite
The most
common
form ends
in
-ate
One more oxygen
per-…-ate
The most common form of a polyatomic
ion usually ends with….
A.
B.
C.
-ide
-ite
-ate
As you already know, monatomic anions
always end with….
A.
B.
C.
-ide
-ite
-ate
The Common Polyatomic Ions

Most common polyatomic ions are
anions and have a negative charge



There is one common cation…
Ammonium
NH4+
Most polyatomic anions end
in –ate or –ite



There are two name exceptions…
Hydroxide
OHCynide
CN-
Which of these is the correct formula
and charge of a nitrate ion?
A.
B.
C.
D.
NO2
NO3
NO2
NO3
charge = 1+
charge = 1+
charge = 1charge = 1-
Which of these is the correct formula
and charge of an amonium ion?
A.
B.
C.
D.
NH3
NH4
NH3
NH4
charge = 1+
charge = 1+
charge = 1charge = 1-
Which of these is the correct formula
and charge of a sulfite ion?
A.
B.
C.
D.
SO3
SO4
SO3
SO4
charge = +2
charge = +2
charge = -2
charge = -2
What is the name and charge of the CrO4
ion?
A.
B.
C.
D.
E.
F.
Carbonate
Chromate
Dichromate
Perchlorate
Chromate
Dichromate
charge = -2
charge = 4
charge = -2
charge = -1
charge = -2
charge = 2
Writing Formulas with Polyatomic Ions
Use the cross-cross method to find the formula for ionic
compounds that contain polyatomic ions.
 Put (parentheses) around each polyatomic ion, and
NEVER CHANGE THE IONS SUBSCRIPTS!!!
 (parenthesis) are usually dropped if the polyatomic ion’s
subscript is 1.
 Formula Examples:
a) calcium nitrate
b) sodium hydroxide

Writing Formulas with Polyatomic Ions
Use the cross-cross method to find the formula for ionic
compounds that contain polyatomic ions.
 Put (parentheses) around each polyatomic ion, and
NEVER CHANGE THE IONS SUBSCRIPTS!!!
 (parenthesis) are usually dropped if the polyatomic ion’s
subscript is 1.
 Formula Examples:
a) calcium nitrate
b) sodium hydroxide

Ca2+
(NO3)Ca(NO3)2
Na+
OHNaOH
Which of these is the formula for the
compound formed from magnesium ions
and hydroxide ions?
A.
B.
C.
D.
Mg(OH)2
Mg2OH
MgOH2
MgO2H2
Which of these is the formula for the
compound formed from Al3+ and SO32-?
A.
B.
C.
D.
Al3(SO3)2
Al2SO3
Al2(SO3)3
Al3(SO2)3
Which of these is the formula for
potassium hypochlorite?
A.
B.
C.
D.
P(ClO)2
K2CO
PClO2
KClO
Naming Ionic Compounds with Polyatomic Ions

Naming examples:
Simply name the cation and polyatomic anion with no
change to either name.

Examples

Mg3(PO4)2
magnesium phosphate
Li2SO4
lithium sulfate
Name the compound….. LiCN
A.
B.
C.
D.
lithium cyanate
lithium cyanite
lithium cyanide
lithium nitride
Name the compound….. (NH4)2SO3
A.
B.
C.
D.
E.
ammonium sulfur trioxide
hydro nitrogen sulfate
ammonium sulfate
ammonium sulfite
diammonium sulfite
Name the compound….. Fe(ClO3)2
A.
B.
C.
D.
iron(II) chlorate
iron(III) chlorite
iron(I) dichlorate
iron(II) chloride
What did we learn?





Polyatomic ions are charged molecules that are very
tightly bound together, so that they retain their identity
within ionic compounds and over the course of many
chemical reactions.
Monoatomic ions end in –ide as we well know.
Polyatomic ions add –ate, -ite or per- -ate
Polyatomic ions behave as a unit and carry a charge as a
group of atoms.
In formulas, use (parenthesis) around polyatomic ions like:
Ca(NO3)2
Additional Practice
1. Write the symbol and charge for the cation and anion
then use criss-cross method to write the formula.
a) potassium sulfate

K2SO4
b) ammonium chloride
NH4Cl
Additional Practice
Name these compounds
a) Na2CO3

Sodium Carbonate
b) AgNO3
Silver Nitrate