Polyatomic Ions and
Their Compounds
Polyatomic Ions
A polyatomic ion is an electrically charged
group of two or more chemically bonded
atoms that functions as a single ion.
 The atoms of the ion are bonded together
with covalent bonds, but the group of
atoms as a whole has a charge.
 Polyatomic ions combine with other
monatomic or polyatomic ions to form
ionic compounds according to the same
rules as monatomic ions.
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Nitrate (NO3-)
http://www.chemistry.oregonstate.edu/courses/ch121-3/ch121/ch121latestnews/polyatomic%20ions.htm
Sulfate ion (SO42-)
http://www.chemistry.oregonstate.edu/courses/ch121-3/ch121/ch121latestnews/polyatomic%20ions.htm
Writing the Formula for a Compound
Containing a Polyatomic Ion
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Just as in binary compounds, cations and anions
must be combined such that the total positive
charges equal the total negative charges.
Determine the ions and their charges.
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The charges of monatomic ions may be determined from
their position on the periodic table or, in the case of
transition elements, from the Roman numeral in the
name of the compound.
The charges of the polyatomic ions are given on the
chart on page 178.
Combine the ions in such a way that the overall
compound is neutral.
Example: Sodium phosphate
Na is found in the first column. Its charge
is therefore +1.
 The phosphate ion is PO43-.
 To make a neutral compound, there must
be three Na+ for every one PO43-.
Therefore, the formula is Na3PO4.
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Example: Copper (II) hydroxide
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The copper ion is Cu+2 (The Roman numeral
indicates this).
The hydroxide ion is OH- (From the chart on page
178).
There must be two OH- ions for every one Cu+2.
Therefore, the formula must be Cu(OH)2.
Note the parentheses that enclose the polyatomic
ion. Parentheses are used to show that
everything inside the parentheses are doubled by
the subscript 2.
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Cu(OH)2 has 2 O and 2 H. To write CuOH2 would mean
that the compound has 1 O and 2 H.
Example: Ammonium sulfate
The cation is the polyatomic ion
ammonium, NH4+.
 The anion is the sulfate ion, SO42-.
 Two NH4+ are needed for every one SO42-,
 Therefore, the formula is (NH4)2SO4.
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Naming Compounds Containing
Polyatomic Ions
The monatomic ions in the compound will
be named just as they are in binary
compounds.
 Use the chart on p.178 of your text to
identify the polyatomic ion. Its name is
given next to its formula.
 Name the cation first, then the anion.
 If the cation is a transition metal, be sure
to indicate its charge with a Roman
numeral.
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Example: K2CO3
The cation is the K+, potassium ion.
 The anion, CO32-, is listed in the chart on
page 178. Its name is carbonate.
 Therefore, this compound is potassium
carbonate.
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Example: Cr(OH)2
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The cation is chromium, which can have more
than one possible charge.
The anion is OH-, hydroxide. Its charge is shown
on the table on page 178.
Since there are 2 OH- ions, the total negative
charge is -2.
The total positive charge must therefore be +2.
Since there is only one chromium ion, it must
have a charge of +2.
Therefore, the name of this compound is
Chromium (II) hydroxide.
Example: Fe2(CO3)3
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The cation is iron, which can have more than one
possible charge.
The anion is CO32-, carbonate. Its charge is
shown on the table on page 178.
Since there are 3 CO32- ions, the total negative
charge is -6.
The total positive charge must therefore be +6.
Since there are two ions, each one must have a
charge of +3.
Therefore, the name of this compound is
Iron (III) carbonate.
Hydrates
Hydrates are ionic compounds with water
molecules incorporated into their crystal
lattices.
 These ionic compounds can be binary
compounds or compounds involving
polyatomic ions.
 The water molecules are known as the
water of hydration.
 The remaining compound is known as
anhydrous.
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Formulas of Hydrates
The formulas for hydrates indicate the
number of water molecules incorporated
into the crystal lattice
 For example, CaSO4·2H2O means that
there are two water molecules for every
formula unit of CaSO4 (two water
molecules for every one Ca2+ and SO42-)
 The formula for anhydrous Calcium sulfate
is CaSO4.
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Names of Hydrates
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Hydrates are named by adding the
appropriate Greek prefix to the word
hydrate and then adding this word to the
end of the name.
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CaSO4·2H2O is Calcium sulfate dihydrate.
CoSO4·7H2O is Cobalt (II) sulfate
heptahydrate.