Notes/WKSTs Polyatomic Ions:
Names and Formulas
Name
Date___________
Block_____
As the name suggests, polyatomic ions contain many atoms.
A polyatomic ion is an ion consisting of two or more atoms chemically bonded together by
covalent bonds and carrying a net electric charge.
Remember the definition of an ion: an atom or molecule that has gained or lost one or more
valence electron and has become a charged particle.
The ions involved in the binary ionic compounds are single atoms that have become ions.
Polyatomic ions are molecules that have become ions. Almost all polyatomic ions carry a
negative charge. The original molecules have gained electrons.
The exception is the ammonium ion, NH4+1. The ammonium ion is the only polyatomic ion
that carries a positive charge. The ammonium ion is formed when an ammonia molecule, NH3,
forms a coordinate covalent bond with a hydrogen ion, H+1. The ammonia molecule doesn’t
lose electrons but rather gains a positively charged particle: the single proton of the hydrogen
ion. It is this “additional” proton – or rather the additional positive charge – that gives the ion its
+1 charge.
Most of the polyatomic ions we will be using are called oxoanions which consist of oxygen
covalently bonded to another nonmetallic element (called the characteristic or central element).
For example, the oxoanion carbonate consists of a central carbon atom with three oxygen
atoms: CO3-2.
Sulfur, for example, forms the oxoanions sulfate, SO4-2 , and sulfite, SO3-2. Two or more
different oxoanions of the same central element can be distinguished from each other by suffizes
added to the stem name of the central element. The suffix -ate denotes the anion with the
greater number of oxygen atoms; the suffix -ite denotes the anion with fewer oxygen atoms.
The sulfate ion has four oxygen atoms; the sulfite ion has three oxygen atoms.
The oxoanion with the greatest number of oxygen atoms is given the prefix per- and the suffix
-ate. The oxoanion with the least number of oxygen atoms is given the prefix hypo- and the
suffix -ite. For example:
ClO4 –1 perchlorate ion
ClO3 –1 chlorate ion
ClO2 –1 chlorite ion
ClO –1
hypochlorite ion
Polyatomic Ions You MUST Know
acetate
C2H3O2 –1 (named after acetic acid HC2H3O2(aq))
ammonium
NH4 +1
cyanide
CN –1
carbonate
CO3 –2
chromate
CrO4 –2
dichromate
Cr2O7 –2
hydrogen carbonate
HCO3 -1
(also called bicarbonate)
hydroxide
OH –1
nitrate
NO3 –1
nitrite
NO2 –1
phosphate
PO4 –3
sulfate
SO4 –2
sulfite
SO3 –2
(do not switch the order of the elements!)
Writing Formulas Containing Polyatomic Ions
The polyatomic ion behaves as a SINGLE UNIT. Treat the formula of a
polyatomic ion as you would an ion of a single atom. Remember, the total number
of positive charges must equal the total number of negative charges. There is zero
net charge on the ionic compound. NEVER CHANGE THE FORMULA OF A
POLYATOMIC ION: do NOT omit or change any subscript in the formula!
Na +1 +
OH
-1
there is one positive charge and one negative charge;
therefore, only one of each ion is required: NaOH
Notice that the symbols of the ions involved are written with no space between
them. Writing Na OH is incorrect.
Na +1 +
SO4 –2
there is one positive charge, but there are two negative
charges; therefore, two positive ions are needed to
balance the negative charges: Na2SO4
Again, notice the placement of the symbols of the ions and subscript. The formula
is not written Na2 SO4. Be very careful when writing formulas of compounds.
Na +1 +
PO4 –3
NH4 +1 + Cl –1
there is one positive charge, but there are three negative
charges; therefore, three positive ions are needed:
Na3PO4
only one of each ion is needed: NH4Cl
So far so good. But in the examples above, only one polyatomic ion was involved.
How do we write the formula of a compound that requires MORE than one
polyatomic ion? Figure out how many total positive and total negative charges are
needed, and how many of each ion are needed. Then use parentheses ( ) around
the entire polyatomic formula and write the necessary subscript OUTSIDE the
parentheses.
Ca +2 + OH –1
Al +3
+
NH4 +1 +
Here we have two positive charges and only one negative
charge. Two negative ions are required: Ca(OH)2
OH –1
Here we have three positive charges and one negative
charge. Three positive ions are needed: Al(OH)3
S –2
We need two positive ions to balance the two negative
charge on the sulfide ion: (NH4)2S
And what do we do if the positive ion is a polyatomic ion and the negative ion is
also a polyatomic? Nothing different! Use parentheses!! Just remember to treat
the polyatomic ions as single units.
NH4 +1 + OH –1
NH4 +1
+ CO3 –2
only one of each ion is needed: NaOH
two ammonium ions are needed to balance the charge on
the carbonate ion: (NH4)2CO3
Complete the following table:
OH -1
Li +1
Mg +2
Al +3
NH4 +1
Pb +2
Pb +4
Fe +2
Fe +3
Zn +2
NO3 -1
SO4 -2
PO4 -3
C2H3O2 -1
Now that you can write the formulas of compounds containing polyatomic ions, let’s try naming
the compounds from the formulas. And then we’ll write formulas from the names of
compounds.
Name each ion in the formula. Do not include any reference to the subscripts either in the
polyatomic ion formula or in the compound formula.
Examples:
NaOH is sodium hydroxide
Ca(OH)2 is calcium hydroxide
NH4Cl is ammonium chloride
(NH4)2S is ammonium sulfide
(NH4)2SO4 is ammonium sulfate
Pb(OH)2 is lead(II) hydroxide
Pb(OH)4 is lead(IV) hydroxide
Al2(SO4)3 is aluminum sulfate
Name the following compounds:
1.
LiOH
______________________________________________
2.
FeSO4
______________________________________________
3.
Cu2SO4
______________________________________________
4.
CuSO4
______________________________________________
5.
NH4F
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6.
(NH4)3PO4
______________________________________________
7.
Co(OH)2
______________________________________________
8.
ZnSO4
______________________________________________
Write the correct formula for each compound:
1.
sodium sulfate
____________________
2.
potassium hydroxide
____________________
3.
copper(I) phosphate
____________________
4.
copper(II) phosphate
____________________
5.
barium hydroxide
____________________
6.
ammonium carbonate
____________________
7.
iron(II) phosphate
____________________
8.
iron(III) sulfate
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9.
magnesium phosphate
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10. ammonium phosphate
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