Learning Polyatomic Ions
“The impossibility of separating the nomenclature of a science from the science itself, is owing to this,
that every branch of physical science must consist of three things; the series of facts which are the objects
of the science, the ideas which represent these facts, and the words by which these ideas are expressed.
Like three impressions of the same seal, the word ought to produce the idea, and the idea to be a picture
of the fact. “
-Antoine-Laurent Lavoisier
Hundreds and hundreds of polyatomic ions exist! It would be a daunting task to try to memorize all of
them. In fact, some textbooks simply present lists of fifty or more ions to learn. It is MUCH easier to
learn a system that will permit you to learn just a little, yet be able to generate the name, formula, and
charge for many common polyatomic ions.
THE BIG FIVE
Here is a very short list of ions to learn. The beginning of each name contains a stem representing the
non-metal atom at the center of the ion. Notice that each name ends in –ATE, a suffix that indicates that
the polyatomic ion contains oxygen. The suffix does not tell you how many oxygen atoms are included in
the ion; nor does it indicate the charge. You must learn these!
Chlorate
Nitrate
ClO3NO3-
Carbonate
Sulfate
CO32SO42-
Phosphate
PO43-
Once you have learned The Big Five, you may use this short list to expand your knowledge of ions.
IONS BY ANALOGY: PERIODICITY
As you read down the column on the Period Table known as the halogens (group VII), note that bromine
(Br) and iodine (I) are found directly under chlorine (Cl). Since the elements in a family are known to
have similar chemical characteristics, knowing the Big Five member chlorate (ClO3-) means that you
automatically know bromate (BrO3-) and iodate (IO3-). You can use this comparison technique in many
situations.
Exercise
Give the formula and charge for the ions selenate and arsenate.
Solution: Consult your Periodic Table. Since you have already learned that sulfate is SO42-, then selenate
must be SeO42-. Since you know that phosphate is PO43-, then arsenate must be AsO43-.
IONS BY EXTENTION
There are a number of oxyanions containing chlorine as the central atom. As long as you know the
formula and charge of the ion chlorate (ClO3-), then you can predict the others. Note that although the
central atom and overall charge do not change, the number of oxygen atoms does. The suffix –ite means
one fewer oxygen than the -ate form. The prefix hypo- means one oxygen less than the -ite form, while
the prefix per- (short for hyper-) means one more oxygen than the –ate form.
Hypochlorite
Chlorite
Chlorate
Perchlorate
ClOClO2ClO3ClO4-
Exercise
Give the formula for each name: bromite; periodate; hypochlorite; peroxide.
IONS BY HYDROGENATON
Any anion with a charge of 2- (or more) can be modified by adding a proton (H+) to it, thus changing the
charge by one. If more than one proton can be added to the parent anion to form new anions, then the
anions are distinguished by adding the Greek counters mono-, di-, tri-, and so on.
SO42- sulfate
HSO4- hydrogensulfate
PO43HPO42H2PO4-
phosphate
monohydrogenphosphate
dihydrogenphosphate
Exercise
Give the formula for each name: hydrogenselenate; dihydrogenstibnate; hydrogentellurite;
dihydrogenphosphite.
THE LITTLE FIVE
Here is a small group of additional anions that you will see frequently. Learn them now.
1) Polyatomic cations
NH4+
ammonium
Hg22+
mercury(I)
2) Polyatomic ions named as if monoatomic
OHhydroxide
CNcyanide
3) Polyatomic anions with characteristic colors
MnO4permanganate (purple)
CrO42chromate (yellow)
Cr2O72dichromate (orange)
4) Organic polyatomic anions
C2H3O2acetate
C2O42oxalate
5) Anions with the prefix thio (indicating that a sulfur atom has displaced an oxygen atom)
S2O32thiosulfate
CNSthiocyanate