Name ___________________________________ Date ______________________ Class ______________________
Chapter 9 – Chemical Names and Formulas
Polyatomic Ions
As the name suggest, 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. 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, NH41+. The ammonium ion is a 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 suffixes
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:
perchlorate ion (ClO4-1)
chlorate ion (ClO3-1)
chlorite ion(ClO2-1)
hypochlorite ion (ClO-1)
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 a 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!!
Example 1: Na+1 + SO4-2 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.
Example 2: 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.
Example 3: Na+1 + PO4-3 there is one positive charge, but there are three negative charges; therefore, three positive
ions are needed: Na3PO4.
Example 4: NH4+1 + Cl-1
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.
Example 1: Ca2+ + OH-1
required: Ca(OH)2.
Here we have two positive charges and only one negative charges. Two negative ions are
Example 2: Al3+ + OH-1
Al(OH)3.
Here we have three positive charges and one negative charge. Three positive ions are needed:
Example 3: NH41+ + S2-
We need two positive ions to balance the two negative charges 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.
Example 1: NH4+1 + OH-1
only one of each ion is needed: NH4OH.
Example 2: NH41+ + CO32-
two ammonium ions are needed to balance the charge on the carbonate ion: (NH4)2CO3.
**Work the following problems – 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
Writing Names from Formulas Containing Polyatomic Ions
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
ZnSO4 is zinc sulfate
**Work the following problems – name the following compounds.
Formula
LiOH
FeSO4
Cu2SO4
CuSO4
NH4F
(NH4)3PO4
Co(OH)2
ZnSO4
CaCO3
KOH
Compound Name
**Work the following problems – write the correct formula for each compound.
Compound Name
sodium sulfate
copper (I) phosphate
barium hydroxide
iron (II) phosphate
magnesium phosphate
Formula
Compound Name
potassium hydroxide
copper (II) phosphate
ammonium carbonate
iron (III) sulfate
ammonium phosphate
Formula