Names and formulas for atoms with more than 1 ionic charge
Some metals are able to form more than one kind of ion. For example, the element copper forms two
completely different compounds when it reacts with chlorine. One of the compounds is white; the other is
yellow. Chemists have found that the ionic charge on the copper in the white compound is 1+. Its chemical
formula is CuCl, since the ionic charge of chlorine is always 1-. The ionic charge of the copper in the
yellow compound is 2+. Its formula is therefore CuCl2. The table below shows the names and ionic charges
of some metals that have more than one ionic charge.
Table 1: Names and multiple ionic charges of some metals
Name of element
copper
iron
lead
tin
Symbol
Cu
Fe
Pb
Sn
Ionic charges
1+, 2+
2+. 3+
2+, 4+
2+, 4+
Roman Numeral
I, II
II, III
II, IV
II, IV
These compounds are named in the same way as other ionic compounds, except that a Roman numeral (in
the above table) is added in round brackets after the metal to indicate its ionic charge. For example, CuCl
is called copper (I) chloride because the ionic charge on the copper is 1+. CuCl2 is called copper (II)
chloride because the ionic charge on the copper is 2+. Remember that you have to use the Roman numeral
system only when naming the ions of metals that can have more than one ionic charge.
Polyatomic Compounds
In the lab we have used calcium carbonate, sodium bicarbonate, calcium hydroxide, and copper (II)
sulphate. These names do not seem to fit the naming patter we have been examining so far. What are
these compounds?
Such compounds are pure substances that involve combinations of metals with polyatomic
ions (see table 2 below). Polyatomic ions are groups of atoms that tend to stay together
and carry an overall ionic charge. An example of a polyatomic ion is the nitrate ion (to the
right). When a compound containing this ion is dissolved in water, the positive metal ion
and the nitrate ion separate from each other, but the nitrate ion itself stays together Above: The nitrate ion
contains one nitrogen atom
tightly bonded to three
oxygen atoms. The overall
ionic charge is 1-.
as a unit surrounded by water molecules.
Table 2 – Common Polyatomic Compounds
Compound
calcium carbonate
magnesium hydroxide
sulfuric acid
sodium hypochlorite
copper (II) sulfate
sodium carbonate
ammonium nitrate
Formula
CaCO3
Mg(OH)2
H2SO4
NaClO
CuSO4
Na2CO3
NH4NO3
Use or source
Chalk and building materials
Stomach antacids
Car battery acid
Bleach for clothes
fungicide
Laundry detergent
fertilizer
Writing Formulas for Polyatomic Compounds
The ionic charges of polyatomic ions make it possible for them to form ionic compounds. Table 3 lists
some common polyatomic ions and their ionic charges.
Table 3: Common polyatomic ions and their ionic charges.
Name of polyatomic ion
Ion formula
nitrate
hydroxide
bicarbonate (hydrogen carbonate)
chlorate
carbonate
sulfate
phosphate
-
NO3
OHHCO3ClO3CO32SO42PO43-
Ionic charge
1111223-
When a polyatomic ion such as nitrate or sulphate combines with other elements, we follow the same rules
for writing formulas that we learned with ionic bonding, just as if it were an individual non-metal ion.
Consider the following example:
What is the formula for the ionic compound formed by sodium and a sulfate ion?
Rule 1: Write the symbols of the metal and of the polyatomic group.
Na
SO4
Rule 2: Write the ionic charges.
1+
2Na
SO4
Rule 3: Choose the number of ions to balance the charge.
Two Na+ ions will balance the charge of one SO42- ion.
Rule 4: Write the formula using subscripts.
The formula is Na2SO4.
The total ionic charge is 2(1+) + (2-) = 0
Naming Polyatomic Compounds
Above: Sodium sulphate is made up of two sodium
ions (each with an ionic charge of 1+) combined with
a sulfate ion (one sulphur atom tightly bonded to
four oxygen atoms, with a total ionic charge of 2-).
The overall compound is neutral.
The name of the ionic compound in the previous example is simply sodium sulfate. This stable compound is
shown below. The name for the polyatomic compounds are relatively straightforward. The name is simply a
combination of the name of the metal and the name of the polyatomic ion.
For example, the compound formed by a carbonate ion (CO32-) and a potassium ion (K+) has the formula
K2C03 and its name is potassium carbonate. You will note that the positive part of the compound is always
written first in both the formula and the name. Ammonium (NH4+) and nitrate (NO3-) combine to form
ammonium nitrate (NH4NO3).
Polyatomic ions make it possible to have an even wider range of ionic compounds, especially when they
combine with metals that may have more than one ionic charge.
Do you get what I am saying?
Understanding Concept Questions on Polyatomic Ions
Please answer the following questions in your notebook – we will review the answers tomorrow.
1. In your own words, explain what is meant by the term “polyatomic ion”. Give two examples.
2. What happens to the ions in the compound sodium nitrate when it dissolves in water?
3. Write the formulas for the following compounds:
a. sodium phosphate
b. calcium sulfate
c. potassium chlorate
d. aluminum hydroxide
e. beryllium nitrate
f. magnesium hydrogen carbonate (magnesium bicarbonate)
g. nickel carbonate
4. Write the names for the following compounds:
a. K2CO3
b. Na2SO4
c. Al(HCO3)3
d. AgNO3
5. Why is ammonium nitrate (NH4NO3) not written as N2H4O3?
6. Some polyatomic ions have a positive charge. The ammonium ion (NH4+) is an example. Give
the names and formulas of the compounds formed by this ion with:
a. A chloride ion
b. A sulphate ion