Pisgah High School
M. Jones

Named for Alfred Stock, a
German chemist and teacher who published the system in 1919.
Add a Roman numeral after the name of the metal to indicate the oxidation number.

Using the Stock System:
1. Write the name of the metal
2. Add a Roman numeral corresponding to the oxidation number of the metal (if necessary)
3. Write the name of the negative element, modified to end in ide

When is the Roman Numeral
Necessary?
When the metal has more than one oxidation number.
Metals in groups IA, IIA, and III B do not need Roman numerals.
In addition, omit the Roman numeral for Al, Ga, In, Zn, Cd and Ag.

The oxidation number is a number that tells us how an element combines with other elements.
For ions, the oxidation number is the ionic charge.

What does the Roman Numeral do?
The Roman numeral indicates the oxidation number on a single metal atom, and differentiates between several possible compounds.
Consider: FeO and Fe
2
O
3
Both contain iron and oxygen
But, both cannot be iron oxide
…

What does the Roman Numeral do?
Therefore…
FeO is called iron(II) oxide and
Fe
2
O
3 is called iron(III) oxide.
Consider: FeO and Fe
2
O
3
Both contain iron and oxygen
But, both cannot be iron oxide
…

First, determine the oxidation number of iron in FeO …
… by starting with the negative element, find the oxidation number of the positive element.
Since iron has an oxidation number of +2, FeO is named iron(II) oxide.

Then for Fe
2
O
3
…
First, determine the oxidation number of iron in Fe
2
O
3
…
… by starting with the negative element, find the oxidation number of the positive element.
Since iron’s oxidation number is +3,
Fe
2
O
3 is named iron(III) oxide.

Remember …
The Roman numeral indicates the oxidation number of the metal.
You need not use a Roman numeral for metals in groups
IA, IIA and IIIA, or for Al, Ga, In, Zn, Cd or Ag

Name these compounds:
SrO
CrCl
3
ZnS
Srontium oxide
Chromium(III) chloride
Zinc sulfide
Mn
2
O
3
SnF
4
TiS
2
Manganese(III) oxide
Sin(IV) fluoride
Titanium(IV) sulfide

Writing formulas for binary compounds of a metal and a nonmetal

The sum of the oxidation numbers on all the atoms in a compound must equal zero.
+2 –2 = 0
FeO
Consider iron(II) oxide.
Oxygen is –2 and iron is +2.
The sum is 0.

Now consider iron(III) oxide
+3 -2
Fe
2
O
3
In iron(III) oxide, the iron is +3 and the oxygen is –2.
These don’t add up to zero.

Now consider iron(III) oxide
+6 -6 = 0
+3 -2
Fe
2
O
3
But, since there are two Fe atoms and three O atoms, we can multiply to
2 x 3 = 6 get the totals.
3 x –2 = -6 Now the sum is zero.

Look at it another way:
+6
= 0
+6 -6
+3 +3 -2 -2 -2
Fe
2
O
3
=
O O O

Now you are ready to write formulas

The Crisscross Method
Simple but effective…
…most of the time.
Be aware of the potential problems with this method.

Suppose you are writing the formula for copper(II) chloride
First, write down the symbols
Cu Cl

Then write the oxidation numbers in copper(II) chloride
The oxidation number of copper comes from the
+2 name.
-1
Cu Cl
Get the oxidation number of chlorine from the periodic table.

Crisscross the numbers for the formula of copper(II) chloride
When you crisscross, ignore the signs.
+2 -1
Cu Cl
1 2

Clean up the subscripts in the formula of copper(II) chloride
Subscripts of 1 are invisible.
(don’t even put the 1)
+2 -1
Cu Cl
1 2

Clean up the subscripts in the formula of copper(II) chloride
Subscripts of 1 are invisible.
(don’t even put the 1)
+2 -1
Cu Cl
2

Now you have the formula of copper(II) chloride
Cu Cl
2

When is the “crisscross method” a problem?
When all the subscripts are divisible by a number other than 1.

Look at chromium(VI) oxide
The oxidation number of Cr is
+6
+6 -2
Cr O

Look at chromium(VI) oxide
Now, crisscross the oxidation numbers.
+6 -2
Cr O
2 6
When you crisscross, both subscripts are divisible by 2.

Look at chromium(VI) oxide
Divide each subscript by 2.
+6 -2
Cr O
2 2

Look at chromium(VI) oxide
This is the correct formula
Cr O
3
Remember, 1’s are invisible.

Remember: reduce the subscripts to their smallest whole-number values.

2
Some Exceptions:
4
2
6
4
10
6
6
2
2
2
2

The “ic/ous” method …
… is an archaic method, but still in use today by the chemical industry
… uses the –ic or – ous suffixes on the name of the metal.
… may use the Latin name
… uses prefixes like hypo& per-

Higher oxidation number
Use “ic” for the higher oxidation number, like ferric.
IC
OUS
Use “ous” for the lower oxidation number, like ferrous.
Lower oxidation number

Name and
Latin Root iron
Lower Ox.
Number ferrum copper ferrous
Fe 2+ cuprous cuprum Cu + mercury mercurous
Hg
2
2+
Higher Ox.
Number ferric
Fe 3+ cupric
Cu 2+ mercuric
Hg 2+

…
FeCl
2
CuO
FeN
Cu
3
P
HgS
Hg
2
Cl
2
Ferrous chloride
Cupric oxide
Ferric nitride
Cuprous phosphide
Mercuric sulfide
Mercurous chloride

Write the formula …
Ferric bromide
Cupric nitride
Ferrous chloride
Mercuric oxide
Mercurous iodide
Stannous fluoride
FeBr
3
Cu
3
N
2
FeCl
2
HgO
Hg
2
I
2
SnF
2

Mike Jones
Pisgah High School
Canton NC mjones@haywood.k12.nc.us
Copyright 2012
All rights reserved.