Formulas and Oxidation Numbers
A chemical formula is a combination of symbols and numerical subscripts, which
represent the composition of the compound. The element symbols represent the elements
in the compound the subscripts represent the relative proportion of each element in the
compound. You can predict the proportions by determining the oxidation numbers
(acquired positive or negative charge on the atom) of the element or polyatomic ion and
balancing the positive and negative charges so they total zero.
The following rules are used for writing chemical formulas:
1. The sum of the positive and negative oxidation numbers (charges) of all neutral
substances must equal zero.
2. One positive charge will neutralize one negative charge.
3. Ions with positive charges (cations) are written first and negative ions (anions) are
written second.
4. When the relative proportion of an element in a compound is greater than one, the
symbol for that element must be followed by a numerical subscript indicating the
proportion.
5. When the relative number of polyatomic ions in a compound is greater than one, the
symbol must be enclosed by a parenthesis, followed by the correct numerical
subscript. Example: Al2(SO4)3.
Equipment:
Scissors, pencil and paper, sheet of ions
Procedure:
1. Cut out the “ion” squares.
2. Construct neutral formulas for the combinations listed by gluing the correct number
of ions of each substance alongside each other. For example, the formula for a
compound containing magnesium and chlorine may be determined in the following
way: Place the Mg2+ on a piece of construction paper. Place enough Cl- ions (2)
alongside the Mg2+ ion to balance the charges and make a neutral compound. Glue
the ions.
3. Complete the table 1-10.
Conclusions:
1. What is a binary compound? How are binary ionic compounds named?
List all the binary compounds you named.
2. Some ionic compounds are given names with Roman numerals. The Roman numeral is the
oxidation number of the cation. Often there are two compounds made of the same elements.
They are distinguished from each other by the Roman numeral. Covalent compounds can
also form different compounds made of the same elements, but they are distinguished by
prefixes indicating the numbers of each of the atoms. Write the names of the following
compounds that have “multivalenced” cations.
UF5
SeCl2
UF6
Se2Cl2
3. Predict formulas and names for five additional compounds using the list of ions from this
activity only. Add them to your table (11-15).
Data Table
Combining Substances
Formula of Compound
Name of Compound
1. Aluminum and bromine
2. Iron (II) and sulfur
3. Potassium and sulfate
4. Ammonium and sulfur
5. Iron (III) and sulfate
6. Sodium and oxygen
7. Aluminum and nitrate
8. Iron (III) and chlorine
9. Aluminum and oxygen
10. Sodium and phosphate
11.
12.
13.
14.
15.
Mg2+ +
+
Mg2+ +
+
-
O2-
-
O2-
-
O2-
-
O2-
Al3+
+
+
+
Al3+
+
+
+
Al3+
+
+
+
-
Cl-
-
Cl-
-
Cl-
Fe2+
+
+
Fe2+
+
+
Al3+
+
+
+
-
NO3-
-
NO3-
-
NO3-
Fe3+
+
+
+
Fe3+
+
+
+
Fe3+
+
+
+
-
Br-
-
Br-
-
Br-
Na+
+
Na+
+
Na+
+
-
S2-
-
S2-
K+
+
K+
+
Na+
+
-
SO42- -
SO42- -
SO42-
NH4+ +
Na+
+
-
PO43- -
PO43- -
SO42-
NH4+ +