Name: ___________________________________________________ Period: _________ Date: __________________
Writing Chemical Formulas
Part I: Calcium (Ca) links up with iodine (I) to form a compound called calcium iodide. The oxidation number (charge) of
calcium is 2+. The oxidation number of iodine is 1-.
1) Write down each element and its charge.
(Remember, positive always comes first.)
1.
2.
2) Cross over the numbers.
3) Cancel out numbers. (Skip this step if not needed.)
4) Write the formula.
3.
4.
5) The name of this compound is _____________________
6) One molecule of calcium iodide has ________ atom(s) of calcium and ________ atom(s) of iodine.
7) Altogether, how many atoms does one molecule of calcium iodide have? _________
Part II: Gold and sulfur combine to form the compound gold sulfide. A gold ion has a charge of 1+, while sulfur is 2-.
8) Write down each element and its charge.
(Remember, positive always comes first.)
8.
9) Cross over the numbers.
9.
10) Cancel out numbers. (Skip this step if not needed.)
10.
11) Write the formula.
11.
12) The name of this compound is _____________________
13) One molecule of gold sulfide has ________ atom(s) of gold and ________ atom(s) of sulfur.
14) Altogether, how many atoms does one molecule of gold sulfide have? _________
Part III: Fill in the blanks by writing the correct formulas in the empty space below.
NONMETALS
Cl 11.
H 1+
METALS
Al3+
S 22.
O 23.
Br 1-
4.
5.
9.
10.
H2O
HCl
6.
I 1-
7.
8.
Al I3
11.
12.
13.
14.
15.
16.
17.
18.
19.
20.
21.
22.
23.
24.
25.
26.
27.
28.
29.
30.
Ca 2+
Cu 1+
Mg 2+
Na 1+
Part IV: Six compounds are listed below. Read the example below. Then, figure out the oxidation number (charge) of the
elements in each compound.
Oxidation numbers can help you find a compounds chemical formula, but you can also do the
reverse (find the charge based on the formula). For example, NiI2 is the formula for nickel iodide.
The formula tells us that:

The charge of nickel is 2+ (Remember, the metal always comes first and has a positive charge).

The charge of iodine is 1-
Chemical
Formula
Cation
Anion
(and its charge)
(and its charge)
1
CaF2
Ca 2+
F 1-
2
KBr
3
Mg3N2
4
CCl4
5
H2S
6
FeCl3