Chemical Compounds Worksheet printable pdf download

Formulas of ionic compounds.

To figure out the formula of an ionic compound you need to 1) identify the ions, then 2) create

a neutral compound from those ions.

Example. To write the formula for sodium sulfide requires the following steps:

1. Recognize that sodium ions are Na

and sulfide ions are S

2. Combine these two ions so that the total charge is zero. In this case, it take

two 1+ charges per one 2- charge. Thus the formula for sodium sulfide is

This handout focuses on the second step. The main conceptual issue is that ionic compounds

are neutral.

Ions have charge, such as the +1 charge on a sodium ion, Na

. Ionic compounds, however, are

neutral. If you know the ions, you can figure out the formula of an ionic compound by

calculating how many of each ion you need to achieve neutrality -- to have the same number

of positive charges and negative charges.

People use various mental tricks to figure out how many ions are needed to reach neutrality.

After a little practice, you will do many of these “by inspection.”

One trick is to “cross multiply.” (This is similar to finding the least common denominator

when adding fractions.)

Example. Write the ionic compound between Mg

and Cl

(magnesium and

chloride ions). The charges are 2+ and 1-. Take 1 of the 2+, 2 of the 1-. The

result: MgCl

(magnesium chloride), which has 2 + charges and 2 - charges.

Caution. Check that your formula is the simplest formula possible (i.e., the empirical

formula). This is especially important if you use the cross multiply method.

Example. Write the ionic compound between Ca

and O

(calcium and oxide

ions). The charges are 2+ and 2-. Using the cross multiply method, take 2 of

the 2+, 2 of the 2-. The result: Ca

. This is neutral (4 each + and - charges),

but it is not the simplest -- or proper -- formula. Simplify to CaO (calcium

oxide).

In the problems that follow (see back of page), you are given two ions, and asked to write the

formula of the ionic compound that could form between them. Since you are given the ions,

including the correct ion charges, all you need to do is to achieve neutrality. You can do these

sections in any order; do problems that help you. If you have trouble at first, check yourself as

you go. Seek help if necessary.

Chemical Compounds Worksheet