## Chapter 2

## Q. 2.3

**Writing an Ionic Formula, Given the Ions**

a. Chromium(III) oxide is used as a green paint pigment (**Figure 2.22**). It is a compound composed of Cr^{3+} and O^{2-} ions. What is the formula of chromium(III) oxide?

b. Strontium oxide is a compound composed of Sr^{2+} and O^{2-} ions. Write the formula of this

**PROBLEM STRATEGY**

Because a compound is neutral, the sum of positive and negative charges equals zero. Consider the ionic compound CaCl_2, which consists of one Ca^{2+} ion and two Cl^- ions. The sum of the charges is:

\underbrace{1 \times(+2)}_{\text {Positive charge }}+\underbrace{2 \times(-1)}_{\text {Negative charge }}=0Note that the number of calcium ions in CaCl_2 equals the magnitude of charge on the chloride ion (1), whereas the number of chloride ions in CaCl_2 equals the magnitude of charge on the calcium ion (2). In general, you use the magnitude of charge on one ion to obtain the subscript for the other ion. You may need to simplify the formula you obtain this way so that it expresses the simplest ratio of ions.

## Step-by-Step

## Verified Solution

a. You can achieve electrical neutrality by taking as many cations as there are units of charge on the anion and as many anions as there are units of charge on the cation. Two Cr^{3+} ions have a total charge of 6+, and three O^{2-} ions have a total charge of 6-, giving the combination a net charge of zero. The simplest ratio of Cr^{3+} to O^{2-} is 2 : 3, and the formula is Cr_2O_3. Note that the charge (without its sign) on one ion becomes the subscript of the other ion.

b. You see that equal numbers of Sr^{2+} and O^{2-} ions will give a neutral compound. Thus, the formula is SrO. If you use the units of charge to find the subscripts, you get

The final formula is SrO, because this gives the simplest ratio of ions.