Question 4.3: Writing Formulas for Ionic Compounds Write the formula for ...

1. Write the symbol for the metal cation and its charge followed by the symbol for the nonmetal anion and its charge. Determine charges from the element’s group number in the periodic table (refer to Figure 3.12).

Al^{3+}          O^{2-}

2. Adjust the subscript on each cation and anion to balance the overall
charge.

Al^{3+}             O^{2-}

\overset{\downarrow }{Al_2O_3}

3. Check to make sure the sum of the charges of the cations equals the sum of the charges of the anions.

cations: 2(3+) = 6+
anions: 3(2-) = 6-
The charges cancel.