element. Therefore the calcium ion, Ca2+, is called calcium and its chemical
symbol appears first.- The monoatomic anion with which the cation combines is named by taking
the root of the element’s name and adding –ide. You must know this rule. The
anion’s name comes second. Therefore, the chlorine ion, Cl−, is called
chloride. - The name of this compound is calcium chloride.
A quick way to determine the formula of a binary ionic compound is to use the
crisscross rule.
Example 1
To determine the formula for calcium chloride, first write the ionic forms with
their associated charges.
Next move the numerical value of the metal ion’s superscript (without the
charge) to the subscript of the nonmetal’s symbol. Then take the numerical value
of the nonmetal’s superscript and make it the subscript of the metal as shown
above.
Note that the numerical value 1 is not shown in the final formula.
You now have the chlorine’s 1 as the subscript of the calcium and the
calcium’s 2 as the subscript of the chloride. As a result, you have CaCl 2 as the
final formula for calcium chloride.
Example 2
Write the name and formula for the product formed when aluminum reacts with
oxygen. First write the name.
- Name the cation first and then the anion.
- The monoatomic (one-atom) cation takes its name from the name of the
element. Therefore the aluminum ion, Al3+, is called aluminum and its
chemical symbol appears first. - The monoatomic anion with which the cation combines is named by taking
the root of the element’s name and adding –ide. You must know this rule. The
anion’s name comes second. Therefore, the oxygen ion, O2−, is called oxide. - The name of this compound is aluminum oxide.