C3.
Tes
Example 8
Name each ionic compound.
CaCl2
AlF3
Co2O3
Solution
Using the names of the ions, this ionic compound is named calcium chloride. It is not calcium(II) chloride because calcium forms only one cation when it forms an ion, and it has a characteristic charge of 2+.
The name of this ionic compound is aluminum fluoride.
We know that cobalt can have more than one possible charge; we just need to determine what it is. Oxide always has a 2− charge, so with three oxide ions, we have a total negative charge of 6−. This means that the two cobalt ions have to contribute 6+, which for two cobalt ions means that each one is 3+. Therefore, the proper name for this ionic compound is cobalt(III) oxide.
Test Yourself
Name each ionic compound.
Sc2O3
AgCl
Answers
scandium oxide
silver chloride
How do you know whether a formula—and by extension, a name—is for a molecular compound or for an ionic compound? Molecular compounds form between nonmetals and nonmetals, while ionic compounds form between metals and nonmetals. The periodic table (Figure 3.2 “A Simple Periodic Table”) can be used to determine which elements are metals and nonmetals.
There also exists a group of ions that contain more than one atom. These are called polyatomic ions. Table 3.7 “Common Polyatomic Ions” lists the formulas, charges, and names of some common polyatomic ions. Only one of them, the ammonium ion, is a cation; the rest are anions. Most of them also contain oxygen atoms, so sometimes they are referred to as oxyanions. Some of them, such as nitrate and nitrite, and sulfate and sulfite, have very similar formulas and names, so care must be taken to get the formulas and names correct. Note that the -ite polyatomic ion has one less oxygen atom in its formula than the -ate ion but with the same ionic charge.
Table 3.7 Common Polyatomic Ions
NameFormula and Charge NameFormula and Charge
ammoniumNH4+ hydroxideOH−
acetateC2H3O2−, or CH3COO−nitrateNO3−
bicarbonate (hydrogen carbonate)HCO3−nitriteNO2−
bisulfate (hydrogen sulfate)HSO4−peroxideO22−
carbonateCO32−perchlorateClO4−
chlorateClO3−phosphatePO43−
chromateCrO42−sulfateSO42−
cyanideCN−sulfiteSO32−
dichromateCr2O72−triiodideI3−
The naming of ionic compounds that contain polyatomic ions follows the same rules as the naming for other ionic compounds: simply combine the name of the cation and the name of the anion. Do not use numerical prefixes in the name if there is more than one polyatomic ion; the only exception to this is if the name of the ion itself contains a numerical prefix, such as dichromate or triiodide.
Writing the formulas of ionic compounds has one important difference. If more than one polyatomic ion is needed to balance the overall charge in the formula, enclose the formula of the polyatomic ion in parentheses and write the proper numerical subscript to the right and outside the parentheses. Thus, the formula between calcium ions, Ca2+, and nitrate ions, NO3−, is properly written Ca(NO3)2, not CaNO32 or CaN2O6. Use parentheses where required. The name of this ionic compound is simply calcium nitrate.
Write the proper formula and give the proper name for each ionic compound formed between the two listed ions.
cause the ammonium ion has a 1+ charge and the sulfide ion has a 2− charge, we need two ammonium ions to balance the charge on a single sulfide ion. Enclosing the formula for the ammonium ion in parentheses, we have (NH4)2S. The compound’s name is ammonium sulfide.
Because the ions have the same magnitude of charge, we need only one of each to balance the charges. The formula is AlPO4, and the name of the compound is aluminum phosphate.
Neither charge is an exact multiple of the other, so we have to go to the least common multiple of 6. To get 6+, we need three iron(II) ions, and to get 6−, we need two phosphate ions. The proper formula is Fe3(PO4)2, and the compound’s name is iron(II) phosphate.
Test Yourself
Write the proper formula and give the proper name for e
