All ionic compounds consist of a cation bonded to an anion. Binary ionic compounds are the result of monatomic metal cations bonding with monatomic nonmetal anions (e.g., NaCl or MgF2). However, you've now learned that not all ions are monatomic. Groups of atoms can also form ions, which are referred to as polyatomic ions.
Ionic compounds formed by polyatomic ions are called ternary ionic compounds. The term “ternary” means these compounds contain at least three different elements. Ternary ionic compounds are usually composed of a monatomic metallic cation bonded with a polyatomic anion.
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Watch the following video to learn how ternary ionic compounds are formed and named.
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So far in our study of ionic compounds, we’ve primarily looked at binary ionic compounds, which are composed of two elements. The next step is to look at ternary ionic compounds. But first, let’s look at one of the building blocks of these compounds: units called polyatomic ions.
As their name suggests, polyatomic ions are composed of multiple elements. But unlike molecules, which are electrically neutral, polyatomic ions carry a net charge. Most polyatomic ions are anions, that is to say, they carry a negative charge, with an important exception to this being ammonium, which carries a plus one charge.
Let’s look at a few of these polyatomic ions. The first example is sulfate, which is composed of one sulfur and four oxygens, and it carries a charge of two minus. Another common and important ion is the hydroxide ion, composed of a single oxygen and a single hydrogen, and carries a one minus charge. The last example, as mentioned before, is the ammonium polyatomic ion, composed of a single nitrogen and four hydrogens, and it carries a plus one charge. These are just three of the dozens of polyatomic ions that exist. And they can be used, just like monatomic ions, in the formation of ionic compounds. But the ionic compounds that polyatomic ions form are called ternary ionic compounds.
As you may have guessed from the name, they’re called this because they contain three different elements. They’re usually made from a polyatomic anion and a metallic cation.
When we write the formula units for, and name, these ternary ionic compounds, we use a similar process as we used for binary ionic compounds. Let’s look at how that process applies here. As an example, in this process, we’re going to look at the ternary ionic compound formed by the combination of the calcium ion and the carbonate polyatomic ion. The first step in this process is to write the formula for each ion. In this case, that’s calcium two plus, and CO3 two minus. Next, criss-cross the charges, dropping ones and signs, and treating polyatomic ions as indivisible units. So, in this case, each of our ions is going to get a subscript of two, but with the carbonate ion, we’re going to put the chemical formula for carbonate in parentheses and write the subscript of two on the outside of those parentheses. The next step is to reduce the subscripts and simplify them. In this case, both of our ions have a subscript of two, so we can divide those by two, and we leave off any subscript of one. So we just get Ca and CO3. Since there’s no subscript on the carbonate polyatomic ion, we can drop the parentheses and just write this as CaCO3. Lastly, to name this compound, we just state the name with the cation first. In this case, our cation was calcium, so we would call this compound calcium carbonate.
Let’s look at one more example. This example reads, “What is the formula unit and name for the ternary ionic compound created by the combination of tin (IV) ion and the sulfite ion?” So our first step is to write down both of our ions. Tin four is Sn four plus, and sulfite is SO3 two minus. Next, we criss-cross the charges, writing each one as the subscript for the other ion. That gives us Sn two and SO3 four. Next we reduce those subscripts. Both two and four can be divided by two, giving us a formula unit of Sn(SO3)2. Because there are two sulfite ions, we have to keep the parentheses around the SO3. To name this, we say the cation first – our cation was tin four – and then the anion, which is sulfite. So this ternary ionic compound would be called tin (IV) sulfite.
Question
What is the formula unit and name for the ternary ionic compound formed by the bonding of the iron (III) ion and the hydroxide ion?
The ternary ionic compound created has a formula unit of Fe (OH)3 and is called iron (III) hydroxide.
