Polyatomic Ions

Definition of Polyatomic Ions

These ions essentially act as one, (mostly) inseparable unit. So what do I mean by that? Well, let's talk about dissociation.

When compounds that are soluble in water are placed in water, they will dissociate (i.e., break apart). For example, here is the dissociation for NaCl:

$$NaCl \rightarrow Na^+ + Cl^-$$

Basically, the compound is breaking down into its ions. Now, what about a compound containing a polyatomic ion? Let's look at the dissociation of sulfuric acid (H₂SO₄), which contains the polyatomic ion sulfate (SO₄^2-)

$$H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$$

As you can see, the sulfate ion stays intact, this is what I meant by "inseparable".

Polyatomic Ions Examples

Now that we have a general idea of what a polyatomic ion is, let's look at some examples.

Here's calcium carbonate (CaCO₃), which contains the polyatomic ion carbonate (CO₃^2-):

Fig.1-Calcium carbonate (CaCO3)

Here we see that carbonate acts as one unit with a -2 charge. The calcium ion (Ca²⁺) is attracted to this charge and forms a bond with the polyatomic ions.

Polyatomic ions can form compounds with each other, as shown below with ammonium carbonate ((NH₄)₂CO₃):

Fig.2-Structure of ammonium carbonate ((NH4)2CO3)

Each ammonium ion (NH₄⁺) has a +1 charge, while carbonate, as we saw earlier, has a -2 charge. Each ammonium ion bonds to one side of the carbonate ion.

Naming of Compounds with Polyatomic Ions

Ionic compounds with polyatomic ions are named the same way as other ionic compounds: just put the name of the cation and the name of the anion together. If there is more than one polyatomic ion, don't add a number to the name. The only time this is okay is if the name of the ion already has a number in it, like dichromate or triiodide.

For example, H₂SO₃ is hydrogen sulfate.

There is one important difference in how the formulas for ionic compounds are written. If more than one polyatomic ion is needed to balance the overall charge in the formula, put the formula of the polyatomic ion in parentheses and write the correct numerical subscript to the right of the parentheses. So, the right way to write the formula for the beryllium ion (Be²⁺) and the hydroxide ion (OH^-) is Be(OH)₂, not BeOH₂ or BeO₂H₂. When they are needed, use parentheses. This ionic compound is called "beryllium hydroxide," which is its only name.

Common Polyatomic ions

There are plenty of polyatomic ions, but there are some that you will encounter a lot in your chemistry career. Since polyatomic ions are considered one unit and not a compound, they do not follow the naming convention for compounds. Instead, we just memorize the names and charges.

While this may seem like a daunting task at first, you'll get the hang of it eventually!

Polyatomic ions chart

Below is a chart showing some common polyatomic ions:

Fig.3-A list of common polyatomic ions in order of charge

You'll notice that some of these ions, like nitrate (NO₃^-) and nitrite (NO₂^-) look very similar. This type of ion is called an oxyanion since it contains oxygen and is an anion (negatively charged ion).

When we name these ions, the suffix is based on the number of oxygen:

• Most oxygen: Per + name + ate
• More oxygen: name + ate
• Less oxygen: name + ite
• Least oxygen: Hypo + name + ite

This convention is based on the number of possible oxygen atoms, as shown below:

Fig.4-Chart for naming oxyanions

Polyatomic ion charges

Polyatomic ions can have a wide variety of charges. Negatively charged species (anions) are much more common, however, there are still some that have positive charges (cations).

So where do these charges come from? These charges are based on the component element's Oxidation Number.

Fig.5-Chart with common oxidation states