Many substances are made up of just two types of ions—a metal cation and a nonmetal anion. Each ion is made up of only one type of atom. There are other ways that atoms can join together, either as larger ionic complexes or as full molecules. Polyatomic ions are found in some of the most useful things we work with. So, what are polyatomic ions? Keep reading to find out!
- This article covers the topic of polyatomic ions.
- First, we will learn the definition of polyatomic ions.
- Next, we will look at some examples of polyatomic ions.
- Then, we will learn how to name compounds with polyatomic ions.
- After that, we will look at a chart of some of the most common polyatomic ions.
- Lastly, we will cover oxidation numbers to see how polyatomic ions get their charge.
Definition of Polyatomic Ions
A polyatomic ion is a group of two or more covalently bonded atoms (or a metal complex), that acts like a single unit and has a net charge.
Depending on the definition used, the word "molecule" may or may not be used to describe an ion with more than one atom. The Greek prefix poly- means "many," but even ions with only two atoms are often called "polyatomic."
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 (SO42-)
$$H_2SO_4 \rightarrow 2H^+ + SO_4^{2-}$$
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 (CO32-):
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.
The dashed lines on carbonate show that each double bond is really a "1 1/3 bond". Carbonate has several resonance forms, where each C-O bond can be a C=O bond.
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.
A species' oxidation number is the theoretical charge it will have during bonding. This number tells us how many electrons the species will lose (positive number) or gain (negative number) during bonding.
Fig.5-Chart with common oxidation states
Now that we know what oxidation states are, let's look at how to determine the charge of a polyatomic ion.
Find the charge of the polyatomic ion carbonate (CO3?)
Oxygen commonly has an oxidation state of -2. Generally speaking, the species on the left has a positive charge, while the species on the right have a negative charge. Because of this, carbon's oxidation state is +4 here instead of -4.
Adding these, we get,
$$+4+3(-2)=-2$$
Therefore, the charge on carbonate is -2.
Polyatomic Ions - Key takeaways
- A polyatomic ion is a group of two or more covalently bonded atoms (or a metal complex), that acts like a single unit and has a net charge
- Polyatomic ions will not dissociate in water
- Polyatomic ions can form compounds with each other
- 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.
- A species' oxidation number is the theoretical charge it will have during bonding. This number tells us how many electrons the species will lose (positive number) or gain (negative number) during bonding.
- Polyatomic ions get their charges from the combined oxidation numbers of their components
References
- Fig.3-Chart with common oxidation states (https://upload.wikimedia.org/wikipedia/commons/thumb/a/aa/Oks%C3%BCdatsiooniastmed.png/640px-Oks%C3%BCdatsiooniastmed.png) by Collective on Wikimedia Commons licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/)
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