Polyatomic Ions: Study with Video Lessons, Practice Problems & Examples

Polyatomic ions are groups of atoms with an overall charge, often categorized as negatively charged oxyanions. Trioxides, like borate (B_O3^3-), contain three oxygens, while tetraoxides, such as sulfate (S_O4^2-), have four. Reducing the number of oxygens alters the name from -ate to -ite, creating new ions like sulfite (S_O3^2-). Additionally, halogen oxyanions, derived from halogens, follow specific naming conventions based on oxygen count. Positive polyatomic ions, like ammonium (NH₄⁺), are less common.

Polyatomic Ions

Polyatomic ions represent tightly bound groups made of different elements that possess an overall charge. That charge can be either positive or negative. For now, we're going to focus only on the negatively charged ones. So we're going to take a look at our polyatomic oxyanions. These are the negatively charged polyatomic ions that end with oxygen. And when it comes to these polyatomic oxyanions, we can describe them as either being trioxides or tetraoxides. Let's focus on the trioxides first. Let's focus on the fact that they have the letter t. If we look at this portion of the periodic table, we're going to say that these 4 in blue represent possible trioxides, and they also form the letter t. Now the trioxides themselves, we say when their name ends with 8, they possess 3 oxygens. So, again, use this to help you reinforce this: trioxides with t, when they end with 8, they have 3 oxygens, they form the letter t on the periodic table. So they all have 3 oxygens, so that's BO3, CO3, NO3 and SiO3. Then we have our tetraoxides. Tetra, some of you may know what that stands for, tetraoxides. Tetraoxides. When their name ends with 8, they possess 4 oxygens because tetra stands for 4. And those are represented by phosphorus and sulfur. So they would have 4 oxygens, so PO4 and SO4. Alright. We said oxyanion, so we talked about a negative charge, but we don't see it quite yet. Click on to the next video and let's take a look when we start incorporating charges with these trioxides and tetraoxides.

Polyatomic Ions

Up to this point, we know how many oxygens are found within our trioxides and tetraoxides. Now it's time to take a look at how we determine their charges. We know that our trioxides are here within this group, and we know that they are called trioxides because they have 3 oxygens to them. To think about their charges, there's a pattern when we take a look at the periodic table. So starting with group 3a, we have minus 3. So this would be B2O3 minus or minus 3. Then we go minus 2, minus 1. So in minus 2, all those in group 4a would be XO2 minus, and then N3O3 would be minus 1. So here we determine the charges for the trioxides when they end with the name 'ate'. We'll get to that later on.

With the tetraoxides, the tetraoxide here, we're gonna start over again. So minus 3, minus 2, minus 1. So minus 3 here for P4O13 minus 3. We've determined that they have 4 oxygens because they're tetraoxides and this will be SO42-. Notice here we have another slot here for minus 1, which represents elements within this group 7a. We'll talk about them later on.

So at this point, we determine who our trioxides are and who our tetraoxides are, how many oxygens they have, and what their charges are. Now that we've taken a look at these different types of polyatomic ions, let's continue onward and take a look at additional ones.

Polyatomic Ions

With our trioxides and tetraoxides, we've determined the number of oxygens and the charges associated with them. Now let's associate these structures with names. With our trioxides, we have borate. So borate would tell us that we're dealing with boron, so that'd be B_4O_3^{3-}, carbonate, carbon, so that's the CO_3^{2-} that we discovered. Nitrate is dealing with the nitrogen, so NO_3^{-}. And then silicate must be the silicon that we have here, so SiO_3^{2-}. This is their full polyatomic ion form with the name associated with it.

Now let's look at our tetraoxides. So with our tetraoxides we have phosphate, which must be dealing with our phosphorus, so that is PO_4^{3-} and then finally sulfate, which deals with our sulfur, so SO_4^{2-}. Now these represent our most common types of polyatomic oxyanions, and it's important to remember them as trioxides and tetraoxides because from here, we can slightly change their structures and introduce ourselves to new polyatomic ions and with them, new names. So click on the next video and let's take a look at some of these situations.

Polyatomic Ions

We now know the most common trioxides and tetraoxides. But what happens when we start messing with the number of oxygens they possess? Well, this is going to open up a whole new avenue of other polyatomic ions. We're going to say here decreasing the number of oxygens by 1 changes the ending to "ite" while keeping the overall charge the same. So up above, we saw that a common tetraoxide was sulfate. It's a tetraoxide because it has 4 oxygens, and we knew that based on where it's located on the periodic table, its charge is 2−. Now in this form, its ending is "ate". Now I'm going to decrease the number of oxygens by just 1. It becomes SO₃. The overall charge stays the same, so it's 2−. Our "ate" ending now changes to "ite". So SO₃²⁻ is sulfite, so "ite" now. So that is the difference in these polyatomic ions. So just remember, once we start manipulating the number of oxygens, we can create a whole new polyatomic ion and a name associated with it. Now that we've seen this example, let's move on and look at some other polyatomic ions.

Polyatomic Ions Example 1

In this example question, it says, "Give the formal or systematic name for the following polyatomic ion, PO3-3." Alright. So, it has phosphorus within it. Looking back up, you know that phosphorus was one of our tetraoxides and it came in the form of PO4-3. Its name was phosphate. We just learned that if I decrease the number of oxygens by just 1 oxygen, it'll give me a new polyatomic ion. So, reducing it by 1 oxygen gives me this PO3-3, and remember, what that does is it changes the "ate" ending to "ite". So, this would represent our phosphite ion. So, it all hinges on the fact that you remember your trioxides and your tetraoxides. And then from there, changing the number of oxygens changes the ending of your polyatomic ion.

Give the systematic name for the following polyatomic ion.

NO₂^–

• A. nitrate
• B. nitrite
• C. nitrous
• D. pernitrate

Give the systematic name for the following polyatomic ion.

CO₂^2–

• A. hypocarbonate
• B. carbonate
• C. carbonite
• D. carbonide
• E. carbide

Give the systematic name for the following polyatomic ion.

AsO₄^2–

• A. arsenate
• B. arsenite
• C. arsenic
• D. hypoarsenate
• E. arsenide

The formula for the sulfate ion, SO₄^2–. If the term of 'thio' means the replacement of an oxygen by a sulfur, what is the formula for the thiosulfate ion?

• A. SO₄^3-
• B. SO₃^3-
• C. SO₃^2-
• D. S₂O₃^2-
• E. S₂O₃^-

Polyatomic Ions

Now let's take a look at the halogenoxy anions. These are polyatomic ions, which still contain oxygen, but now are containing halogens. Remember, your halogens are the elements that are in group 7a or group 17 of the periodic table. They're referred to as also oxyhalogens, besides being called your halogenoxyanions. Some key things to keep in mind when dealing with these types of polyatomic ions include, first of all, their base name. The base name is just the beginning of the non-metal's name that is unchanged. The non-metal here we're talking about is the halogen. The number of oxygens in these polyatomic ions affects either the prefix, which is the beginning of the name, and/or suffix, which is the end of the name. We're going to say here all the halogen oxyanions possess a -one charge, so that's what they all have in common. Let's take a look at our halogens. We have fluorine, chlorine, bromine, and iodine. The base name is just the beginning of their names. Fluorine would be "Fluor" for its base name, Chlorine would be "Chlor" for its base name, Bromine would be "Brom" for its base name. Now to this base name, we can add an ending, which we'll talk about next. Iodine is just "Iod" as its base name. Now we said that the number of oxygens can affect both the beginning and/or end of the name. If it has 4 oxygens, it uses the prefix "per" and the suffix "ate." So let's say we're looking at bromine. So bromine, Br, has 4 oxygens. They all possess a minus one charge. So the name of this would be perbromate. If they have 3, 2, and 1 oxygens, we're going to see how the name changes. So here, let's say we have Cl3-. Here, the beginning of the name, the prefix drops "per" no longer exists, but the ending of "-ate" is still around. Because we're using chlorine, we use the base name of "chlor." So this would be chlorate. Next, let's look at I2- so this would be iodite, ending changes from "ate" to "ite," as you can see. And then let's say we have FO-, so that would be hypofluorite. So just see, when we have 4 oxygens, we have the prefix "per." When we have 1 oxygen, we have the prefix "hypo." When we have 3 or 4 oxygens, we have the ending of "ate," but once we drop down to 2 and 1, the ending changes to "ite." These are things that you need to keep in mind when naming these different types of polyatomic ions, or in this case your oxyhalogens or your halogenoxianions. So the number of oxygens determines the beginning and end of the name. Also, the type of halogen you use determines the base name.

Polyatomic Ions Example 2

In this example question, it says, name each of the following compounds. So for a we have ClO₄^- and for b we have BrO₂^-. Alright. Remember, we have just learned that if your halogen oxyanion has 4 oxygens in it, it's going to have the prefix of "per." Because it is a chlorine, we're going to use its base name which is "chlor," and because it has 4 oxygens, its suffix name is "ate." Remember, all of these halogen oxyanions possess a charge of minus 1. So ClO₄^- is called perchlorate. This would be the perchlorate ion. Now let's do b. For b, we have BrO₂^-. When we have 2 oxygens, we're going to say that there is no prefix because it is bromine, we're going to use the base name of "brom," and then remember, when we have 2 oxygens, we use the suffix of "ite". So BrO₂^- would be the bromite ion. So these would be the two names for these halogen oxyanions. Just remember, it's based on the number of oxygens that they possess, which affects both their prefix and/or suffix names.

Polyatomic Ions

Up to this point, we've just examined polyatomic ions that possess a negative charge. Now it's time to look at the positively charged ones. We're going to say most polyatomic ions are negatively charged except for NH4+ ion and Hg22+ ion. We're going to say NH4+ (Ammonium ion) is the only major polyatomic ion that you're going to need to know that has a plus one charge.

Then, we're going to discuss Hg22+. And know that the little 2 is not a typo. What this is, it's two individual Hg+ mercury ions. They combine together, so now there are two of them together, and their combined charge comes out to 2 plus. Because each one possesses a plus one charge, we're going to say this is called mercury(I) ion. Yes, I know it might be a little confusing because the charge is 2 plus, but realize that the 2 plus comes from the fact that each mercury is plus 1.

So again, a vast majority of the polyatomic ions you're going to encounter possess a negative charge. These two possess positive charges.

Polyatomic Ions

Now the other polyatomic ions don't fit into predictable patterns and so must be memorized. So we're going to start out first with the other tetraoxides. Now these are called the others because they don't quite fit in with the other 2 major tetraoxides we covered in sulfate and phosphate. Now, permanganate. Permanganate's formula is MnO4−1. Then we have chromate, which is CrO42−, and then finally, oxalate, which is C2O42−. Now remember, they're all tetraoxides because they all possess 4 oxygens. But we call them the others because we have manganese, chromium, and carbon. These elements are in different places on the periodic table, so it's hard to form a real pattern with them.

Then we have the other polyatomic ions. We don't really classify them as trioxides or tetraoxides because some of them don't possess that many oxygens or any oxygens at all. So here we have cyanide. Cyanide is CN−. We have hydroxide, which is OH−, peroxide which is O22−. This kind of reminds us a little bit of the mercury 1 ion where we have, in this case, 2 oxygens. Each one is minus 1, so collectively there are 2 minus. Then we have dichromate, which is a little bit similar to chromate. So di meaning that we kind of double things a little bit, but here it's Cr2O72− instead of CrO42−. Then we have cyanate, which is related to cyanide. Cyanate is CNO−, and actually OCN− is the correct way to write it. So, cyanide doesn't possess an oxygen, cyanate does possess an oxygen.

Then finally, we have the acetate ion. Acetate is written as C2H3O2−. Now, this will be the predominant form that you will see, but later on in chemistry, you may see them showing in another form. You may also see it as CH3COO−. So just keep your eyes open. When you see either form, both of them represent the acetate ion.

Polyatomic Ions Example 3

So here if we take a look at this example, it says, based on your understanding of the polyatomic oxyanions, provide the structure for the thiocyanate ion. Alright. So we know that our cyanate ion is OCN−. And remember, we've come across this prefix of thio before. Remember, when we have this prefix of thio, it means that we're going to replace an oxygen with a sulfur. So if we're dealing with thiocyanate, that means we're going to replace one oxygen with a sulfur. In this case, we only have one oxygen, so it's just going to get replaced with a sulfur. So this becomes SCN−. So our cyanate is one of our other polyatomic ions of OCN−. That was the key to knowing what thiocyanate looks like. Remember, thio just means replace an oxygen with a sulfur while maintaining everything else about the polyatomic ion the same. So our answer would be SCN−.

Give the systematic name for the following polyatomic ion.

FO^–

• A. hypofluorite
• B. fluorite
• C. fluoride
• D. fluorate
• E. hypofluorate

Give the systematic name for the following polyatomic ion.

IO₃^–

• A. iodite
• B. hypoiodate
• C. iodide
• D. iodate
• E. periodate

The silicate ion is the silicon version of the carbonate ion. Based on this description, provide the structure of the silicate ion.

• A. SiO₃^4-
• B. SiO₃^3-
• C. SiO₃^2-
• D. SiO₂^2-
• E. SiO₂^3-

Here’s what students ask on this topic:

What are polyatomic ions and how are they classified?

Polyatomic ions are groups of atoms that are covalently bonded together and possess an overall charge, either positive or negative. They are often classified based on their charge and the number of oxygen atoms they contain. Negatively charged polyatomic ions, or oxyanions, can be further categorized into trioxides and tetraoxides. Trioxides, such as borate (BO₃^3-), contain three oxygen atoms, while tetraoxides, like sulfate (SO₄^2-), have four. Additionally, there are halogen oxyanions derived from halogens, which follow specific naming conventions based on the number of oxygen atoms. Positive polyatomic ions, like ammonium (NH₄⁺), are less common.

How do you determine the charges of trioxides and tetraoxides?

The charges of trioxides and tetraoxides can be determined by their position on the periodic table. For trioxides, starting from Group 3A, the charge is -3 (e.g., BO₃^3-), then -2 for Group 4A (e.g., CO₃^2-), and -1 for Group 5A (e.g., NO₃^-). For tetraoxides, the pattern restarts: -3 for Group 5A (e.g., PO₄^3-), and -2 for Group 6A (e.g., SO₄^2-). This systematic approach helps in predicting the charges of these polyatomic ions.

What is the difference between -ate and -ite endings in polyatomic ions?

The difference between -ate and -ite endings in polyatomic ions lies in the number of oxygen atoms they contain. An ion with an -ate ending has more oxygen atoms than its -ite counterpart. For example, sulfate (SO₄^2-) has four oxygen atoms, while sulfite (SO₃^2-) has three. The overall charge of the ion remains the same despite the change in the number of oxygen atoms.

What are halogen oxyanions and how are they named?

Halogen oxyanions are polyatomic ions that contain oxygen and a halogen element (Group 7A or Group 17 of the periodic table). Their names are based on the number of oxygen atoms and the halogen involved. For example, if a halogen oxyanion has four oxygen atoms, it uses the prefix 'per-' and the suffix '-ate' (e.g., perbromate, BrO₄^-). If it has three oxygen atoms, it is named with the suffix '-ate' (e.g., chlorate, ClO₃^-). For two oxygen atoms, the suffix changes to '-ite' (e.g., iodite, IO₂^-), and for one oxygen atom, the prefix 'hypo-' and the suffix '-ite' are used (e.g., hypofluorite, FO^-).

What are some examples of positively charged polyatomic ions?

Positively charged polyatomic ions are less common than negatively charged ones. The most notable examples are the ammonium ion (NH₄⁺) and the mercury(I) ion (Hg₂²⁺). The ammonium ion consists of one nitrogen atom and four hydrogen atoms, carrying a +1 charge. The mercury(I) ion is unique because it consists of two mercury atoms, each with a +1 charge, resulting in a combined charge of +2.