Polyatomic Ions: Videos & Practice Problems

Polyatomic ions are groups of atoms that carry a charge, with negatively charged oxyanions categorized as trioxides and tetraoxides. Trioxides, like borate (B_O3^3-), contain three oxygens, while tetraoxides, such as sulfate (S_O4^2-), have four. The naming convention changes with the number of oxygens, where reducing oxygen by one changes the suffix from -ate to -ite. Additionally, halogen oxyanions, like chlorate (ClO₃^-), follow specific naming rules based on their oxygen count. Understanding these patterns is crucial for mastering polyatomic ions.

Polyatomic Oxyanions

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 tetraxides.

Let's focus on the trioxides first. Let's focus on the fact that it has 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 "ate," they possess 3 oxygens. So again, use this to help you reinforce this. Trioxides with T. When they end with "ate," they have 3 oxygens. They form the letter T on the periodic table, so they all have three oxygen. So that's BO3, CO3, NO3, and SiO3.

Then we have our tetroxides. Tetra, some of you may know what that stands for. Tetra oxides. When their name ends with "ate," they possess 4 oxygens because tetra stands for four, and those are represented by phosphorus and sulfur. So they would have 4 oxygens, so PO4 and SO4.

All right, we've said oxyanions, 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 tetroxides.

Polyatomic Ion Charges

Up to this point, we know how many oxygens are found within our trioxides and Tetra oxides. Now it's time to take a look at how do we determine their charges. We know that our trioxides are here within this T 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 -3. So this would be BO3-3 or -3. Then we go -2 -, 1. So if -2, all those in Group 4A would be 2 minus and then no three would be -1 O. Here we determine the charges for the trioxides when they end with the name 8, We'll get to that later on with the truettroxides.

The tetraxides here we're going to start over again. So -3 -, 2, -, 1, so -3 here for PO4-3. We've determined that they have 4 oxygens because they're tetraoxides and this would be SO4-2. Notice here we have another slot here for -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 are Tetra oxides 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 one.

Polyatomic Oxyanion Names

With our trioxides and tetroxides, we've determined the number of oxygens and the charges associated with them. Now let's associate those structures with names. With our trioxides, we have borate, So borate would tell us that we're dealing with boron. So that would be BO3-3 carbonate carbon. So that's the CO3-2 that we discovered. Nitrate is dealing with a nitrogen, so NO3-1 and then silicate must be the silicon that we have here, SI, so SiO3-2. So this is their full polyatomic ion form with the name associated with it.

Now let's look at our Tetra oxides. So with our Tetra oxides we have phosphate which must be dealing with our phosphorus. So that is PO4-3 and then finally sulfate, which deals with our sulfur SO4-2. Now these represent our most common types of polyatomic oxyanion. And it's important to remember them as trioxides and Tetra oxides 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 Ion Variations

We now know the most common trioxides in Tetra oxides, but what happens when we start messing around with the number of oxygens they possess? Well, this is going to open up a whole new Ave. of other polyatomic ions. We're going to say here, decreasing the number of oxygens by one changes the ending to 8, while keeping the overall charge the same.

So up above we saw that a common tetraloxide was sulfate. It's a Tetra oxide because it has four oxygens and we knew that based on where it's located on the periodic table, its charges to minus. Now in this form its ending as eight. Now we're going to decrease the number of oxygens by just one. It becomes SO32-. The overall charge stays the same, so it's 2 minus.

Our eight ending now changes to 8 so, SO32- is solved fight so it 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.

Naming Polyatomic Oxyanion Example

In this example question, it says give the formal or systematic name for the following polyatomic ion PL₃^3-. All right, so it has phosphorus within it. Looking back up, you know that phosphorus was one of our Tetra oxides and it came in the form of PO₄^3-. Its name was Foss fate.

We just learned that if I decrease the number of oxygens by just one oxygen, it'll give me a new polyatomic ion. So reducing it by one oxygen gives me this PO₃^3-. And remember what that does is it changes the eight ending to 8. 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 oxygen 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

Answer: (B)

Give the systematic name for the following polyatomic ion.
CO₂^2–

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

Answer: (C)

Give the systematic name for the following polyatomic ion.
AsO₄^3–

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

Answer: (A)

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^-

Answer: (D)

Halogenoxy Anions Naming

Now let's take a look at the halogen Oxy anions. So 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 Oxy halogens, besides being called your halogen oxyanions.

Now some key things to keep in mind when dealing with these types of polyatomic ions. First of all, they're basining. The base name is just the beginning of the non metals name that is unchanged. The nonmetal here we're talking about is the halogen. The number of oxygens in these polyatomic ions affect either the prefix, which is the beginning of the name, and Oregon suffix, which is the end of the name.

We're going to say here. All the halogen oxy anions possess a -1 charge, So that's what they all have in common. So 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 core for its base name. Brom would be Brom, 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 Oregon end of the name. If it has four oxygens, it uses the base name of PER and the suffix ending of eight. So let's say we're looking at, let's talk about bromine. So bromine branch has four oxygens. They all possess a -1 charge. So the name of this would be PER. Insert the base name in the middle per bromate.

If they have 3, two and one oxygens, we're going to see how the name changes O here. Let's say we have ClO3-, O here, the beginning of the name. The prefix drops per no longer exist, but the ending of eight 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 IO2-. So this would be iodine ending changes from 8:00 to 8:00 as you can see.

And then let's say we have FO1-, so that would be Hypo for the beginning of the name, the prefix and then floorite. So just see when we have 4 oxygens we have the prefix of per. When we have one oxygen we have the prefix of hypo. When we have 3 or 4 oxygens we have the ending of eight. But once we drop down to two and one, the ending changes to. So these are things that you need to keep in mind when naming these different types of polyatomic ions, or in this case your Oxi halogens or your halogen oxyanions.

OK, so the number of oxygens determined the beginning and end of the name. Also, the type of halogen you use determines the base name.

Naming Halogen Oxyanion Example

In this example question, it says name each of the following compounds. So for A we asked yellow 4 minus and for B we have Bro 2. All right. Remember, we just learned that if your halogen Oxy anion has four oxygens in it, it's going to have those prefix of PER because it is a chlorine. We're going to use its base name, which is chlor, and because it has four oxygens, its suffix name is 8.

Remember all of these halogen oxy anions possess a charge of -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 two oxygens, we're going to say that there is no prefix because it is a branch bromine. We're going to use the base name of Brom and then remember when we have two oxygens we use the suffix of eight.

O BrO2- would be the bromite ion. So these would be the two names for these halogen Oxy anions. Just remember it's based on the number of oxygens that they possess, which will affect both their prefix and or suffix names.

Polyatomic Cations

Up to this point, we 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+ is called the ammonium ammonium ion, and it's going to be the only major polyatomic ion that you're going to need to know that has a +1 charge. Then we're going to say we have Hg22+ and know that little 2 is not a typo. What this is, It's two individual Hg1 mercury ions. They combine together.

So now there's two of them together and their combined charge comes out to two plus because each one possesses a +1 charge. We're going to say this is called Mercury One ion. Yes, I know it might be a little bit confusing because the charge is 2 plus, but realize that that two plus comes from the fact that each Mercury is +1.

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

Other 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 Tetra oxides. Now these are called the others because they don't quite fit in with the other two major Tetra oxides we've 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 tetraloxides because they all possess 4 oxygen, 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 Tetra oxides 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-. So kind of reminds us a little bit of mercury one ion where we have in this case 2 oxygens, each one is -1, so collectively they're 2 minus. Then we have dichromate, which is a little bit similar to chromate, so dye meaning that we kind of double things a little bit, but here it's Cr2O72- instead of O8.

Then we have cyanate which is related to cyanide. Cyanate is CNO- and actually it's OCN-, the correct way to write it. So sine I doesn't possess an oxygen, cyanate does possess an oxygen. Then finally we have 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 Chapter 3, COO-. So just keep your eyes open when you see either form. Both of them represent the acetate.

Polyatomic Ion Example

So here if we take a look at this example, it says based on your understanding of the polyatomic oxy anions 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 file 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- and in the rest of it stays the same S-. So our cyanate is one of our other polyatomic ions of OCN-. That was the key to knowing what thiocyanate looks like. Remember thiol 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

Answer: (A)

Give the systematic name for the following polyatomic ion.
IO₃^–

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

Answer: (D)

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-

Answer: (C)

Here’s what students ask on this topic:

What are polyatomic ions?

Polyatomic ions are ions that consist of two or more atoms covalently bonded together, that function as a single ion. Unlike monatomic ions, which are composed of single atoms with a positive or negative charge, polyatomic ions have a charge that is distributed over the entire group of atoms. The charge results from the total number of electrons being different from the total number of protons in the molecules, leading to an overall positive or negative charge.

An example of a polyatomic ion is sulfate, which has the chemical formula:

${\mathrm{SO}}_{4}2-$

In this ion, one sulfur atom is covalently bonded to four oxygen atoms, and the entire molecule carries a -2 charge. Polyatomic ions can participate in ionic bonds with other ions to form ionic compounds, and they play a crucial role in various chemical reactions and processes. It's important to memorize the common polyatomic ions and their charges as they frequently appear in chemical equations and are essential for understanding chemical reactivity and properties.

How can I memorize polyatomic ions?

Memorizing polyatomic ions can be challenging, but there are several effective strategies you can use to make the process easier. Here are some tips:

1. Flashcards: Create a set of flashcards with the name of the ion on one side and its chemical formula and charge on the other. Regularly test yourself with these flashcards to reinforce your memory.
2. Mnemonics: Use mnemonic devices to help remember the ions. For example, for the sulfate ion ${\mathrm{SO}}_{4}2-$, you might use 'Sulphur's Oxygen 4' to remember the composition and charge.
3. Grouping: Group ions with similar endings together, like -ate ions (nitrate, sulfate, phosphate) and -ite ions (nitrite, sulfite, phosphite). This helps you see patterns in their naming and composition.
4. Practice Writing: Write down the ions repeatedly. This kinesthetic activity can help cement the information in your memory.
5. Association: Associate each ion with a particular fact or image. For instance, you might link the carbonate ion ${\mathrm{CO}}_{3}2-$ with an image of a piece of chalk (calcium carbonate).
6. Regular Review: Go over the ions regularly, not just in one cramming session. Spaced repetition is key to long-term memorization.

Remember

How to name polyatomic ions?

Polyatomic ions are ions composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit. Naming them follows specific rules based on the elements present and the ion's charge.

For most polyatomic ions, the name of the ion is derived from the root name of the central element, with a suffix that indicates the number of atoms involved and the ion's charge. Here are the common suffixes:

• "-ate" is used for the most common or typical polyatomic ion with a certain number of oxygen atoms.
• "-ite" is used for an ion that has one fewer oxygen atom than the "-ate" ion.
• "hypo-" and "-ite" together indicate one fewer oxygen than the "-ite" ion.
• "per-" and "-ate" together indicate one more oxygen than the "-ate" ion.

For example, the sulfate ion is:

${\mathrm{SO}}_4²-$

and the sulfite ion is:

${\mathrm{SO}}_3²-$

When the number of oxygens changes, the name reflects this change with the appropriate prefix or suffix.

For ions with hydrogen, you prefix the name with "hydrogen" or "dihydrogen," depending on the number of hydrogen atoms present, to indicate that hydrogen ions (H⁺) have been added to the oxyanion.