Understanding Polyatomic Ions and Ionic Compound Formulas

We learned so far about different types of ions. We know that again metals like sodium make sodium ions. Metals like

chlorine or even oxygen makes negative ions like oxide or chloride. So far all the ions including magnesium can be

tungsten 3+ ion 2 plus whatever all these cations and everything has only one atom. We can say one is mono. We

will see later. These are like monatomic ions. What if an ion has more than one atom? There are certain type of ions

called polyatomic ions and they have more than one type of atom and also they have more than one atom. That's why poly

means poly is more than one. We are trying to learn polyatomic ions and these are the polyatomic ions we have to

know and we have to remember and yeah it looks like a lot. It is a lot but it is not that hard to remember because

there's a pattern not all of them for certain ions. First thing I want you to emphasize is this is NH4 is ammonium

ion. This is the only positive or only cation polyatomic cation we are going to learn. All other polyatomic ions are

negatively charged. You have remember all these but don't try to say that acetate is this carbonate is this

nitrate is this nitrite like that. If you just look carefully, there are certain patterns. For example, this is

NO3 minus. It says nitrate. The next one below is NO2 minus. It is nitrite. Nitrate. Nitrite. Look at this one. SO4

2 minus it is sulfate. And when SO3 that means there are one less oxygen sulfide nitrate nitrite sulfate sulfide. So you

can see number of oxygen in this group reduce and the name also change. Another one you can see here this is chloride.

This is chlorate. Chlorate has more oxygen. And if you see that this is a whole group it has only chlorine and

oxygen. This chloride has two oxygen atoms. Hypocchloride hypocchloride has only one. And we have perchlorate here.

It has one more oxygen chloride. Right now I'm just trying to show you these names are not just random. There's a

certain pattern at least for some of them that makes you it easier for you to understand and remember. Here we have

hydrogen sulfide HSO3 or bulfides. Bulfate is just SO4 feroxide cyanide like that chromate dromate phosphate the

these are things you should remember there are certain things is not that easy especially you have to remember the

charges as well. Whenever there is a hydrogen, hydrogen sulfate or here we have hydrogen carbonate. It is derived

from the previous ion. In this case, let's look at this is sulfite. Next one is hydrogen sulfite or bulfite. You just

add one H to sulfite. You are adding one H+. Therefore, charge of bulfite is just -1 instead of -2 here. So, I'm just

trying to show you there are certain patterns. It's easy for you to remember. It is not just random. Whenever you say

hydrogen sulfide, you should know the charge. If you know the charge of sulfite similarly, if this is hydrogen

carbonate or bicarbonate, it is coming from carbonate. You just add one hydrogen to carbonate. Then the charge

is just one less than carbonate. Carbonate is -2 bicarbonate or hydrogen carbonate is ne1. There are certain

patterns. Therefore, it is not that hard to remember all of them. And you need this for polyatomic ions making

compounds and also we will learn about oxy ions. You need these formulas for that. Let's look at one example just

sulfate. So basically it's showing that this sulfate group is an ion polyatomic ion. It has one sulfur atom and four

oxygen atoms. We need that information later when we are writing chemical formulas. There are four oxygen atoms

and one sulfa atom. when we are writing the formula masses right now don't try to I mean remember just try to

understand the polyatomic ions and this is the only positive one we are going to learn any questions let's have a

compound magnesium and chlorine ion when you form a compound using these two ions we know that we can exchange the cation

and anion magnitude we will get mg we don't write one and cl2 this two is coming from here this is The compound

formed by magnesium and chloride ion. How about the compound formed by magnesium? Let's say nitrate. This is a

polyatomic ion. The charge of nitrate is ne1. Now we have a anion and a cation. Anion and a cation. When they form a

compound, we should know that it is an ionic compounds because these are ions. Cation and anion. What is the formula of

this compound? We do the same thing we just did for magnesium chloride. we exchange the magnitude of charges with

an and cation. In this case, we will get mg one because there's negative one. I'm not going to write one here. This

negative one and then I will write NO3 and there's a two here right from magnesium. This has to go here and this

two belongs to whole nitrate group. Therefore, I just cannot write two here. It is incorrect since it belongs to

whole nitrate groups. I'm going to write a parenthesis and write two in here. We know that MGCL2 means there are one

magnesium ion and two chloride ions. That's what it shows from this Mgl2. Similarly, this is showing there's one

magnesium ion and there are two nitrate groups in this compound or in this formula. That is important to

understand. And also it's showing that we know that there's one nitrogen in nitrate group. It's showing that there

are three oxygen in a nitrate group and there are two nitrate groups in magnesium nitrate. We just found

magnesium and nitrate ions use magnesium nitrate. Many occasions you need to find the number of atoms especially when

you're writing reactions and when we are finding masses formula masses we need to count the number of atoms in a formula.

Let's try to count number of atoms. Here we have three types of atoms. Magnesium, nitrogen and oxygen. It's easy, right?

How many magnesium atoms are there in this formula? There was only one magnesium because when you look at this

formula, when you don't see any number in front of the ion in this case, that means it has only one. This two does not

belongs to magnesium. This two belongs to only nitrate because it is just after the parenthesis of nitrate. Now we don't

write one here because chemists don't write once in many cases. You will see when we write reactions we don't write

one. And now we know we don't write one in charges as well. Right? Now we know one magnesium atom. How many nitrogen

atoms are there? There are two there are two ways to think about that. One way is you can see two here in the parenthesis.

It's a mess here. There are two here. That means this two belongs to whole nitrate groups. That means this two

belongs to this nitrogen and this two belongs to this oxygen as well. Therefore there are two nitrogens here.

Another way you can think is this two means there are two nitrate groups NO3 minus NO3 minus. So since this two means

two nitrate groups you can easily see there are two nitrogen atoms in this formula. How many oxygen atoms are

there? Six. Yes. Yes. There are three oxygen per nitrate group. But in this formula you can see there are two

nitrate groups. Therefore, we have six oxygen atoms. You can write six. Another way is there's two and there's three. 2

* 3 is six. That's how you get the number of atoms. And this really important for you to understand how we

can get the number of atoms. So, this is the same thing and it's explaining what is what. Now, it's saying naming of

compounds containing polyatomic ions. It's very simple. It's very simple. You just need to understand the name of the

polyatomic ions. Let's do an example. Write the formula for an ionic compound containing polyatomic ion. Ion 3

phosphate. Now you should understand that when you see ion 3, this is a type two compound. That's why they are

showing the charge of this cation. It is type two compound. It doesn't matter. Let's try to write the formula. The

first step is write the symbol of the cation and it charge. This is the cation and it charge. Symbol is Fe 3+. The next

one is write the symbol for the anion and it charge. Anion is phosphate. Phosphate phosphate is this one. This is

phosphate. It is P43 minus. P43 minus. This is phosphate. Charge is -3. It has four oxygen atoms in that ion.

Next charge without sign become subset of the other. We know that when you are making formulas, you just change this

thing and I'm going to write Fe3 oops Fe3. That three is coming from this phosphate. This three is coming from

this phosphate and P4. Now I'm going to write three for P4 because ion 3 this comes here that mean this comes to

phosphate. Please, please, please don't forget about the parenthesis. In this case, it might not make a big

difference. You will see. Can you subscribe to the smallest whole number ratio? That means these are the

subscripts. Fe3 P43. We can make it much simpler. Ratios. You can divide both sides by three. It will become Fe P4.

Since you are learning, it's okay to write a parenthesis here. It's going to be very useful when you are writing

especially equations. And also it will help you to recognize that P4 is a group. It behaves independently.

Whenever you see a reaction in our level, these phosphate groups or any of these polyatomic ions act as a group.

Therefore, it will help you to understand and identify groups if you put parenthesis here. Even though that

is not the correct way to write it in chemistry. For example, sodium hydroxide in AOH. O is a polyatomic ion. Here we

don't show the parenthesis but still when you are learning it's okay to write parenthesis just you know that O is a

different group it act as a group it behaves like a group it will help you last step is check that total charge of

the kion cancels total charge of annions this is positive3 how do I know it is given ion three phosphate I just told

you it act as a group whole group I need to find the charge of whole group is -3 how do I know we have remember that it

is here positive 3 and -3 cancels out. This seems like a correct formula. Fe P4. What are the formulas of compound

made from following? Aluminium ion and sulfate ion. Aluminum is a metal. Sulfate is a polyatomic ion. It has to

be ionic compound. I'm going to find the charge of aluminum. It is constant. It is positive3 plus. Sulfate ions42 minus

sulfate SO42 minus. The next step is to exchange without the charge. It's a mistake. You should not put the charge.

Then I will put a L2 2 is coming from sulfate charge SO4. Now I'm going to write the parenthesis. Please don't

forget the parenthesis and three. That would be the formula. Next one is chromium 2 with hydrogen carbonate.

Chromium 2 already charge is given CR2 plus. Chromium is a metal and charge is given hydrogen carbonate. Hydrogen

carbonate. We just discuss about that a little bit. H CO3 minus CR1 I don't write one then write H

CO3 and I write two here coming from this chromium 2 you should know chromium is a metal by looking at the periodic

table and charge has to be positive the main reason is when we learn about compounds we write the metal or cation

and the charge of the cation this is the charge of the cation that means it is always positive