Ionic Compounds
What are ionic compounds?
In chemistry, Ionic compounds are basically defined as being compounds where two or more ions are held next to each other by electrical attraction. One of the ions has a positive charge (called a "cation") and the other has a negative charge ("anion"). Cations are usually metal atoms and anions are either nonmetals or polyatomic ions (ions with more than one atom). The same thing that makes the positive and negative ends of a magnet stick to each other is what makes cations and anions stick to each other. Usually, when we have ionic compounds, they form large crystals that you can see with the naked eye. Table salt is one example of this - if you look at a crystal of salt, chances are you'll be able to see that it looks like a little cube. This is because salt likes to stack in little cube-shaped blocks.
So, what are the main properties of ionic compounds? Well, I'm glad you asked...
The first word in the name is the name of the cation, and the second word is the name of the anion. There is no exception to this rule.
The best way to go about naming ionic compounds is to take a look at the formula and figure out the names of the cation and anion. When you've got that, just stick them together and you've got the name of the compound.
Handy methods for naming compounds
Let's go through this using an example: Fe2(SO4)3
Step One: Name the cation and anion
The cation is always the first thing you see in the name, and the anion is always the second thing. In this case, you should recognize that Fe is "iron", and that SO4 is the "sulfate" ion. Generally, if one of these ions has more than one atom in it, you'll need to look it up in a chart.
Step Two: Figure out if you need a Roman numeral in the name.
If the cation in the compound you're naming is not a transition metal, then you definitely don't need to use a Roman numeral and the naming is done. If there is, then you need to figure out whether or not the cation can exist in more than one charge. If not, then you don't need a Roman numeral. If so, then move on to Step Three...
Step Three: Figure out what the Roman numeral should be
Basically, this should be fairly easy. A good rule of thumb is that usually the number of anions you have in the molecule is equal to the charge on the cation, and that the number of cations you have is equal to the number of anions. Using our example, there are three sulfate ions, meaning that iron has a charge of +3. Likewise, since there are two iron atoms, the sulfate has a charge of -2. Since iron has a charge of +3 in this compound, the name in this example is iron (III) sulfate.
Step Four: Check your work
Look at the answer from the last step, and ask yourself whether the charges are OK. Is +3 a charge that iron can have? Is -2 the charge of the sulfate ion? In this case, the answer to both questions is "yes", so we're finished, and the answer of iron (III) sulfate stands
Giving the formula of an ionic compound if you're given the name
We'll use an example to find the formula of an ionic compound: copper (II) fluoride
Step One: Translate the name into the ions
In copper (II) fluoride, the cation is the copper (II) ion and the anion is the fluoride ion. Hopefully, you realize that the copper (II) ion is simply Cu2+ and the fluoride ion is F-. If not, then you need to go back and review the rules for naming ions above.
Step Two: Put brackets around the ions, but leave the charges on the outside
In this case, the copper (II) ion would be [Cu]2+ and the fluoride ion would be [F]-1. Never change anything in these brackets, ever!
Step Three: Put the ions next to each other.
When we do this here, we get [Cu]2+[F]-1
Step Four: Cross the charges:
The charge on the cation will be equal to the number of anions you have, and the charge on the anion will be equal to the number of cations you have. In our example, you should realize that we have one copper atom (because the charge on fluorine is -1) and two fluoride ions (because the charge on copper is +2). This gives us a formula of: [Cu][F]2
Step Five: Take the brackets away.
The final formula for copper (II) fluoride is then CuF2
Exceptions:
IF we have a polyatomic ion, such as sulfate or ammonium, you need to replace the brackets with parentheses in step five. For example, if you end up with [NH4]2O as the formula for ammonium oxide at the end of step four, you'd simply replace the brackets with parentheses in step five to give you (NH4)2
Here's some other stuff about ionic compounds that you might have wondered about:
In chemistry, Ionic compounds are basically defined as being compounds where two or more ions are held next to each other by electrical attraction. One of the ions has a positive charge (called a "cation") and the other has a negative charge ("anion"). Cations are usually metal atoms and anions are either nonmetals or polyatomic ions (ions with more than one atom). The same thing that makes the positive and negative ends of a magnet stick to each other is what makes cations and anions stick to each other. Usually, when we have ionic compounds, they form large crystals that you can see with the naked eye. Table salt is one example of this - if you look at a crystal of salt, chances are you'll be able to see that it looks like a little cube. This is because salt likes to stack in little cube-shaped blocks.
So, what are the main properties of ionic compounds? Well, I'm glad you asked...
- All ionic compounds form crystals So far as I know, there are no exceptions to this. Again, salts like to form crystals because when you have a whole bunch of little electrical positive and negative charges all stuck together, they seem to like to bunch into little stacking groups.
- Ionic compounds tend to have high melting and boiling points. When I say "high", what I mean is "very, very high." Most of the time, when you work with ionic compounds in a chemistry class, the melting point is hot enough that you can't melt them with a Bunsen burner.
- Ionic compounds are very hard and very brittle. Again, this is because of the way that they're held together. Above, I said that it takes a lot of energy to break the positive and negative charges apart from each other. This is the reason that ionic compounds are so hard - they simply don't want to move around much, so they don't bend at all. This also explains the brittleness of ionic compounds.
- Ionic compounds conduct electricity when they dissolve in water. If we take a salt and dissolve it in water, the water molecules pull the positive and negative ions apart from each other.
The first word in the name is the name of the cation, and the second word is the name of the anion. There is no exception to this rule.
The best way to go about naming ionic compounds is to take a look at the formula and figure out the names of the cation and anion. When you've got that, just stick them together and you've got the name of the compound.
Handy methods for naming compounds
Let's go through this using an example: Fe2(SO4)3
Step One: Name the cation and anion
The cation is always the first thing you see in the name, and the anion is always the second thing. In this case, you should recognize that Fe is "iron", and that SO4 is the "sulfate" ion. Generally, if one of these ions has more than one atom in it, you'll need to look it up in a chart.
Step Two: Figure out if you need a Roman numeral in the name.
If the cation in the compound you're naming is not a transition metal, then you definitely don't need to use a Roman numeral and the naming is done. If there is, then you need to figure out whether or not the cation can exist in more than one charge. If not, then you don't need a Roman numeral. If so, then move on to Step Three...
Step Three: Figure out what the Roman numeral should be
Basically, this should be fairly easy. A good rule of thumb is that usually the number of anions you have in the molecule is equal to the charge on the cation, and that the number of cations you have is equal to the number of anions. Using our example, there are three sulfate ions, meaning that iron has a charge of +3. Likewise, since there are two iron atoms, the sulfate has a charge of -2. Since iron has a charge of +3 in this compound, the name in this example is iron (III) sulfate.
Step Four: Check your work
Look at the answer from the last step, and ask yourself whether the charges are OK. Is +3 a charge that iron can have? Is -2 the charge of the sulfate ion? In this case, the answer to both questions is "yes", so we're finished, and the answer of iron (III) sulfate stands
Giving the formula of an ionic compound if you're given the name
We'll use an example to find the formula of an ionic compound: copper (II) fluoride
Step One: Translate the name into the ions
In copper (II) fluoride, the cation is the copper (II) ion and the anion is the fluoride ion. Hopefully, you realize that the copper (II) ion is simply Cu2+ and the fluoride ion is F-. If not, then you need to go back and review the rules for naming ions above.
Step Two: Put brackets around the ions, but leave the charges on the outside
In this case, the copper (II) ion would be [Cu]2+ and the fluoride ion would be [F]-1. Never change anything in these brackets, ever!
Step Three: Put the ions next to each other.
When we do this here, we get [Cu]2+[F]-1
Step Four: Cross the charges:
The charge on the cation will be equal to the number of anions you have, and the charge on the anion will be equal to the number of cations you have. In our example, you should realize that we have one copper atom (because the charge on fluorine is -1) and two fluoride ions (because the charge on copper is +2). This gives us a formula of: [Cu][F]2
Step Five: Take the brackets away.
The final formula for copper (II) fluoride is then CuF2
Exceptions:
IF we have a polyatomic ion, such as sulfate or ammonium, you need to replace the brackets with parentheses in step five. For example, if you end up with [NH4]2O as the formula for ammonium oxide at the end of step four, you'd simply replace the brackets with parentheses in step five to give you (NH4)2
Here's some other stuff about ionic compounds that you might have wondered about:
- Ionic compounds are usually formed when metal cations bond with nonmetal anions. The only common exception I know to this is when ammonium is the cation - there's no metal in ammonium, but it forms ionic compounds anyhow.
- Ions are atoms that have satisfied the octet rule (which for those of you who've been sleeping the last couple of months, states that every atom wants to have eight valence electrons, just like the nearest noble gas). If you have two neutral elements, and one wants to gain electrons to be like the nearest noble gas and the other wants to lose electrons to be like the nearest noble gas, chances are that they will react with each other and make an ionic compound.