Important things to know about ionic compounds


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What are ionic compounds?

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). Think back to grade school: 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.

Sometimes when you see a salt, it looks like a powder instead of a cube. This doesn't mean that the salt is not a crystal - it means that the crystals are just so small that you can't see them with the naked eye. If you were to put the powder under a microscope, chances are that you would see little geometric blocks. 

So, what are the main properties of salts? Well, I'm sure 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. The arrangement that these ions like to stack into is different, and is referred to as the "unit cell". There are ten or so different general shapes of unit cells. When you get to graduate school, ask me about them. For high school classes, it's really not all that important.
  • 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.  So, why are these temperatures so high? Well, it has to do with the way that ionic materials are held together. Remember how we said above that ionic compounds form crystals? These crystals are basically just great big blocks of positive and negative charges all stuck together. To break the positive and negative charges apart, it takes a huge amount of energy. This means that if we heat up the compound to add energy, it takes a huge amount of energy to break it apart.
  • Ionic compounds are very hard and very brittle. Again, this is because of the way that they're held together. Above, we 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. It takes a lot of energy to pull ionic charges apart from each other. However, if we give a big crystal a strong enough whack with a hammer, we usually end up using so much energy to break the crystal that the crystal doesn't break in just one spot, but in a whole bunch of places. Instead of a clean break, it shatters. 
  • 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. (This is because of the unusual properties of water, but that's a different story for a different time). Instead of the ions being right next to each other, they wander all over the water.  Now, think back to what electricity is - hopefully, you remember that electricity is just the movement of electrons through metals (or anything else). Now, electrons are just negatively-charged particles, and metals have the property that they're good at letting them wander around. Dissolved salts are the same way. When you dissolve the salt in the water, the positive and negative ions in the water allow electrons to flow much better than if you just had water by itself. Voila! Salt water conducts!  A question you might have is "Does electricity travel through salt crystals?" Nope. It doesn't. Because the ions are stuck in one place due to the structure of the crystal, the electricity doesn't move around very well.  Another good question: "Does water without salt in it conduct electricity?" The answer: Not very well. Water by itself is a lousy conductor. The reason that boneheads who put the hairdryer in the bathtub with them turn into human fritters is that when they wash themselves, all the crud on them gets dissolved in the water. Some of the crud is ionic, so when the dryer hits the water, they get zapped. An interesting "thought experiment" would be to wash all the salt off yourself and then drop a hairdryer in the bathtub with you. In theory, you would be fine. In real life, you'd still become a crispy critter because tap water by itself has ionic compounds dissolved in it anyway. Thus, my warning:

If you put a hairdryer in the bathtub with you under ANY conditions, you will fry yourself! 


How do we name ionic compounds?

For practice problems with complete solutions, click here.
Most ionic compounds (and any I would ever give on a test) have two word names. 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.

So, how do we name cations? If the cation is a main block element, the name of the cation will just be the name of the element. So, the Na+ ion is the "sodium" ion. Not too challenging. However, if the cation is a transition metal, what you need to do is to check out whether or not there is more than one possible charge for that element. For example, iron can have a charge of either +2 or +3.  As a result, you need to specify whether the cation has a +2 or a +3 charge. When you've done this, just put the number after the name of the element in Roman numerals. For example, the Fe+3 ion just has the name "iron (III)".

How about anions? If the anion has only one atom in it, then the name of the anion is the same as the name of the element EXCEPT the end of the element name is taken off and "-ide" is added to the end. Thus, oxygen becomes "oxide", sulfur becomes "sulfide", phosphorus is "phosphide", et cetera. If the anion has more than one atom, then we'd say that it's a "polyatomic ion", meaning (not surprisingly) that the anion has more than one atom. Look up the polyatomic ion in a table (or pull it out of your... uh... memory), and you've got the name. Thus, OH- is "hydroxide", SO42- is sulfate, et cetera.

Handy methods for naming compounds


Naming ionic compounds if you're given the formula

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. If you're one of my students, you need to have eight of the polyatomic ions memorized: hydroxide, nitrate, nitrite, sulfate, sulfite, carbonate, phosphate, ammonium. If you don't know the formulas, look 'em up.

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.

But... what if we find a mistake when we check our work?
In this case, you have to find another way to solve the problem. Take the example of FeS. If we solve the problem using the first three steps, we find that the formula should be iron (I) sulfide. However, if we check this work as we should in step four, we find that iron cannot have a charge of +1, only +2 or +3, and sulfur can only have a charge of -2. In a case like this, you need to find another way to solve the problem.

When this happens, look at the anion. In our example of FeS, the anion is the sulfide anion, S-2. If we have one sulfide ion, this means that the total negative charge in the molecule is -2. As a result, iron must have a charge of +2 to counterbalance the -2 charge of sulfur. Since iron has a charge of +2, the name of the compound is iron (II) sulfide.

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


IF the charges on the ions can be divided by the same number, then do it before you do step four. For example, if you were to find the formula for manganese (IV) oxide, you'd realize in step three that both manganese (IV) and oxygen have charges that can be divided by two. Instead of crossing the +4 for manganese and the -2 for oxygen, you'd simplify it so that you cross a +2 for manganese and a -1 for oxygen.

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


Other stuff I might have forgotten above

In no particular order, 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.

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