Naming Ionic Compounds
Yesterday we talked about how to use the crossing over method to figure out the formulas for ionic compounds,
and today we’re going to start to talk about what the names of these formulas are.
What happens to electrons in an ionic compound?
They are transferred
What types of atoms are they usually being transferred from and to?
From a metal to a nonmetal
So, ionic compounds form between a metal and a nonmetal. Which one is the positive ion?
The metal
That leaves the nonmetal to be the negative ion.
There are two parts to an ionic compound, and there are two parts to its name. Here are the two steps to follow
to name a binary ionic compound:
1. The metal has the same name as its element
2. The nonmetal has the same name as its element, except the suffix has been replaced with an -ide
Once we do some examples this should seem pretty easy.
Let’s start with NaCl. What two elements are in this compound?
Sodium and chlorine
Which one is the metal?
Sodium
That leaves the chlorine for the nonmetal.
So, let’s follow the rules to name this:
1. Sodium stays the same
2. Chlor-ine becomes chlor-ide
So, what’s the name of this compound?
Sodium chloride
Try these next few in your notebook:
MgO, KF, CaBr2, BeS
Can you do it the other way around? Look on the periodic table for the charges of the ions and use them to write
the formulas for these ionic compounds in your notebooks:
Sodium iodide, hydrogen chloride, silver fluoride
Those were specifically for binary ionic compounds. Remember those polyatomic ions? Well, they can form
ionic compounds with metals also, but naming them is easy. Look at the table and tell me what you think the
name of this compound is: AgNO3
Silver nitrate
So, to name an ionic compound with a polyatomic ion in it, you still just keep the name of the metal, and then
you just add the name of the polyatomic ion that you’re given on the table after it. Try these:
Mg(OH)2, CaS2O3
The trick here is just to know to suspect a polyatomic ion if there are more than two elements in the ionic
compound.
Let’s practice naming and formula writing.
For each of the following, write the name and draw the dot diagram of the compound: MgS, SrCl2, Li2O
Write the formula for each of these compounds:
Hydrogen cyanide, Potassium chloride, Barium sulfate, Calcium fluoride
The Stock System
Let’s review what we did yesterday: Try to write the formulas of these from their names:
Sodium cyanide, calcium carbonate, copper nitrite
What’s the problem with copper?
It has more than one oxidation state
What’s one possible formula for copper nitrite?
What charge does the copper have there?
CuNO2
+1
What formula could copper nitrite have if the copper has a +
2 charge?
Cu(NO2)2
Now there is just one more thing that you need to know about naming ionic compounds. You’ve seen that some
elements can be polyvalent – they can have more than one possible charge. This is true of most of the transition
metals – the elements in the middle of the periodic table. For elements like those the type of name that we’ve
been giving to the compound is not descriptive enough. The name copper nitrite could mean either CuNO2 or
Cu(NO2)2 so there is one more thing that we have to add in order to say which one we mean. If you are naming
a compound with a polyvalent ion in it you must use the stock system. This system is the same as what we had
been doing by keeping the name of the metal and adding an –ide on the end of the nonmetal, but now we’re
going to add in a Roman numeral after the metal to show which oxidation state that it has. So, for example,
instead of just calling Cu(NO2)2 copper nitrite we can call it copper (II) nitrite to show that we’re using the +2
oxidation state of the copper, which would need 2 nitrites to go with it.
Let’s try some examples. What are the names of two compounds that could be formed by iron and chlorine?
Iron (II) chloride and iron (III) chloride.
Knowing that the oxidation numbers have to add up to zero, we can figure out the formula for iron (II) chloride
because we’re told what the charge on the iron is. If iron is +2, how many chlorines do we need to go with it to
make the whole compound neutral? 2
So how do you write the formula for iron (II) chloride? FeCl2
What is the formula for iron (III) chloride? FeCl3
Here is a worksheet for practice (Do “The Stock System” worksheet)