Naming Compounds (Section 5)
This chapter is on naming Covalent and Ionic compounds. If I had to pick one individual concept that I struggled with in my first quarter of college chemistry, naming compounds would get the award. There are so many terms you need to memorize and so many exceptions that I almost went crazy. I'll do my best to keep this as simple as possible, but it really is fairly complicated.
Before I kick off how to name Covalent and Ionic compounds, It'd be a good idea to define them.
Covalent Compound - Consists of atoms bounded together which joined together by electron pairs. They are generally Metal bonded to another Metal.
Ionic Compound - Consists of Cations (positive) and Anions (negative).
They are generally Nonmetal bonded to a Metal.
So when we talk about naming an Ionic Compound, in general we can assume we are talking about a Nonmetal bonded to a Metal. If you look at your periodic table you will see it states which elements are metals and nonmetals. You will also notice that nonmetals make up only a very small portion of the periodic table. Surprisingly however, there are many covalent compounds.
We're going to first talk about naming binary covalent compounds.
First to name covalent compounds we need to know some Greek numerical prefixes. It'd be a good idea to memorize these since they will be useful in many other classes.
| Number | Prefix |
| 1 | -mono |
| 2 | -di |
| 3 | -tri |
| 4 | -tetra |
| 5 | -penta |
| 6 | -hexa |
| 7 | -hepta |
| 8 | -octa |
| 9 | -nona |
| 10 | -deca |
Naming Binary covalent compounds:
There are a few simple rules for naming Binary covalent compounds. A Binary covalent compound is something like PCl5. These are the rules:
1. The element to the farthest left (lowest group number) on the periodic table comes first, followed by the higher group number (farther right).
2. Once the first term has been decided, if the first term has only one atom of the element in the compound, it gets named it's elemental name. (N for example gets named nitrogen)
3. If the first term has more than one atom, it gets the Greek prefix representing the number put in front of the elemental name. (N2 for example, if it was the first term, would get the name dinitrogen)
4. The second term always gets the elemental name with the suffixe "ide".
5. If the second term ALWAYS gets a Greek prefix in front of the elemental name. Even if it only has one atom.
Let's try naming a few and see how you do -
| Problem 5.1 |
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| Problem: Name the binary covalent compound P4O6
Plan: We will check the periodic table to ensure it's a binary covalent compound, which it is, and then use the process listed above to name the compound. Since the first term has four atoms, it just gets named tetra + its elemental name, phosphorus. The second term always gets a Greek prefix so it gets named hexoxide. Answer: tetraphosphorus hexoxide Discussion: You will notice that the greek prefix on the oxygen got cut short to hexoxide instead of hexaoxide. This is because they both end in vowels so the "o" stays and the "a" gets removed. See what I mean by exceptions? |
That's how you name binary ionic compounds. Now let's get into the more challenging part. Naming covalent compounds
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| Problem 5.2 |
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| Problem: Dinitrogen tetroxide is used as a rocket propellent. What is the molecular formula for Dinitrogen tetroxide?
Plan: This is similar to the problem above, only in reverse form. We are given the name and need to predict the formula. Since the Di prefix is in front of nitrogen, we know it has a two subscript. Since the oxide has a tetra prefix, we know it has 4. Answer: N2O4 Discussion: You'll notice that again the tetra prefix was shortened. This is because tetraoxide is a bit cumbersome to type and pronounce, therefore scientists have decided to shorten tetra with oxide to tetroxide. |
As like anything in chemistry there are more exceptions! There are a few binary covalent compounds that don't get named like the above process. Most of these you already know or have at least heard of. These are the few exceptions you should know. Just memorize these.
Water (H2O)
Ammonia (NH3)
Methane (CH4)
Ethane (C2H6)
THESE DO NOT GET NAMED BY THE BINARY COVALENT FORM ABOVE! We don't call water dihydrogen monoxide. It's simply water.
Well that's about all we can cover on naming binary covalent compounds. Click the link below to go to the next page which talks about naming Ionic Compounds.
| NEXT PAGE! Naming Ionic Compounds |
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