Naming Covalent Compounds

Naming covalent (molecular) compounds is simply a matter of writing

how-many-what how-many-what"ide"

That may not sound simple, but it's actually a very simple idea. You tell how many atoms of the first element and what it is, then how many atoms of the second element are present, what it is, and then put the "ide" ending on.

A Simple Example:

P₂O₅ is called diphosphorus pentoxide.

If we pick that apart we can see that it first tells us that there are 2 (di) of something and then tells us that it is phosphorus (P). Next we have 5 (pent) of something, then we learn that it is oxygen, and we end with "ide."

Of course to do this, you need to know the number prefixes...

1	mono	6	hexa
2	di	7	hepta
3	tri	8	octa
4	tetra	9	nona
5	penta	10	deca

Here are a few more examples:

• N₂O₃ - dinitrogen trioxide
• As₄O₁₀ - tetrarsenic decoxide
• S₂Cl₂ - disulfur dichloride

Dealing with Vowels

You may have noticed in the second example above that some vowels are missing. If you follow the system described above blindly, you would have gotten tetraarsenic decaoxide.

It was decided that too many vowels make the names harder to pronounce and a general rule was developed. If an element starts with an A or an O, then you can drop the A or O at the end of the number word.

Notice that we NEVER drop a letter if an I is involved. That means that we shorten hepta-arsenic to heptarsenic, but tri-iodide is written as triiodide.

Single Atoms

If there is only one of the first element in a compound, you may drop the "mono" from the name. However we NEVER drop the mono for the second element. That means that CO₂ is carbon dioxide (not monocarbon dioxide) but that CO is carbon monoxide (not carbon oxide).

Your Turn
To make sure you understand this, try these on your own. The answers are here.

1. BrO₃
2. BN
3. N₂O₃
4. NI₃
5. SF₆
6. XeF₄
7. PCl₃
8. PCl₅
9. ICl₂
10. SO₂
11. P₄O₁₀
12. OF₂
13. ClO₂
14. SiO₂
15. BF₃
16. N₂S₅
17. SO₃
18. KrF₆
19. BrCl₅
20. SCl₄
21. XeO₃