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Jump to one of the following inorganic nomenclature parts:   Choose One Introduction to Inorganic Nomenclature Part 1- Molecular Compounds Part 2- Monatomic Ions Part 3- The Stock System Part 4- Polyatomic Ions Part 5- Ionic Compounds Part 6- Binary Acids and Oxoacids Part 7- Quizzes

A molecular compound is made up of a nonmetal and a nonmetal. We can also use metalloids (also called semimetals) so to make things easier on me, when I say "nonmetal" I'm also talking about metalloids. I'm going to treat germanium and antimony as nonmetals for the purpose of naming compounds. Let's take a look at the Periodic Table to see which portion of the Table we're working with-

Here are the nonmetals.

We'll only be talking about binary molecular compounds right now so we will only have 2 nonmetals bonded together. The uncertainty comes when determining how many of each of the two nonmetals we have in the molecular compound. With molecular compounds the only way to know how many of each element is to use the following Greek prefixes-

With N₂O₄, we have 2 nitrogens which is "di" and 4 oxygens which is "tetra". When we name the compound, the naming is done in the same order that the elements appear in the formula along with the modifications for the prefix. The second element in the formula needs a modified ending. Let's look at the a number of the possibilities-

Note that we usually drop the last 3 letters as in fluorine to fluoride, but this depends. Sometimes it's 2 letters- sulfur to sulfide. And other times it's 4 letters- Nitrogen to nitride.

A reminder- I'm choosing to treat metalloids like nonmetals. Not that this works all the time but it's not a bad approximation. The prefix is directly attached to the element name. N₂O₄ is named dinitrogen tetroxide (I know, it's also written tetraoxide... I'll get to that... keep your shirt on!).

What about CO₂. Using the rules we set up suggests that the name of it is monocarbon dioxide.

"Wait a minute! That can't be right! I don't remember seeing it written that way!"

I bet you just yelled that, right? OK. You're correct... basically. It's not that it's incorrect to call it monocarbon dioxide. It's just that we call it carbon dioxide. By convention, if we only need 1 of the first element in the formula, we drop the "mono" prefix. You can't drop the "mono" if it belongs in the second element, however. CO is carbon monoxide and not carbon oxide.

And this leads right into a number of other naming conventions. One was illustrated above with "mono" on the first element. The second deals with situations when you've got certain vowels slammed together. If a prefix ends in "o" and the element it precedes begins with an "o", we drop one of them. As in monoxide and not monooxide. And, if a prefix ends in an "a" and the element it precedes begins with an "o", we drop the "a". As in heptoxide and not heptaoxide. If you're thinking ahead, we keep things like triiodide and tetraiodide. These are not absolute rules but this is how I'm choosing to deal with 'em.

We've already dealt with the third one and that's how we modify the ending of the second element in the formula.

The fourth naming convention has to do with binary hydrogen compounds. H₂S could be named dihydrogen sulfide but it's generally not. This is actually named hydrogen sulfide and it's understood that there are 2 hydrogens. How do you know whether there are 1, 2, or 3 hydrogens? If it's a group VIIA binary hydrogen compound then there's only 1 hydrogen. Example- HF or hydrogen fluoride. If it's a group VIA binary hydrogen compound then there are 2 hydrogens as in the hydrogen sulfide compound. There are 2 additional exceptions and this has to do with what's called common names. A "comon name" is one that is generally accepted even though it doesn't follow current naming conventions. While you could call H₂O hydrogen oxide, it's far more likely that you'd call it water. I suppose you could call H₃N hydrogen nitride but I doubt you've ever seen it written that way. It's more likely that you've seen it written as NH₃ and called it ammonia. There's a reason you don't write ammonia with leading hydrogens but we'll save the concept of molecular bases for a later time.

Let's look at a some more examples (name to formula)-

And let's try a couple more examples (formula to name) before letting you scamper away to try some quizzes.

If you are wondering how you decide which one to put first in the compound and the name, don't worry about it. I don't require you to know how to predict the order. Some instructors require this and I'll refer you to your notes and/or your textbook if this is required. You'll know the order based on what you're given. If you are given the name, the order in the formula is the same as the order in the name. The same goes if you need to write the name when given the formula. No guesswork required!

The quizzes you should try are quizzes 1 and 2.