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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

What if you were asked to go to the chemical stockroom and grab a bottle of chromium bromide? The first thing you might think is that we haven't talked about chromium. Also, if all we've talked about was how to name ions and not compounds, you might be a touch confused. However, you're an enterprising student with a ton of motivation. And, it certainly wouldn't hurt if I were to tell you that this project was a group exam and you've been picked by the entire class to either pass or fail the exam.

So, you look at the name- "chromium bromide". Your first assumption might be that it's probably made up of ions (good one since chromium is a metal!). Your second one would be to look for a bottle that has chromium and bromine (from the "bromide" in the name). And off you go!

Luckily, the stockroom is impeccably organized and so it takes you only a short period of time to find what appears to be the compound. However, you glance at the label and see that it says "chromium(II) bromide". Since you still have some time to burn, you look back on the shelf and make a horrifying discovery... there's also a chromium(III) bromide! What to do? The first is a white crystalline form while the second is a black crystalline form and they both have compl;etely different uses. One is used for chromizing while the other is used for separating olefins.

Confused, you start to ask what to do but your instructor (that would be me) cackles with glee and tells you to make your one-and-only pick. And make it snappy! With the weight of your classmates' faith on your shoulders, you close your eyes, grab one of the two, and offer it up as the correct choice.

And what was the result? Did the class pass or were you doomed to spending the rest of the semester as an exile among your peers?

That's beside the point since it wasn't a fair question to begin with.

There was a reason for initially neglecting the transition metals (including lead and tin). Many of the elements in these areas can form ions with different charges. Lead can form ions with a 2+ or a 4+ charge. Copper can form ions with a 1+ or 2+ charge. And more. "Why" is too detailed for the moment. We'll get to that when you go into more detail about electronic configurations. The main thing to know is that you'll need to be careful when dealing with cations made from elements other than those from group IA, group IIA, group IIIB, or the metals of group IIIA. With ions made from these groups there's no doubt as to their charge. So, we need a way to indicate the charge when we write the name so there's no doubt.

And that's what we call the "Stock System". Roman numerals are used to indicate the charge on many ions made from a transition or post-transition metal. 1+ gets a I. 2+ gets a II. 3+ gets a III. 4+ gets a IV. 5+ gets a V. And so on. You will not be asked (well, at least you won't in my class!) to guess what the possible charges are on the ion. Let's take a look at the Periodic Table again to make sure you understand which area we're talking about.

Here are the transition metals.

There are so many to worry about that if it will make you feel any better, I'll list the ions that will be on the online quizzes and that you'll be responsible for. Believe me when I say that you won't need to memorize them when it comes to naming ions, writing symbols from the name, and writing ionic compounds in an upcoming section. Trust me for right now!

Even though lead and tin are not transition metals, you need to use the Stock system with them. The explanation involves a discussion of d electrons and we're not ready for that.

And... oops. I fibbed a little. Here's where we run into the first of our exceptions. Look at the last two rows of the above table. Specifically, the cadmium, silver, and zinc ions. We don't use the Stock System for naming them because there is only one common cation for each and therefore no doubt as to the charge. Remember, the reason for using the Stock System was to eliminate doubt as to the charge. "Get me the lead(II) ion" has no doubt whereas "get me the lead ion" has doubt. Those three previously named ions are ones that you'll simply have to commit to memory.

There's one other exception which usually is the one that many students have a tough time handling. The mercury(I) ion actually exists as a polyatomic ion ("many atoms") and not as a single ion. It's incorrect to write the mercury(I) ion as Hg¹⁺. Since it is believed that it is a mercury atom paired with a mercury(II) ion, together they have a total charge of 2+. That's why it's written Hg₂²⁺.

There's another way of handling these ionic compounds with differently charged ions made from the same element and that's called the "common" naming system (names like cuprous, cupric, ferrous, and ferric). I'm not going to go into any detail because I don't require my students to memorize it. However, it's still in wide use and if you get a job working in a chemical storeroom you'll need to become familiar with it. I have chosen to stick with the IUPAC naming system.

Now would be a good time to jump into the quizzes and run through quizzes 3-6. Remember, all we've worked on so far is how to name ions and how to write the symbols given the name of the ions. We have done nothing about naming compounds. There's one more section to go until we get to naming compounds.