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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

Ah, now we're ready to put together ions to make compounds. Tackle this as if it were a combination of a math problem and a mystery. Let's just close our eyes and jump- it will make sense very soon.

Chemical Formula to Compound Name

The most difficult part is recognizing the ions. Let's begin with binary ionic compounds. Binary compounds are those with two monatomic ions. When you see a chemical formula of you will know that the first ion is the cation while the second ion is the anion. Additionally, with binary ionic compounds you also know that the first one will be the metal (metal make cations) and the second one will be the nonmetal (nonmetals make anions).

For example, NaI. You know that this compound has ions made from the element sodium and the element iodine. Sodium is a metal and is the cation. The sodium ion is Na¹⁺ and the iodide ion is I^1-. Just as the chemical formula is in order of metal/cation first and nonmetal/anion second, the compound's name goes in the same order. Cation name followed by the anion name with the words "ion" left off. Here's a few examples-

Look back at the table. See how the name has nothing to do with the number of cations or anions? Magnesium nitride has 3 cations and 2 anions but it's still named in the same manner as barium oxide which has 1 cation and 1 anion. Also, the charge of the cations and anions does not have any bearing on the compound's name. Compare potassium bromide to barium oxide to see what I mean. The ions in the 2 compounds have different charges but the naming is still done the same way. This is a very important point and we'll go over it in more depth when we discuss writing the compound's formula from the name.

Each of those examples is a binary ionic compound. Binary means two elements and not necessarily only two atoms. NaI, sodium iodide, has only two atoms in the formula while Mg₃N₂, magnesium nitride, has a total of 5 atoms in the formula. But they each only have two elements.

Polyatomic ionic compounds, those compounds with 3 or more different elements, are named the same. The difference is that you can't split up the polyatomic ion into corresponding elements which is a very common mistake done by students who don't drill much in this area. Let's take NaC₂H₃O₂. The first thing to do is to look at the chemical formula and realize that it has more than 2 different elements in it. Therefore, it's not a binary ionic compound but it is a polyatomic ionic compound. The ions in it are the sodium ion, Na¹⁺, and the acetate ion, C₂H₃O₂^1-. That's as far as you take the acetate ion. Here's another table with a few examples.

There are a number of flies in the ointment that we need to go over. The first is the use of parentheses. Just like in math, you use them when you need to group something together. Notice that for calcium hydroxide we wrote it as Ca(OH)₂ and this tells us we have 1 calcium ion and 2 hydroxide ions. CaOH₂ would be something completely different and have 1 calcium ion, 1 atom of oxygen, and 2 atoms of hydrogen. Not to mention that it's both confusing and wrong! For the compound ammonium nitrate, no parentheses were placed about the ammonium ion, NH₄¹⁺, because there's only one of them. A point of focus, remember that we haven't not discussed why we have the number of ions we have in each compound. Hang on a sec until we get there!

Notice that the endings for the polyatomic anions have not been modified. They are used just as they are written until the monatomic anions.

The last example shows how you sometimes need to do a little detective work. The first bump with that one should be when you recognize that manganese is a transition metal (and not one of the exceptions to the Stock System) and thus can have multiply-charged anions. The detective work comes into play when you work backwords from the anion to get the cation. The anion is the hydroxide ion which has a charge of 1-. But, the formula shows 2 of them so the total charge is 2-. Since the positive charge must balance the negative charge, the compound must have a total positive charge of 2+. With only one manganese(??) ion to worry about, it must have a charge of 2+. Therefore, the cation's name is manganese(II) ion (the "II" comes from the 2+ charge associated with the cation). And we have the compound's name- manganese(II) hydroxide.

Just like binary ionic compounds, the number of each of the ions plays no part in naming the compound. It's just cation followed by anion.

Compound Name to Chemical Formula

Getting the chemical formula from the name can be a bit tricky. This time we have to pay attention to the charges on the ions in the compound. The ionic compound has to be electrically neutral- just as much positive charge as negative charge. You need to keep your accounting straight on these! Additionally, we use the smallest whole number coefficients possible.

Let's start with the easiest examples and work our way up.

Sodium chloride. The first thing to do is to identify the cation and the anion. We've got the sodium ion, Na¹⁺, for the cation and the chloride ion, Cl^1- for the anion. The next thing to do is to see what it would take to balance the positive charge with the negative charge with the least number of ions. The sodium ion has a charge of 1+ and the chloride ion has a charge of 1-. In this case, one cation would exactly balance with 1 anion. Therefore, the chemical formula would be NaCl. Although 2 cations would also balance electrically with 2 anions (total positive charge of 2+ and a total negative charge of 2-), writing the compound's formula as Na₂Cl₂ would be wrong. We need to balance the charges with the least number of ions.

Sodium chloride is far more complex than the formula NaCl suggests. It's a huge three dimensional lattice of cations and anions arranged in what's called a face-centered cubic arrangement. In case you just started sweating, this is an advanced topic but I'm simply mentioning it right now. If you want more information, search in the glossary of your text. Here's a two-dimensional slice with the chloride ions depicted as green spheres and the sodium ions depicted as magenta spheres. The face-centered arrangement results in a net of one sodium ion relative to one chloride ion-

Sodium oxide. There's the sodium ion, Na¹⁺, and the oxide ion, O^2-. The sodium ion has a charge of 1+ and the oxide ion has a charge of 2-. Now we can't just put one cation with one anion since the total charge wouldn't be neutral. However, if we were to take 2 of the cations, we'd have a total positive charge of 2+ and this would balance with the total negative charge due to the anion. The chemical formula for sodium oxide is Na₂O.

Calcium bromide. We have a calcium ion, Ca²⁺, and a bromide ion, Br¹⁺. This one is very similar to the sodium oxide example except the cation has a charge of 2+ and the anion has a charge of 1-. So, we need 2 of the anions to charge balance with the cation. Thus, the chemical formula is CaBr₂.

Barium phosphide. The barium ion is Ba²⁺. The phosphide ion is P^3-. This example isn't as simple as taking one or two of each of the cations and the anions. Now we see why you need to be somewhat careful and also why you need to do some drilling to get comfortable writing chemical compounds. A different perspective is to look at it only as a math problem. If you need to take some multiple of 2+, add it to some multiple of 3-, and come up with 0, how would you do it? You'd take the lowest common multiple of 2 and 3 which is 6. So you need 3 cations (total charge of 6+) and 3 anions (total charge of 6-) to electrically balance everything. Or, Ba₃P₂.

The ionic compounds with polyatomic ions are done the same way. Let's take two examples and then I'll turn you loose to practice on the quizzes.

Sodium carbonate. Again, the sodium ion is Na¹⁺. This time, we have the carbonate ion, CO₃^2-. We have 1+ due to the cation and 2- due to the anion and so we need 2 sodium ions with 1 carbonate ion. Or, Na₂CO₃. Notice that we didn't use parentheses because we only need one carbonate ion. Parentheses aren't needed for the sodium ion because it's monatomic and just the subscript will do.

Mercury(I) phosphate. Bells should go off with this one not only because of the transition metal cation but because of the mercury(I) cation. Remember, it's the "goofy" one- Hg₂²⁺. If you have trouble with it, memorize it! The mercury(I) ion actually has a total charge of 2+ since it's a polyatomic ion. The phosphate ion (not phosphide!) is PO₄^3- and has a total charge of 3-. What's the lowest common multiple of 2 and 3? Yep. It's 6. To get 6+ we need 3 of the mercury(I) ions and to get 6- we need 2 of the phosphate ions. Thus, (Hg₂)₃(PO₄)₂. Yikes! Parentheses galore in that one...

I think at this point it might be best to let you work on quizzes 7 through 12. These 6 quizzes will give you ionic compounds and formulas of any combination of ions (monatomic and polyatomic).