Naming Ionic Compounds

In my time as a teacher, probably the most common question people have for me is “What’s the deal with your beard?”  The next common question people have for me is “How do I name ionic compounds?”  We’ll explore the answer to one of these questions in this tutorial.

1-beard

But seriously, what’s up with the beard?


Before we name:  Is it ionic?

Before you can write the names of ionic compounds, it’s important that you make sure you’re actually dealing with an ionic compound.  After all, if you do a great job of naming your ionic compound but the compound is actually covalent, you still won’t get the answer right.

If you’ll recall the tutorial about ionic compounds, ionic compounds are compounds in which cations (ions with positive charge) are attracted to anions (ions with negative charge) in great big crystals.  If you’ve got only nonmetals, and if the ammonium ion isn’t the cation, you’re probably looking at a covalent compound.  Go check out the tutorial for those (links to the right of this page).


Writing the names of ionic compounds

If you’d like to learn to name ionic compounds, follow these steps.  If you follow these steps, you’ll get everything right and your family will love you.  If you don’t follow these steps, your life will descend into misery and bitterness until you eventually end up having a heart attack when you’re 45.

2-heart attack

“…and while we’ve got him on the operating table, it wouldn’t be a bad idea to check his titration gland, either.”

With that totally realistic scenario bouncing around in your head, let’s get started with your stepwise ticket to Namingburg:

1)  Ionic compound names consist of two words.  The first word is the name of the cation (the first ion) and the second word is the name of the anion (the second one).

First word:  naming the cation:

The cation is the first thing in the chemical formula.  Here’s how you figure out what word to write down to identify it:

  • If the cation is a metal, write down the name of the metal.
  • If the cation is the ammonium ion (NH4+), just write “ammonium” as the first word.

Using these, NaCl has the first word “sodium” and “Li2SO4” has the first word “lithium.” It’s simple!

Second word:  naming the anion:

The anion is the second thing in the chemical formula.  Here’s how you figure out what word to write down to identify it:

If the anion is just one element (as in the case of NaCl or CaF2), just write the name of that element with “-ide” at the end.  For example, the second word in NaCl would be “chloride” and the second word in CaF2 is “fluoride”.

If the anion has more than one element, then you’ve got yourself a polyatomic ion.  A polyatomic ion is just a fancy way of referring to an ion that has more than one atom in it, like the nitrate ion (NO3-) or the sulfate ion (SO4-2) ion.  In the case of polyatomic ions, just write down the names of the ions by consulting either a chart that tells you what they are or your brain (if your teacher is making you memorize them).  In case you don’t know what the polyatomic ions are, here’s a list of some common ones you might want to familiarize yourself with:

3-polyatomicions

This makes NaNO3 into “sodium nitrate” and PbCO3 into “lead carbonate.”  No biggie, right?

Step 2:  Are we done yet?

It’s possible that you’re finished, and that you can stop writing your formula and go have a congratulatory Fresca¹ soda in one of seven flavors.  Fresca:  It’s a taste and a feeling all wrapped up in one!²

4-fresca

Admit it:  Like all people, you love the fresh, crisp flavor of Fresca.

However, it may be the case that you need to do some more stuff before you’re finished naming the ionic compound.  Let’s find out if you’re one of those lucky folks:

  • Transition metal cations need Roman numerals.  Exceptions:  Zn, Ag, and Cd, which don’t require Roman numerals.
  • Main block element cations don’t need Roman numerals.  Exceptions:  Pb, Sn, and Bi, which do require Roman numerals.
  • Ammonium doesn’t need a Roman numeral.

What is a Roman numeral?  Move to Step 3 to find out…


Step 3:  Finishing the formula with a Roman numeral

If the rules above have told you that you need a Roman numeral, then congratulations! You’ve got a little more work ahead of you.  If they say that you did not need a Roman numeral, please stop reading.  Your formula is finished and adding a Roman numeral will actually make it wrong.

The Roman numerals in formulas indicate the amount of positive charge on a cation.  For example, the iron(II) ion has a charge of +2, and the copper(I) ion has a charge of +1.  The reason some cations need this and some do not is that some cations can have more than one possible charge.  For example, you can have copper with a charge of +1 or with a charge of +2, and it’s important that you be able to distinguish them.

An example: CuCl vs. CuCl2

The first compound, using the rules above, is called “copper chloride.”  The second compound, using the rules above, is also called “copper chloride.”  Though the compounds are clearly related to one another, it’s just as clear that they are not the same thing. As a result, we use Roman numerals to tell people which one we’re dealing with.  How do we find the right one?  I’m glad you asked…

How to find the Roman numeral:

Use the steps below to figure it out:

  • Find out how many cations you have.  This is the number written as a subscript below the formula of the cation.  For example, FeF2 has one iron atom, while Cu2O has two copper atoms.
  • Find out how many anions you have.  If your anion is an element, this is the number written as a subscript below its formula.  If your anion is a polyatomic ion, we can assume it’s always “1” unless we see parentheses around the anion formula – in this case, the number after the parentheses is the number of anions.  As a result, CuCl has one chlorine atom, CuCl2 has two chlorine atoms, CuNO3 has one nitrate ion, and Cu(NO3)2 has two nitrate ions.
  • Find out the charge on the anion.  If the anion is an element, just count on the periodic table to the next noble gas (for example, Cl is one away from Ar, giving it a charge of -1).  If it’s a polyatomic ion, you should either have the charge memorized or have a table on which you can look it up.
  • The grand finale:  The Roman numeral for this compound is found by using the numbers we found in this equation:

6-romannumeral

As an example, we would find that Fe(OH)2 has a Roman numeral of (2)(-1)/1 = -2. We’ll just leave off the negative sign to call it 2, or II.  As a result, Fe(OH)2 is iron(II) hydroxide.

Other examples to test your prowess:

  • CuCl = copper(I) chloride
  • MnO2 = manganese IV) oxide
  • Pb3(PO4)2 = lead(II) phosphate

Because Roman numerals sometimes give people problems, here are some tips you can use to determine if your Roman numeral is wrong.  I can’t say for sure without seeing the example if you’ve done it right, but these are good signs that you need to try again:

  • If your Roman numeral isn’t a whole number, it’s wrong.
  • If your Roman numeral is greater than seven (VII), it’s wrong.
  • If you wrote a Roman numeral and you didn’t have to, it’s wrong.
  • If the Roman numeral is 1 (I) and you left it off, it’s wrong.
  • If you don’t remember what the Roman numerals are for each number, don’t just write the number in regular numerals – you need to write them using the following numerals: I, II, III, IV, V, VI, VII.³

Anyway, that’s how to name compounds.  Because this whole tutorial took longer than I thought, I’ll include writing ionic formulas in the next one.


Footnotes:

1.  Fresca is a registered trademark of the Coca-Cola Company.  The Coca-Cola Company neither sponsors nor endorses this website or anything included herein.  It would be awesome if they did, but they don’t.

2.  The slogan “It’s a taste and a feeling all wrapped up in one!” is copyrighted by the Coca-Cola Company, who neither sponsors nor endorses this website or anything included herein.  I, on the other hand, would like to tell you that I think Fresca is an absolutely fantastic product.  I particularly like the Original Citrus flavor, though they’re all quite good.  Except peach.  I don’t care for that one.

3.  You can practice learning the Roman numerals by watching Super Bowl reruns or by getting a really old wristwatch.  Or by taking Latin, which has become inexplicably popular lately.


Photo credits:

  • Me and my beautiful beard:  My dad has no beautiful beard and is secretly jealous of my beard’s awesomeness.
  • X-Ray:  By Hellerhoff (Own work) [CC BY-SA 3.0], via Wikimedia Commons
  • Fresca:  17 USC Section 7 allows for fair use of commercial photographs, under certain, very limited circumstances.  Kids, please don’t assume that you can use any picture from the Internet under “fair use” – it doesn’t work that way.  That’s why your teachers keep bugging you about copyright and plagiarism.  For more information about fair use, visit this site.

The information on this page was written on December 19, 2014 by Ian Guch and is copyrighted according to U.S. law.  However, you may use these resources subject to the the Creative Commons Attribution-NonCommerical-ShareAlike 4.0 International license (CC BY-NC 4.0). For more information about this license and how it affects how you can use the contents of this site, visit http://creativecommons.org.

Please note, however, that this Creative Commons license does not apply to any works that are linked to on this webpage.  For information about using those works, please contact the copyright holder directly.

 

 

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