Naming Salts (Ionic Compounds)

Salts are ionic compounds which, when dissolved in water, break up completely into ions. They arise by the reaction of acids with bases, and they always contain either a metal cation or a cation derived from ammonium (NH₄⁺).

Examples of salts include NaCl, NH₄F, MgCO₃, and Fe₂(HPO₄)₃.

Salts are named by listing the names of their component ions, cation first, then anion. This involves three distinct steps.

Step 1: Split the Formula in Two

Start by making a vertical slice through the formula just after the metal or ammonium:

NaCl	Na|Cl
NH4F	NH4|F
MgCO3	Mg|CO3
Fe2(HPO4)3	Fe2|(HPO4)3

Step 2: Determine the Charges On the Ions

Determine the ions and their charges on each half. This is definitely the tricky part. Seven rules here are helpful:

• Rule 1: Group 1 metals (Li – Fr) are all 1+

• Rule 2: Group 2 metals (Be – Ra) are all 2+

• Rule 3: Aluminum is 3+; Ammonium is 1+

• Rule 4: All other metals require a Roman numeral

• Rule 5: Group 7 nonmetals (F – I) are all 1–

• Rule 6: Group 6 nonmetals (O – Te) AS ANIONS are usually 2–

• Rule 7: The overall charge must be 0

For example:

NaCl	Na|Cl	Na+|Cl–
NH4F	NH4|F	NH4+|F–
MgCO3	Mg|CO3	Mg2+|CO32–
Fe2(HPO4)3	Fe2|(HPO4)3	Fe3+|HPO42–

Step 3: Name the Ions

Then name those ions:

NaCl	Na+|Cl–	sodium chloride
NH4F	NH4+|F–	ammonium fluoride
MgCO3	Mg2+|CO32–	magnesium carbonate
Fe2(HPO4)3	Fe3+|HPO42–	iron(III) hydrogen phosphate

Those ions, by the way, are called the principal species in solution for the salt. Figuring out the principal species in solution just this way gets to be REALLY important when you study equilibrium. You'll need to know those charges too, so you might as well learn them now and get it over with.

Tips for Success

A few more tips may be helpful:

• There's no way around memorizing element names. Just do it.

• Rule 7 is far more valuable than most beginners realize.

  • Can't remember or figure out the charge on the cation? Figure it out for the anion and make it all add up to 0.

  • Can't figure out the charge on the anion? Any chance Rule 1, 2, or 3 applies? If so, figure out the anion charge from the cation charge using Rule 7.

• Stuck because you have a transition metal, such as Fe or Mn, and can't remember the charge on the anion? Look around for other examples of the anion being used. For example, say you have to name FeSO₄ and you can't remember the charge on SO₄. If you find "Na₂SO₄" somewhere else on the exam, quiz, or in the book, you're home free. With this information you'll know that SO₄ must be 2–, and therefore the charge on Fe must be 2+.

• If you know your strong acids, then you know "H₂SO₄." H here is H⁺, and the overall charge is 0. So SO₄ must be 2–. Similarly,

  • HNO₃ gives NO₃^–,

  • HClO₃ gives ClO₃^–, and

  • HClO₄ gives ClO₄^–.

• This works with weak acids, too, if you can remember them, such as H₂CO₃ and H₃PO₄.

• Learn lots of acid names, because they help here.

  • X-ic acids give X-ate anions (sulfuric/sulfate, nitric/nitrate)

  • X-ous acids give X-ite anions (nitrous/nitrite)

Summary

The key to remember is that the system is designed to be unambiguous. We must be able to get one and only one formula from a name, and that name should be a standard one rather than some cutesy name like nutrasweet.

In summary, memorize the more common element names and symbols, memorize the seven rules, have a periodic table handy, learn lots of acid names and formulas, and practice, practice, practice!

YOU CAN DO IT!