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Ternary Ionic Compounds: Formula Writing

When we have a metal and a non-metal we have an ionic compound.

Metal + Non-Metal = Ionic Compound

If we have a metal and a polyatomic ion we have a ternary ionic compound.

For example,
write the formula for:
  • Sodium nitrate
  • Calcium carbonate
  • Aluminum hydroxide

Practice: The key to success.

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Keys for Writing Formulas for Ternary Ionic Compounds:

  • Write the element symbol for the metal and its charge using Periodic Table.
  • Find the name and charge of the polyatomic ion on the Common Ion Table.
  • See if the charges are balanced (if they are you're done!)
  • Add subscripts (if necessary) so the charge for the entire compound is zero.
  • Use the crisscross method to check your work.


  • Don’t write the subscript '1'.
  • If you use the criss-cross method and end up with something like Ca2S2 you'll need to reduce the subscripts to Ca1S1 which we write CaS.

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