Learning to Name and Write Formulas:

- Determining the type of compound you are working with.

- Applying the rules for naming or formula writing for that type of compound.

- Practicing until it becomes second nature.

How to Write Formulas for Ionic Compounds with Polyatomic Ions

  • Write the element symbol for the metal and its charge using Periodic Table.
  • Find the name and charge of the polyatomic ion from the list of polyatomic ions.
  • 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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When writing formulas for compounds that have Polyatomic ions, you need to use a list of common ions. This list is usually given to you by your instructor. Ask which of the Polyatomic ions you need to memorize and if you'll be able to use the list of the ions on an exam.

Remember also, when writing formulas for ionic compounds with Polyatomic ions it is necessary to balance the charges between the metal and a Polyatomic ion.

For help memorizing polyatomic ions watch this video.