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 Ions

Writing chemical formulas for ions is relatively straightforward. Since we are dealing with ions here you are going to have to be able to determine the ionic charge in order to write the formula. Be careful with ions that have transition metals as well, the Roman Numerals will tell you the ionic charge. Finally, if you have a polyatomic ion all you need to do is look the chemical formula up on a table of ions.

The video below described how to write chemical formulas for ions.

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Single Element Ions
(e.g. Potassium ion, Magnesium ion, Fluoride ion)

  • Find and write the element symbol using the Periodic Table.
  • Determine the ionic charge for the element.
  • Write the charge as a superscript above and to the right of the element symbol.

For Single Element Ions of Transition Metals
(e.g. Iron (II) ion, Copper (I) ion, Cobalt (II) ion)

  • Write the element symbol.
  • Write ionic charge based on the Roman Numeral in the name. For example, the Iron (II) ion would be Fe2+.

For Polyatomic Ions
(e.g. sulfate ion,  phosphite ion  ammonium ion)

  • Polyatomic ions are made up of two or more elements (they will be non-metals).
  • Most often polyatomic ions will have a negative charge but there a few with positive charges.
  • Either memorize the formulas of look them up on a Common Ion Table.