How to Write the Formula for Ionic Compounds with Polyatomic Ions

In this video we'll write the correct formulas for ionic compounds with polyatomic ions (ionic compounds with three different elements). These compounds have polyatomic ions and we’ll use the Common Ion Table (a list of polyatomic ions) to help us write the name when given the formula.

Printable list of common polyatomic ions:

https://www.breslyn.org/chemistry/nam...

To write the formula for ionic compounds with polyatomic ions we’ll use the Periodic Table and the Common Ion Table and follow some simple rules.

--Keys for Writing Formulas for Ternary Ionic Compounds --

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

Notes:
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.

Note: It is possible to have two polyatomic ions such as NH4NO3. In this case find and write both names as found on the Common Ion Table.

For a complete tutorial on naming and formula writing for compounds visit:

http://www.breslyn.org/chemistry/naming

Drawing/writing done in InkScape. Screen capture done with Camtasia Studio 4.0. Done on a Dell Dimension laptop computer with a Wacom digital tablet (Bamboo).