In this video we'll write the correct formula for Zinc iodide (ZnI2).

To write the formula for Zinc iodide we’ll use the Periodic Table, a Common Ion Table (see https://youtu.be/N4N1Njh7nCo), and follow some simple rules.

Because Zinc iodide has a polyatomic ion (the group of non-metals after the metal) we'll need to use a table of names for common polyatomic ions, in addition to the Periodic Table.

---Keys for Writing Formulas for Compounds with Transition Metals---

1. Write the symbol and charge for the transition metal. The charge is the Roman Numeral in parentheses.
2. Write the symbol and charge for the non-metal. If you have a polyatomic ion, use the Common Ion table to find the formula and charge (see https://youtu.be/N4N1Njh7nCo).
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.
- 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.
---Formula Writing Resources---

Finding Ionic Charge: https://youtu.be/N4N1Njh7nCo
Memorizing Polyatomic Ions: https://youtu.be/vepxhM_bZqk
Criss-Cross Method: https://youtu.be/VnzIqpdEims
For a complete tutorial on naming and formula writing for compounds, like Zinc iodide and more, visit:

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

Drawing/writing done in InkScape (http://www.InkScape.org). Screen capture done with Camtasia Studio 8.0