How to Write the Formula for Mercury (II) iodide
In this video we'll write the correct formula for Mercury (II) iodide (HgI2).
To write the formula for Mercury (II) iodide we’ll use the Periodic Table, a Common Ion Table (see • Figuring out the Charge on Ions ), and follow some simple rules.
Because Mercury (II) 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 • Figuring out the Charge on Ions ).
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: • Figuring out the Charge on Ions
Memorizing Polyatomic Ions: • Memorizing and Using the List of Polyatomi...
Criss-Cross Method: • The Criss-Cross Method for Writing Chemica...
For a complete tutorial on naming and formula writing for compounds, like Mercury (II) iodide and more, visit:
http://www.breslyn.org/chemistry/naming
Drawing/writing done in InkScape. Screen capture done with Camtasia Studio 8.0