MHS Chemistry
Nomenclature Flow Chart 02
Writing Formulas

These seven steps will allow you to write the proper formula for thousands of simple compounds.  Each step is shown for three examples.

    iron (III) sulfide barium chloride ammonium nitride
1. Memorize the polyatomic ions. OK OK OK
2. Write down the formula for the first part of the name. Fe Ba NH41+
3. If there is a roman numeral in parentheses, it's the positive charge on the first part. Fe3+ Ba NH41+
4. If there isn't a roman numeral, use the charge/oxidation state specified on the periodic table. Fe3+ Ba2+ NH41+
5. Write down the formula for the second part of the name, including the negative charge. Fe3+   S2– Ba2+   Cl1– NH41+   N3–
6. Since the net charge is zero, all the positives and negatives need to cancel.  Choose subscripts by finding the least common multiple of the charges.  Write them in place, and leave out all charges.

[LCM = 6]


[LCM = 2]


[LCM = 3]


(eww! 43?!)

7. If you need more than one of a polyatomic ion, put the ion in parentheses.  See how this clears up some confusion?     (NH4)3N

That wasn't so bad, was it?

[Names from Formulas][Nomenclature Note Sheet][MHS Chem page]