Writing Formulas for Ionic Compounds
The idea behind writing formulas for ionic compounds is relatively simple, although the application can get a little confusing at times.
The basic rule is that you are trying to combine ions in such a way as to end up with a total charge of zero. For example, if I wanted to make a compound out of sodium ions (Na+1) and chloride (Cl-1) I would only need one of each (+ 1 and -1 make 0). This gives the formula NaCl. If I wanted to make a compound out of sodium ions (Na+1) and sulfide (S-2), using one of each would not work (+1 and -2 make -1 not 0). So, I would need 2 sodium ions to match the sulfide, leading to the formula Na2S.
The basic process then is to:
- translate the name of the compound into symbols (that means you need to understand roman numerals and latin prefixes and the polyatomic ions)
- assign a charge to each ion
- combine the ions so that the total charge =0
Here are some examples:
- Na and Cl
- Na+1 and Cl-1
- (+1) + (-1) = 0
- so, NaCl
- Ba and Cl
- Ba+2 and Cl-1
- (+2) + (-1) = 1 (not 0) so... try 2 chlorines
- (+2) + 2(-1) = 0
- so, BaCl2
- Al and S
- Al+3 and S-2
- (+3) + (-2) = 1 (not 0)...least common multiple = 6, so try 2 aluminum ions and 3 sulfides
- 2(+3) + 3(-2) = 0, so Al2S3
- NH4 and O
- NH4+1 and O-2
- (+1) + (-2) = -1 (not 0)...try two ammonium ions
- 2(+1) + (-2) = 0, so (NH4)2O
NOTE: polyatomic ion formulas cannot be changed and they are kept "pure" by enclosing in parentheses when the subscript is more than 1.
If this process works for you...great. Don't look any farther. However, there is another way to write these formulas. It is quick and easy, but can also lead to mistakes if you aren't careful. If you're curious, here is the "other way"