# 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:

Here are some examples:

sodium chloride

• Na and Cl
• Na+1 and Cl-1
• (+1) + (-1) = 0
• so, NaCl

barium chloride

• Ba and Cl
• Ba+2 and Cl-1
• (+2) + (-1) = 1 (not 0) so... try 2 chlorines
• (+2) + 2(-1) = 0
• so, BaCl2

aluminum sulfide

• 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

ammonium oxide

• 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"

Try these to make sure that you understand. The answers are here.