Ionic Formulas by Switching Charges

The "other way" to write ionic formulas involves switching charges. Let's look at a previous example to see how that works.
Barium Chloride
First we'll translate that into symbols and determine the charges;

Next we "cross" the charges, so that the charge on the positive ion becomes the subscript for the second ion and vice versa.

Of course, we don't need to write a 1 in a formula, so that simplifies down to BaCl₂. Note: we do NOT move the charge (+ or -) only the number.
 
Let's look at another example: copper II phosphate

Translating that into symbols with charges gives us

Crossing the charges...gives...

Note the need for the parentheses around phosphate to preserve the polyatomic ion's identity.

An example that comes with a word of warning: barium oxide

translated...

Crossing charges...gives...

Unfortunately, that's WRONG! Remember that our goal was to find a formula that would have a charge of 0. That means that ionic compounds are always written in the simplest formula (the easiest way to get a 0 charge). That means that the correct formula is BaO, not Ba₂O₂.

SO, if you are using this method (of crossing charges) you MUST simplify your formula at the end if possible.