# Question #bf0f1

Mar 5, 2015

Remember that in ionic formulas, the cation, which is another name for positive ion, is written first, and the anion, or negative ion, follows.

If you apply this rule to potassium oxide, the cation will be $\text{K}$ and the anion will be $\text{O}$. Now for the charges, since ions are charged particles.

In the case of ionic compounds, the subscripts are written using the cross rule, which means you can work backwards to determine the charges on your cation and anion.

Here's an example of how to do that

If you start with the formula $\text{K"_2"O}$, the 2 subscript that potassium has will be the negative charge on oxygen and the 1 subscript that oxygen has will be the positive charge on the potassium.

This means that the negative ion will be ${\text{O}}^{2 -}$ and the positive ion will be ${\text{K}}^{+}$.

Take another example: ${\text{Fe"_2"O}}_{3}$. Using this approach will get you the positive ion ${\text{Fe}}^{3 +}$ and the negative ion ${\text{O}}^{2 -}$.

You can use this method for any ionic formula. Remember that the subscript of an atom is equal to the charge on the other atom. You can extend this method to polyatomic ions as well. Read more on that here: