Video: Deducing the Ionic Formula of an Ionic Compound Where Both Ions Have Greater-Than-One Charge

Iron(III) ions have a charge of 3+ and oxide ions have a charge of 2−. What is the formula of iron(III) oxide?

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Video Transcript

Iron(III) ions have a charge of three plus and oxide ions have a charge of two minus. What is the formula of iron(III) oxide?

The first thing we need to do is find the symbol for iron on our periodic table. The symbol for iron is Fe. The next thing we need to do is recall that oxide is derived from oxygen. Oxygen has the symbol O. So we know that the formula for iron(III) oxide has the form Fe something O something.

For the next part of the question, we’re going to need to remember that ionic compounds are neutral overall. Even though they contain positively and negatively charged ions, these charges must balance out perfectly. Iron(III) has a charge of three plus, while the negatively charged oxide ion has a charge of two minus.

In order to produce a neutral ionic compound, we need to make these charges balance. Because the charges are different, one iron ion and one oxide ion cannot balance each other out. We’re left with the charge overall. Add again another oxide anion takes the charge through zero out to the other side. So we can move in the other direction by adding another iron ion. This leaves us with a two plus overall charge. This can be balanced out by adding one more oxide ion giving us an overall neutral ionic compound. That’s two iron(III) ions and three oxide ions.

This means that the formula for iron(III) oxide is Fe₂O₃. This way of incrementing the positive and negative charges is one of the easiest ways to get the right answer. But another way of looking at it is we’ve taken two from the two minus and multiplied it by the three plus and three from the three plus and multiplied it by the two minus.

This way, we’ve ended up with the same magnitude of charge in both directions. So that when we sum them together, we get an overall neutral ionic compound. Of course, Fe₄O₆ is also neutral because four times three equals six times two. However, it’s better to simplify the ratio between the numbers and present the formula in its simplest form. So the formula of iron(III) oxide is Fe₂O₃.

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