What condition must be met in writing a balanced formula for an ionic compound

I just got out of the hospital (Brain Tumor) and I'm expected to complete an entire review (Suffering from mild memory loss). Help me if you can =)

Ionic compounds: metal + non-metal, electron given or taken.

Ionic bonding is the electrostatic between oppositely charged ions.

So for ionic compounds, overall charges on cation(s) (positive) and anion(s) (negative) must cancel out.

For example:

Aluminum, Al, has a +3 charge.
Chloride, Cl, has a -1 charge.

So in order for Cl to balance out the +3 charge, you would need THREE Cl's to balance it out.

-1 x 3 = -3

+3 - 3 = 0

Final formula: AlCl3

That the positive and negative charges cancel out to zero

e.g. Al+3 and O-2 In order to balance the charges find the lowest common multiple between 2 and 3

It is 6. so multiply the +3 by 2 and the -2 by 3 to get 6+ and 6- which equals zero when you ADD them.

So the correct formula for aluminum oxide is Al2O3