If every oxygen ion (combined with an aluminum ion) has a charge of –2, what must the charge of each aluminum ion be? how can you tell this? also, what has to happen to an uncharged aluminum atom so that it will bond with oxygen?

Each aluminium ion should have a charge of +2, because the sum of oxidation numbers must be zero.

The uncharged aluminium atom must lose two electrons to become an ion with +2 charge.
what has to happen to an uncharged aluminum atom so that it will bond with oxygen?

It has to lose electrons to become positively charged, so it can attract a negatively charged oxygen ion

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