A magnesium ion has a charge of +2 and an oxygen ion has a charge of –2. What is the chemical formula of the compound made of these ions?

A magnesium ion has a +2 charge, and an oxygen ion has a −2 charge. One of each ion balances to a total charge of zero, so the simplest formula is MgO. The correct choice is MgO; Mg₂O₂ has the same ratio but is not written in its simplest form.

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