The ionic bond only forms when the charges between the ions are equal and opposite. The positive charge must be equal of that of the negative anion which means their charges will balance each other out and make the compound neutral.
To form sodium chloride (table salt) the positive sodium cation has a charge of
because the +1 charge and the -1 charge are equal and opposite they neutralize each other to a zero charge.
In the case of aluminum oxide, the
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