Given the compound `~Al_2O_3` ,  a) What is the charge of each Al ion if each O ion has a charge of 2-? How do you know? b) What must happen to a neutral Al atom before it can bond with O?

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Expert Answers
sharikendrick eNotes educator| Certified Educator

Ionic Bonding:

The bond formed between Al and O is called an ionic bond. Ionic bonds are formed when when an atom that is a metal (Al) transfers one or more valence electrons to an atom that is a nonmetal (O).

When a metal atom such as Al loses electrons, it becomes a positively charged ion. When a nonmetal atom such as O gains electrons, it becomes a negatively charged ion. When Al forms an ion, it loses three valence electrons, giving it a 3+ charge. When O forms an ion, it gains two valence electrons, giving it a 2- charge.

The ions in an ionic bond are held together by the strong electrostatic attraction between the positively charged metal ions and the negatively charged nonmetal ions.

Charges in Ionic Bonds:

In an ionic compound, the total charge of the metal atom must be equal and opposite to the total charge of the nonmetal atom. The total charge of an atom is equal to the atom's subscript times its charge.

For the compound `~Al_2O_3` , we know that the there are three O atoms (from the subscript), and that each O atom has a charge of 2-. Therefore, the total negative charge in the compound is: 3 x (-2) = -6. 

Since the total negative charge in the compound must be equal and opposite to the total positive charge in the compound, the total positive charge in the compound must be equal to +6. The compound contains 2 Al atoms. In order for there to be a total positive charge of +6, each Al atom must have a charge of 3+: 2 x (+3) = +6.