A chemistry teacher told his students that an ion is an atom that has an unequal number of electrons and protons and that all ions have a charge. A negative charge means that the ion has more electrons than protons. And a positive charge means that it has less electrons than protons. Which ion in the table has a charge that is not the opposite of the charge of one of the other ions?
Oxygen has a charge of negative two. Chlorine has a charge of negative one. Aluminium has a charge of positive three. Magnesium has a charge of positive two. And potassium has a charge of positive one. Negative one is the opposite of positive one. Therefore, chlorine and potassium have the opposite charge of each other. In the same way, negative two and positive two are opposite to each other, as they add to give zero. This means that oxygen and magnesium have the opposite charge of each other.
This leaves us with aluminium or aluminum, which is the only ion in the table that has a charge that is not the opposite of any of the others. The opposite of positive three would be negative three. And none of the other ions have a charge of negative three.