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### Why do nonmetals have both positive and negative oxidation numbers?

The chemical elements can be broadly divided into metals and nonmetals according to their tendency to loose or gain electrons:
• Atoms that belong to metallic elements tend to loose electrons. When they loose electrons, they become cations, positive ions with a charge that equals the number of electrons they have lost. That number is given by the oxidation number. For instance, sodium's oxidation number is +1, while calcium's oxidation number is +2.
• On the other hand, atoms that belong to nonmetallic elements tend to gain electrons, so they become anions, ions with a negative charge that equals the number of electrons they have gain. For instance, fluorine tends to gain one electron and becomes F-. That is why it has oxidation number -1.
But, as we can see in the following periodic table, most nonmetals have both positive and negative oxidation number:

Why do nonmetals have both positive and negative oxidation number if they always tend to gain electrons?