The oxidation number of oxygen (Group VI, period 2) is -2.
The electrical charge of an atom describes its Oxidation number.\
The number of electrons that must be gained or lost in an atom's valence electron shell to make it to be filled or half-filled gives us the oxidation number value.
When the atom gains electrons,it acquires a negative oxidation number. When the atom loses electrons,it acquires a positive oxidation number
Oxygen has 8 electrons with {He] 2s² 2p⁴ configuration.
With 6 electrons in its valence shell, it need to gain 2electrons to achieve stability, When it gains 2 electrons, it acquires a negative oxidation number of -2.
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