Answer:
The oxidation number of oxygen in OH− is −2. This is because the oxidation number of an atom is defined as the charge that an atom appears to have on forming ionic bonds with other heteroatoms1. In OH−, the oxygen atom forms an ionic bond with the hydrogen atom, which has a lower electronegativity than oxygen. Therefore, the oxygen atom gains two electrons from the hydrogen atom and has a negative charge of −2. The hydrogen atom loses one electron and has a positive charge of +1. The sum of the oxidation numbers of all the atoms in a compound is equal to the overall charge of the compound. In OH−, the overall charge is −1, so the oxidation number of oxygen is −2 and the oxidation number of hydrogen is +1.
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