The oxidation state of a pure element is always zero.
The oxidation state for a pure ion is equivalent to its ionic charge.
What is oxidation state?
The oxidation state or oxidation number, is the hypothetical charge of an atom if all of its bonds to different atoms were fully ionic. The oxidation state may be positive, negative or zero.
What are the rules to write the oxidation number?
1. The atoms in pure elements have oxidation number of ZERO.
2. The more-electronegative element in a binary molecular compound is assigned the number equal to the negative charge it would have as an anion. The less-electronegative element is assigned the number equal to the positive charge it would have as a cation.
3. Fluorine always has an oxidation number of -1 in compounds because it is the most electronegative element.
4. Oxygen is usually -2 unless in peroxides (then it is -1) or with fluorine.
5. Hydrogen has +1 in all compounds containing elements that are more electronegative than it, and is -1 in compounds with metals.
6. The algebraic sum of the oxidation numbers of all atoms in a neutral compound is zero.
7. The algebraic sum of the oxidation numbers of all atoms in a polyatomic ion is equal to the charge of the ion.
8. Although rules 1-7 apply to covalently bonded atoms, oxidation numbers can also be assigned to atoms in ionic compound. Monoatomic ions have oxidation numbers equal to their ionic charge.
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