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how to find a compound's oxidation numbers
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For the main group elements, their usual oxidation numbers are as follows:
Group 1 = +1
Group 2 = +2
Group 13 = +3
Group 14 = +4, sometimes -4
Group 15 = -3
Group 16 = -2
Group 17 = -1
Many of the transition metals have varying oxidation states. However there are rules to follow in assigning oxidation numbers to atoms, most especially if they are involved in bonding in a compound or molecule. The usual oxidation numbers of the main group elements is not always the case. Here are the rules.
1. Elements in their free, uncombined states have ON of 0. The ON of Na is 0. The ON of H2 is 0.
2. The ON a monatomic ion is its charge. Na+1 has an ON of +1.
3. The ON of oxygen is -2 except in peroxides (-1), superoxides (-1/2) and when paired with the more electronegative fluorine (+2).
In hydrogen peroxide, H2O2, ON of H is +1 and that of O is -1.
In potassium superoxide, KO2, the ON of K is +1 and that of O is -1/2.
In dioxygen difluoride, O2F2, the ON of F is -1 and that of O is +2.
4. The ON of hydrogen is +1 except in metal hydrides where its oxidation number is -1.
In sodium hydride, NaH, the ON of Na is +1 and that of H is -1.
5. The ON of fluorine is always -1. Other halogens such as chlorine, bromine, and iodine have have oxidation numbers of -1 in binary compounds, except when paired with more electronegative atoms.
In H-O-Cl, the ON of H is +1. O has -2 and Cl has +1.
6. Alkali metals (group 1) has oxidation numbers of +1.
7. Group 2 elements have +2.
8. The sum of the oxidation numbers of atoms in a compound or molecule is 0.
9. The sum of the oxidation numbers of atoms in a polyatomic ion is equal to the charge of the ion.
I hope this helps you.
