Oxidation Numbers
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Core Concept
An oxidation number (or oxidation state) is a value assigned to an atom in a molecule or ion that represents the hypothetical charge it would have if all bonds were completely ionic.
Key Purpose: Oxidation numbers help track the transfer of electrons during chemical reactions.
Practice Tips
Oxidation numbers track electron transfer in reactions.
Use rules systematically to assign oxidation numbers.
Oxidation is an increase in oxidation number, while reduction is a decrease.
Rules for Assigning Oxidation Numbers
Free Elements:
The oxidation number of an atom in its elemental form is 000.
Examples: O2\text{O}_2O2, N2\text{N}_2N2, S8\text{S}_8S8, Na\text{Na}Na all have Ox. number=0\text{Ox. number} = 0Ox. number=0.
Monatomic Ions:
The oxidation number equals the ion's charge.
Examples: Na+=+1\text{Na}^+ = +1Na+=+1, Cl−=−1\text{Cl}^- = -1Cl−=−1, Mg2+=+2\text{Mg}^{2+} = +2Mg2+=+2.
Hydrogen:
In compounds: +1+1+1 (except in metal hydrides where it is −1-1−1, e.g., NaH\text{NaH}NaH).
Oxygen:
In most compounds: −2-2−2 (except in peroxides like H2O2\text{H}_2\text{O}_2H2O2, where it is −1-1−1, and in compounds with fluorine like OF2\text{OF}_2OF2, where it is +2+2+2).
Alkali Metals (Group 1\text{Group 1}Group 1):
Always +1+1+1 in compounds.
Example: NaCl,K2O\text{NaCl}, \text{K}_2\text{O}NaCl,K2O.
Alkaline Earth Metals (Group 2\text{Group 2}Group 2):
Always +2+2+2 in compounds.
Example: CaCl2,MgO\text{CaCl}_2, \text{MgO}CaCl2,MgO.
Halogens (Group 17\text{Group 17}Group 17):
Usually −1-1−1, except when bonded to oxygen or another halogen with a higher electronegativity.
Example: Cl2O=+1\text{Cl}_2O = +1Cl2O=+1 for Cl\text{Cl}Cl.
Neutral Compounds:
The sum of oxidation numbers equals 000.
Example: In H2O\text{H}_2\text{O}H2O: 2(+1)+(−2)=02(+1) + (-2) = 02(+1)+(−2)=0.
Polyatomic Ions:
The sum of oxidation numbers equals the ion's charge.
Example: In SO42−\text{SO}_4^{2-}SO42−: S=+6\text{S} = +6S=+6, 4×(−2)=−84 \times (-2) = -84×(−2)=−8; +6+(−8)=−2+6 + (-8) = -2+6+(−8)=−2.
Steps to Assign Oxidation Numbers
Start with the rules for free elements, ions, hydrogen, and oxygen.
Use the compound's overall charge (neutral or ionic) to determine unknown oxidation numbers.
Verify by checking that the sum matches the total charge.